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Overview

Brief Summary

Swainson’s Hawk (Buteo swainsoni) is a medium-sized slender hawk (Crow-sized), with long, pointed wings and a long tail. Measurements include: length 17-22 inches (43-56 cm); wingspan 47-54 inches (120-137 cm); weight 1.3-2.7 lb (595-1240 g). Females slightly larger than males. Plumage extremely variable, but most individuals are recognizable. Adult-sides of the head and entire upper parts dark blackish brown; feathers obscurely edged with paler brown to cinnamon. Tail gray, basally whitish, with a narrow white tip, and several indistinct blackish bars, the last one broader. Primaries blacker than back; becoming paler basally. Throat white; breast brownish chestnut with weak black shaft streaks. Belly and legs dull white; indistinctly mottled and barred with brown to rufous. Under-wings pale with conspicuous dark marks at ends of coverts. Dark phase more or less sooty all over. Wing and tail as in normal phase, except that wing linings are much more marked with blackish. Rufous phase lighter brown below than the dark phase; and somewhat barred and blotched below with rusty brown. Intermediates occur between all the phases. Eye dark brown; cere pale greenish yellow; bill blackish; legs wax yellow (Brown et al 1968).

The immature plumage, which is worn for two years, is similar to that of adults in its two- toned underwing and finely barred tail, but young birds have a spotted and streaked breast that at times shows a hint of a darker pattern, and the head shows a definite buffy streak above the eye and on the cheek, with a dark eye line and malar stripes. This typical pattern occurs on perhaps half the Swainson’s Hawk encountered in Arizona, and if color pattern alone is used for identification, the other half will be mis-identified. (Glinski 1998).

Found only in the New World; it breeds in North America, in the Great Plains and arid regions, north sparingly to interior Alaska, and south to northern Mexico, and winters in South America. The normal winter range is the Pampas of Argentina, and it has been assumed that any found elsewhere at that season are casuals, probably unable to make the long migration (Brown et al 1968).

Gives a descending shrill, plaintive whistle, kreeeeeeer, trailing off at end. In flight, shows profile like that of Turkey Vulture; the wings are held in a dihedral, or "V", position, which promotes aerodynamic stability in open landscapes where wind can interfere with flight close to the ground. Highly migratory, often seen in large flocks on spring and fall flights. During the breeding season, a soaring, open country hunter. Sometimes hunts high in the air, but more frequently courses low over prairie. Rarely observed flying low at high speed as Ferruginous Hawk does. Often hunts from perches such as tree limbs, poles or posts, rocks, and elevated ground.

  • Arizona Game and Fish Department. 2001. Buteo swainsoni. Unpublished abstract compiled and edited by the Heritage Data Management System, Arizona Game and Fish Department, Phoenix, AZ
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Distribution

The Swainson's Hawk, -Buteo swainsoni-, spends most of the year in the western United States extending into southwest Canada and south to west Texas. In the winter months, these birds migrate over Central America to the La Pampas region of Argentina (Brown 1996, TPWD 1997).

Biogeographic Regions: nearctic (Native ); neotropical (Native )

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occurs (regularly, as a native taxon) in multiple nations

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National Distribution

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Breeding

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Breeding

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Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) BREEDS: known to have bred in east-central Alaska east into Yukon Territory and extreme northwestern Mackenzie; central Alberta, central Saskatchewan, southern Manitoba, western and southern Minnesota, and western Illinois south (mainly east of Cascades and Sierra Nevada) to southern California (rarely), Baja California (formerly), Sonora, Durango, Chihuahua, central and southern Texas and western Missouri; eastern breeding limits unstable. WINTERS: according to AOU (1983), primarily on pampas of southern South America (south to Uruguay and Argentina), irregularly north to Costa Rica and Panama, casually or irregularly north to the southwestern U.S. (especially Texas) and southern Florida.

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Range

W N America; winters to n Argentina, s Brazil and Paraguay.
  • Clements, J. F., T. S. Schulenberg, M. J. Iliff, D. Roberson, T. A. Fredericks, B. L. Sullivan, and C. L. Wood. 2014. The eBird/Clements checklist of birds of the world: Version 6.9. Downloaded from http://www.birds.cornell.edu/clementschecklist/download/

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The breeding range of the Swainson's hawk is restricted primarily to
western North America from interior Alaska and western Canada south
into northern Mexico [12,21].  The Swainson's hawk winters primarily on
the pampas of southern South America, irregularly north to Costa Rica
and Panama, and sometimes north to the southwestern United States and
southern Florida [1,12,21].  During migration the Swainson's hawk occurs
regularly in most of the central states and Canadian provinces, and
rarely, east along the Gulf Coast to Florida. It is occasionally a fall
migrant through the Florida Keys.  The Swainson's hawk is occasionally
found in northeastern North America from southern Ontario, southern
Quebec, New York, and Massachusetts south to Virginia [1].
  • 1.  American Ornithologists' Union. 1983. Checklist of North American birds.        6th ed. Lawrence, KS: Allen Press, Inc. 877 p.  [21234]
  • 12.  DeGraaf, Richard M.; Scott, Virgil E.; Hamre, R. H.; [and others]
  • 21.  Herron, Gary B.; Mortimore, Craig A.; Rawlings, Marcus S. 1985.        Red-tailed hawk. In: Nevada raptors: Their biology and management.        Biological Bulletin No. 8. Reno, NV: Nevada Department of Wildlife:        29-30.  [22694]

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Regional Distribution in the Western United States

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This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):

    1  Northern Pacific Border
    2  Cascade Mountains
    3  Southern Pacific Border
    4  Sierra Mountains
    5  Columbia Plateau
    6  Upper Basin and Range
    7  Lower Basin and Range
    8  Northern Rocky Mountains
    9  Middle Rocky Mountains
   10  Wyoming Basin
   11  Southern Rocky Mountains
   12  Colorado Plateau
   13  Rocky Mountain Piedmont
   14  Great Plains
   15  Black Hills Uplift
   16  Upper Missouri Basin and Broken Lands

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Occurrence in North America


AK AZ CA CO CT DE FL ID IL
IA KS LA MD MA MT NE NV NH
NJ NM NY ND OH OK OR PA RI
SD TN TX UT VA WA WY

AB BC MB NT ON PQ SK YT


MEXICO

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Physical Description

Morphology

This hawk's most unique feature is its variation in color. The light color morph includes white patches on the forehead, the throat and the belly. The rest of the body is a dark brown. The dark color morph, which is the less common type, includes an entirely dark brown body with only a white patch under the tail. Other variations between these two distinct extremes have been observed. These hawks vary in length from 19 to 22 inches, and have a wingspan of 47 to 57 inches. An average weight for a male is 1.8 pounds, while the average for the female is almost 2.5 pounds. This bird is commonly confused with a Red-tailed hawk, but the Swainson's Hawk has a longer wingspan, more variation in color, and flies in a slight dihedral pattern (Brown 1996, AID 1997).

Other Physical Features: endothermic ; bilateral symmetry

Average mass: 980.64 g.

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Size

Length: 53 cm

Weight: 1069 grams

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Ecology

Habitat

This hawk prefers open grasslands and desert-like habitats. It is common to see this hawk perched on a fence post in a prairie or open range. The Swainson's Hawk also inhabits agricultural areas, and is known to follow farmer's tractors in search of insect or rodent prey (Brown 1996, AID 1997).

Terrestrial Biomes: desert or dune ; savanna or grassland ; chaparral

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Habitat and Ecology

Systems
  • Terrestrial
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Comments: Savanna, open pine-oak woodland and cultivated lands (e.g., alfalfa and other hay crops, and certain grain and row croplands) with scattered trees. Tolerates extensive cultivation in nesting area (Schmutz 1989), though vineyards, orchards, rice, corn, and cotton are not suitable foraging habitat. In migration and winter also in grasslands and other open country (AOU 1983). Migrants may roost at night on ground in very large fields (Ridgely and Gwynne 1989). Nests typically in solitary tree, bush, or small grove; many nests on old black-billed magpie nests; sometimes on rock ledge. Readily nests in trees in shelterbelts and similar situations produced by humans (Gilmer and Stewart 1984). Recently reported nesting in city trees and on railway signal gantry in Regina, Saskatchewan (Condor 94:773-774). In the Central Valley of California, nests often are within one mile of a riparian zone; Great Basin nests, usually in junipers, are not near riparian zones (Biosystems Analysis, Inc. 1989). Evidently often returns to area where it nested in previous year.

GREAT BASIN AND MOJAVE HABITAT:
Swainson's Hawks have adapted to agricultural landscapes in Nevada. An ideal landscape for the Swainson's Hawk provides large riparian nesting trees, agricultural fields, and open shrublands within relatively close proximity (GBBO 2010). Swainson's Hawks in the Great Basin occupy the Juniper/Sagebrush community typical to the area. In California, Swainson's hawk habitat generally consists of large, flat, open, undeveloped landscapes that include suitable grassland or agricultural foraging habitat and sparsely distributed trees for nesting (England et al. 1997). Populations in the Great Basin often use juniper trees (Juniperus sp.) for nesting (England et al. 1997), and at least three known nest sites in the Mojave Desert are in Joshua trees (Yucca brevifolia) (California Natural Diversity Database 2009) (PCCP 2010). In addition to Joshua trees, this species was also known historically from the Mojave Yukka (Yucca schidigera) and possibly desert riparian habitats (Bloom 1980).

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Cover Requirements

More info for the terms: cover, density, selection

Swainson's hawk nests are often built in trees that provide shade for
the nest but also afford a good view of the surrounding terrain [33].

The Swainson's hawk is a more efficient predator in open areas than in
areas with high vegetative cover [5].  Bechard [5] found that vegetative
cover is more important than prey abundance in the selection of hunting
sites by the Swainson's hawk.  In Whitman County, Washington, the
Swainson's hawk foraged at sites where vegetative height and density had
been reduced, even though other areas had higher prey density [5].
Alfalfa field use by Swainson's hawk in northern California increased
dramatically during monthly harvests that reduced vegetative heights
[38].
  • 33.  Schlorff, Ronald W.; Bloom, Peter H. 1984. Importance of riparian        systems to nesting Swainson's hawks in the Central Valley of California.        In: Warner, Richard E.; Hendrix, Kathleen M., eds. California riparian        systems: Ecology, conservation, and productive management: Proceedings        of a conference; 1981 September 17-19; Davis, CA. Berkeley, CA:        University of California Press: 612-618.  [5863]
  • 38.  Woodbridge, B. 1991. What do Swainson's hawks really eat?. Journal of        Raptor Research. 25(4): 163. [Abstract]
  • 5.  Bechard, Marc J. 1982. Effect of vegetative cover on foraging site        selection by Swainson's hawk. Condor. 84(2): 153-159.  [22656]

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Preferred Habitat

More info for the terms: shrubs, tree, tundra

The Swainson's hawk inhabits mostly semiopen to open areas in tundra,
valleys, plains, dry meadows, foothills, and level uplands at low to
middle elevations [1,31,40].

Nesting habitat - The Swainson's hawk nests almost exclusively in trees
[37] and will nest in almost any tree species of suitable size (taller
than 10 feet [3 m] with a d.b.h. of 2 inches [5 cm] or more) [6,7].
Nests are constructed in isolated trees (dead or live), in trees in
wetlands and along drainages, or in windbreaks in fields and around
farmsteads [6,12,31].  The Swainson's hawk builds nests from 4 to 100
feet (1.2-30.4 m) above the ground [12,14,35].  They sometimes add to an
existing black-billed magpie (Pica pica) nest [31].  The Swainson's hawk
occasionally nests in shrubs, on the crossbars of telephone poles, or on
the ground, low cliffs, rocky pinnacles, or cutbanks [6,12,31].

In the Central Valley of California, the majority of Swainson's hawk
nests and territories are located in or near riparian systems.  Nests
are found most often in cottonwoods and oaks [33].  In Whitman County,
Washington, Swainson's hawk nests were constructed in black locust
(Robinia pseudoacacia), cherry (Prunus spp.) and hawthorn (Crataegus
spp.) trees [5].  Of 48 Swainson's hawk nests on the Laramie Plains,
Wyoming, 43 were in narrowleaf cottonwood (Populus angustifolia),
peachleaf willow (Salix amygdaloides), or other willows.  None of the
few buttes in the area were used for nesting [15].  In the Centennial
Valley of Montana, Swainson's hawks nest extensively in willows [32].
At 234 Swainson's hawk nest sites in North Dakota, eastern cottonwood
(Populus deltoides) was the most common tree species used (45%).  Other
species included Chinese elm (Ulmus parvifolia), 22 percent; peachleaf
willow, 13 percent; boxelder (Acer negundo), 12 percent; and green ash
(Fraxinus pennsylvanica), 7 percent.  American elm (U. americana) and
Russian olive (Elaeagnus angustifolia) accounted for the remaining 1
percent [20].  In the Lower Sonoran Desert of New Mexico, the Swainson's
hawk often nests and roosts on large yucca plants [30].  In the
southwestern United States, mesquite is commonly used [6].

Foraging habitat - The Swainson's hawk generally forages in open
habitats with short vegetation containing small mammals, reptiles,
birds, and insects [6,40,38].  During the nesting season the Swainson's
hawk usually forages within 1.9 miles (3 km) of the nest.  The
Swainson's hawk has a home range of approximately 3.5 square miles (9 sq
km) [40].  Although the Swainson's hawk does search for prey from
elevated perches, it relies much more on aerial foraging.  Consequently,
it is not tied to habitats containing an abundance of perches, and often
occupy habitats with few or no perches except the nest tree [23].

Winter habitat - The Swainson's hawk generally spends the winter south
of the United States [1,12,31]; no information is available in the
English literature on its habitat in Central and South America.
  • 1.  American Ornithologists' Union. 1983. Checklist of North American birds.        6th ed. Lawrence, KS: Allen Press, Inc. 877 p.  [21234]
  • 12.  DeGraaf, Richard M.; Scott, Virgil E.; Hamre, R. H.; [and others]
  • 14.  DuBois, Kristi; Becker, Dale; Thornbrugh, Joe. 1987. Identification of        Montana's birds of prey. Montana Outdoors. 18(6): 11-31.  [3606]
  • 15.  Dunkle, Sidney W. 1977. Swainson's hawks on the Laramie Plains, Wyoming.        Auk. 94: 65-71.  [22654]
  • 20.  Gilmer, David S.; Stewart, Robert E. 1984. Swainson's hawk nesting        ecology in North Dakota. Condor. 86: 12-18.  [22653]
  • 23.  James, Paul C. 1992. Urban-nesting of Swainson's hawks in Saskatchewan.        Condor. 94: 773-774.  [23120]
  • 30.  Olendorff, Richard R.; Kochert Michael N. 1977. Land management for the        conservation of birds of prey. In: Chancellor, R. D., ed. World        conference on birds of prey; Report of proceedings; [Date unknown]
  • 31.  Palmer, Ralph S., editor. 1988. Handbook of North American birds. Volume        5. New Haven, CT: Yale University Press. 463 p.  [22303]
  • 32.  Restani, Marco. 1991. Resource partitioning among three Buteo species in        the Centennial Valley, Montana. Condor. 93: 1007-1010.  [20663]
  • 33.  Schlorff, Ronald W.; Bloom, Peter H. 1984. Importance of riparian        systems to nesting Swainson's hawks in the Central Valley of California.        In: Warner, Richard E.; Hendrix, Kathleen M., eds. California riparian        systems: Ecology, conservation, and productive management: Proceedings        of a conference; 1981 September 17-19; Davis, CA. Berkeley, CA:        University of California Press: 612-618.  [5863]
  • 35.  Verner, Jared; Boss, Allan S., tech. coords. 1980. California wildlife        and their habitats: western Sierra Nevada. Gen. Tech. Rep. PSW-37.        Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific        Southwest Forest and Range Experiment Station. 439 p.  [10237]
  • 37.  Schmutz, Josef K.; Fyfe, Richard W.; Moore, David A.; Smith, Alan R.        1984. Artificial nests for ferruginous and Swainson's hawks. Journal of        Wildlife Management. 48(3): 1009-1013.  [23122]
  • 38.  Woodbridge, B. 1991. What do Swainson's hawks really eat?. Journal of        Raptor Research. 25(4): 163. [Abstract]
  • 40.  Kochert, Michael N. 1986. Raptors. In: Cooperrider, Allan Y.; Boyd,        Raymond J.; Stuart, Hanson R., eds. Inventory and monitoring of wildlife        habitat. Denver, CO: U.S. Department of the Interior, Bureau of Land        Management, Denver Service Center: 313-349.  [13527]
  • 5.  Bechard, Marc J. 1982. Effect of vegetative cover on foraging site        selection by Swainson's hawk. Condor. 84(2): 153-159.  [22656]
  • 6.  Bednarz, James C. 1988. Swainson's hawk. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]
  • 7.  Bednarz, James C.; Hoffman, Stephen W. 1988. The status of breeding        Swainson's hawks in southeastern New Mexico. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]

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Associated Plant Communities

More info for the term: shrubs

The Swainson's hawk breeds in open grasslands, sagebrush (Artemisia
spp.), shrub-steppe, oak (Quercus spp.) woodlands, open pine (Pinus
spp.)-oak woodlands, pinyon-juniper (Pinus spp.-Juniperus spp.)
woodlands, and cultivated lands [1,3,6,16,40].  In California the
Swainson's hawk favors open blue oak (Quercus douglasii) savannahs and
gray pine (Pinus sabiniana)-oak woodlands [35].  In the Central Valley
of California, populations of Swainson's hawks frequently nest and roost
in riparian communities dominated by valley oak (Quercus lobata),
cottonwoods (Populus spp.), California sycamore (Platanus racemosa), and
willows (Salix spp.)  [22,33].  Foraging habitat for Swainson's hawks in
California includes native grassland communities of oat (Avena spp.),
brome grass (Bromus spp.), ryegrass (Elymus spp. and Lolium spp.), and
barley (Critesion spp.) [33]. West of Laramie, Wyoming, Dunkle [15]
reported that breeding habitat of the Swainson's hawk included irrigated
sedge meadows, shortgrass plains with some sagebrush, and black
greasewood (Sarcobatus vermiculatus) [15].

In the Great Basin, the Swainson's hawk is often found nesting in
juniper-sagebrush and prairie habitats [33].  In Arizona, the Swainson's
hawk generally occurs in sparse semidesert grasslands, plains
grasslands, Great Basin grasslands, and Chihuahuan Desert scrub often
mixed with a few species of shrubs including yucca (Yucca spp.),
creosotebush (Larrea tridentata), mesquite (Prosopis spp.), and fourwing
saltbrush (Atriplex canescens).  In New Mexico and Texas, breeding
Swainson's hawk occur in various types of grasslands including
grasslands with sand shinnery oak (Q. havardii), and are occasionally
found in Chihuahuan Desert scrub.  In Oklahoma, Swainson's hawk breed
primarily in grasslands [6].

The Swainson's hawk sometimes nests in intensively cultivated areas
[4,5,20].  Of the large raptors breeding in northern Colorado, only the
Swainson's hawk regularly nested near cultivated lands [20].
  • 1.  American Ornithologists' Union. 1983. Checklist of North American birds.        6th ed. Lawrence, KS: Allen Press, Inc. 877 p.  [21234]
  • 15.  Dunkle, Sidney W. 1977. Swainson's hawks on the Laramie Plains, Wyoming.        Auk. 94: 65-71.  [22654]
  • 16.  Emmerich, John M.; Vohs, Paul A. 1982. Comparative use of four woodland        habitats by birds. Journal of Wildlife Management. 46(1): 43-49.        [19283]
  • 20.  Gilmer, David S.; Stewart, Robert E. 1984. Swainson's hawk nesting        ecology in North Dakota. Condor. 86: 12-18.  [22653]
  • 22.  Holland, Robert F.; Roye, Cynthia L. 1989. Great Valley riparian        habitats and the National Registry of Natural Landmarks. In: Abell, Dana        L., technical coordinator. Proceedings of the California riparian        systems conference: Protection, management, and restoration for the        1990's; 1988 September 22-24; Davis, CA. Gen. Tech. Rep. PSW-110.        Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific        Southwest Forest and Range Experiment Station: 69-73.  [13511]
  • 3.  Balda, Russell P.; Masters, Nancy. 1980. Avian communities in the        pinyon-juniper woodland: a descriptive analysis. In: DeGraaf, Richard        M., technical coordinator. Management of western forests and grasslands        for nongame birds: Workshop proceedings; 1980 February 11-14; Salt Lake        City, UT. Gen. Tech. Rep. INT-86. Ogden, UT: U.S. Department of        Agriculture, Forest Service, Intermountain Forest and Range Experiment        Station: 146-169.  [17903]
  • 33.  Schlorff, Ronald W.; Bloom, Peter H. 1984. Importance of riparian        systems to nesting Swainson's hawks in the Central Valley of California.        In: Warner, Richard E.; Hendrix, Kathleen M., eds. California riparian        systems: Ecology, conservation, and productive management: Proceedings        of a conference; 1981 September 17-19; Davis, CA. Berkeley, CA:        University of California Press: 612-618.  [5863]
  • 35.  Verner, Jared; Boss, Allan S., tech. coords. 1980. California wildlife        and their habitats: western Sierra Nevada. Gen. Tech. Rep. PSW-37.        Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific        Southwest Forest and Range Experiment Station. 439 p.  [10237]
  • 4.  Bechard, M. J. 1980. Factors affecting the productivity of Swainson's        hawk nesting in southeastern Washington. Pullman, WA: Washington State        University. [Pages unknown]
  • 40.  Kochert, Michael N. 1986. Raptors. In: Cooperrider, Allan Y.; Boyd,        Raymond J.; Stuart, Hanson R., eds. Inventory and monitoring of wildlife        habitat. Denver, CO: U.S. Department of the Interior, Bureau of Land        Management, Denver Service Center: 313-349.  [13527]
  • 5.  Bechard, Marc J. 1982. Effect of vegetative cover on foraging site        selection by Swainson's hawk. Condor. 84(2): 153-159.  [22656]
  • 6.  Bednarz, James C. 1988. Swainson's hawk. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]

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Habitat: Plant Associations

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This species is known to occur in association with the following plant community types (as classified by Küchler 1964):

More info for the terms: cactus, shrub

   K023  Juniper - pinyon woodland
   K024  Juniper steppe woodland
   K026  Oregon oakwoods
   K027  Mesquite bosque
   K030  California oakwoods
   K031  Oak - juniper woodlands
   K032  Transition between K031 and K037
   K033  Chaparral
   K034  Montane chaparral
   K035  Coastal sagebrush
   K037  Mountain-mahogany - oak scrub
   K038  Great Basin sagebrush
   K039  Blackbrush
   K040  Saltbush - greasewood
   K041  Creosotebush
   K042  Creosotebush - bursage
   K043  Paloverde - cactus shrub
   K044  Creosotebush - tarbush
   K045  Ceniza shrub
   K047  Fescue - oatgrass
   K048  California steppe
   K050  Fescue - wheatgrass
   K051  Wheatgrass - bluegrass
   K053  Grama - galleta steppe
   K054  Grama - tobosa prairie
   K055  Sagebrush steppe
   K056  Wheatgrass - needlegrass shrubsteppe
   K057  Galleta - three-awn shrubsteppe
   K058  Grama - tobosa shrubsteppe
   K059  Trans-Pecos shrub savanna
   K060  Mesquite savanna
   K061  Mesquite - acacia savanna
   K062  Mesquite - live oak savanna
   K063  Foothills prairie
   K064  Grama - needlegrass - wheatgrass
   K065  Grama - buffalograss
   K066  Wheatgrass - needlegrass
   K067  Wheatgrass - bluestem - needlegrass
   K068  Wheatgrass - grama - buffalograss
   K069  Bluestem - grama prairie
   K070  Sandsage - bluestem prairie
   K071  Shinnery
   K072  Sea oats prairie
   K074  Bluestem prairie
   K075  Nebraska Sandhills prairie
   K076  Blackland prairie
   K077  Bluestem - sacahuista prairie
   K083  Cedar glades
   K085  Mesquite - buffalograss
   K086  Juniper - oak savanna
   K087  Mesquite - oak savanna
   K088  Fayette prairie

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Habitat: Ecosystem

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This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):

FRES28 Western hardwoods
FRES29 Sagebrush
FRES30 Desert shrub
FRES31 Shinnery
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES37 Mountain meadows
FRES38 Plains grasslands
FRES39 Prairie
FRES40 Desert grasslands
FRES41 Wet grasslands
FRES42 Annual grasslands

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Habitat: Cover Types

More info on this topic.

This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):

    66  Ashe juniper - redberry (Pinchot) juniper
    67  Mohrs (shin) oak
    68  Mesquite
   203  Balsam poplar
   217  Aspen
   220  Rocky Mountain juniper
   221  Red alder
   222  Black cottonwood - willow
   233  Oregon white oak
   235  Cottonwood - willow
   236  Bur oak
   238  Western juniper
   239  Pinyon - juniper
   240  Arizona cypress
   241  Western live oak
   242  Mesquite
   246  California black oak
   249  Canyon live oak
   250  Blue oak - Digger pine
   252  Paper birch
   255  California coast live oak

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Migration

Non-Migrant: No. All populations of this species make significant seasonal migrations.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: Yes. At least some populations of this species make annual migrations of over 200 km.

In migration, occurs regularly in most of Middle America, and rarely east along the Gulf Coast to Florida (AOU 1983). In California, migrates March-early May, with a peak in the first half of April, and September-October (Biosystems Analysis, Inc. 1989). Migrants are greatly concentrated as they pass through Panama (mostly March-early April and October-early November; Ridgely and Gwynne 1989). Migrates through Costa Rica late September-November and late February-early May (Stiles and Skutch 1989). In Colombia, flocks of various sizes reported mainly February-March and September-early November (Hilty and Brown 1986). Main northward migration passes through Panama in mid-March, Veracruz in latter half of March and early April, southern Texas and southwstern U.S. chiefly in April (Palmer 1988); fall concentrations and movements occur in August-September in the north, mainly early October in Texas; peak in migration occurs in September in the southwestern U.S.; arrives in Argentina in late November (Palmer 1988). Annual migration flight may be 18,000-27,000 km, encompasses 4 months of the year. See Houston (1990) for information on migrations of Saskatchewan breeders. Migrates in large, often immense, flocks. Migrates over terrain where updrafts provide needed buoyancy for soaring. May roost at night on ground during migration.

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Trophic Strategy

The Swainson's Hawk is somewhat of a generalist, and eats whatever it can find. During its time in North America, its diet consists of insects, small mammals and birds, and occasional reptiles and amphibians. When these birds migrate to the Argentina area, they feed mainly on insects like grasshoppers and crickets (Brown 1996, TPWD 1997).

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Comments: Vertebrates (mainly mammals) dominate the diet during the breeding season; invertebrates (especially crickets and grasshoppers) are common food at other times and sometimes for nonbreeders in summer. Hawks wintering in Argentina ate mainly dragonflies (Condor 95:475-479, Wilson Bull. 105:365-366). Mammals consumed often include young ground squirrels and pocket gophers. Depending on availability, also eats other small mammals, snakes, lizards, birds, amphibians, and some carrion (e.g., road kills). Hunts for insects on ground; may also catch insects in air. Hunts while soaring or from perch. Does not feed during most of migration (occasional feeding during initial and terminal stages) (Palmer 1988).

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Food Habits

The Swainson's hawk is a versatile and opportunistic predator on
relatively small prey [6].  The Swainson's hawk feeds on small mammals,
large insects, birds, and reptiles [9,14,31,35].  During the breeding
season, the Swainson's hawk primarily preys on small mammals, especially
young ground squirrels (Spermophilus spp.), pocket gophers (Thomomys
spp.), and some microtines [15,20,31,32].  During migration
invertebrates often make up over 90 percent of the Swainson's hawk's
diet [31].

In a North Dakota study, Swainson's hawks preyed primarily on northern
pocket gopher (Thomomys talpoides), Richardson's ground squirrel
(Spermophilus richardsonii), meadow vole (Microtus pennsylvanicus), and
thirteen-lined ground squirrel (S. tridecemlineatus) [20].  To a lesser
extent Swainson's hawks also ate western meadow lark (Sturnella
neglecta), chestnut-collared longspur (Calcarius ornatus), sharp-tailed
grouse (Tympanuchus phasianellus), short-eared owl (Asio flammeus),
American kestrel (Falco sparverius), blue jay (Cyanocitta cristata),
and rock dove (Columba livia) [20].  Toads (Bufo spp.) and various
lizards, mostly desert grassland whiptail (Cnemidophorous uniparens)
and spiny lizards (Sceloporus spp.), were commonly taken by nesting
Swainson's hawk in Arizona.  Mammals, particularly cottontails
(Sylvilagus spp.), ground squirrels, and kangaroo rats (Dipodomys spp.),
were the most common food items brought to Swainson's hawk nestlings in
New Mexico [6].  Rabbits comprised between 40 and 80 percent of the diet
of Swainson's hawk nestlings in New Mexico [7].
  • 14.  DuBois, Kristi; Becker, Dale; Thornbrugh, Joe. 1987. Identification of        Montana's birds of prey. Montana Outdoors. 18(6): 11-31.  [3606]
  • 15.  Dunkle, Sidney W. 1977. Swainson's hawks on the Laramie Plains, Wyoming.        Auk. 94: 65-71.  [22654]
  • 20.  Gilmer, David S.; Stewart, Robert E. 1984. Swainson's hawk nesting        ecology in North Dakota. Condor. 86: 12-18.  [22653]
  • 31.  Palmer, Ralph S., editor. 1988. Handbook of North American birds. Volume        5. New Haven, CT: Yale University Press. 463 p.  [22303]
  • 32.  Restani, Marco. 1991. Resource partitioning among three Buteo species in        the Centennial Valley, Montana. Condor. 93: 1007-1010.  [20663]
  • 35.  Verner, Jared; Boss, Allan S., tech. coords. 1980. California wildlife        and their habitats: western Sierra Nevada. Gen. Tech. Rep. PSW-37.        Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific        Southwest Forest and Range Experiment Station. 439 p.  [10237]
  • 6.  Bednarz, James C. 1988. Swainson's hawk. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]
  • 7.  Bednarz, James C.; Hoffman, Stephen W. 1988. The status of breeding        Swainson's hawks in southeastern New Mexico. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]
  • 9.  Burleigh, Thomas D. 1950. "Give Idaho hawks a break" biologist asks        nimrods; some species helpful to wildlife. Idaho Wildlife Review.        February: 6-7.  [21497]

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Associations

Predators

Large raptors including great horned owl (Buteo virginianus) and golden
eagle (Aquila chrysaetos), and coyote (Canis latrans), bobcat (Lynx
rufus), and striped skunk (Mephitis mephitis) have been known to kill
Swainson's hawk nestlings and fledglings or destroy clutches
[7,11,15,31,38].  Crows (Corvus spp.) sometimes destroy clutches
[15,31].
  • 11.  Cowart, Linda. 1975. The desert hunters. Pacific Discovery. 28(1): 5-9.        [22655]
  • 15.  Dunkle, Sidney W. 1977. Swainson's hawks on the Laramie Plains, Wyoming.        Auk. 94: 65-71.  [22654]
  • 31.  Palmer, Ralph S., editor. 1988. Handbook of North American birds. Volume        5. New Haven, CT: Yale University Press. 463 p.  [22303]
  • 38.  Woodbridge, B. 1991. What do Swainson's hawks really eat?. Journal of        Raptor Research. 25(4): 163. [Abstract]
  • 7.  Bednarz, James C.; Hoffman, Stephen W. 1988. The status of breeding        Swainson's hawks in southeastern New Mexico. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]

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Known prey organisms

  • L. D. Harris and L. Paur, A quantitative food web analysis of a shortgrass community, Technical Report No. 154, Grassland Biome. U.S. International Biological Program (1972), from p. 17.
  • Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web (online). Accessed February 16, 2011 at http://animaldiversity.org. http://www.animaldiversity.org
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Population Biology

Number of Occurrences

Note: For many non-migratory species, occurrences are roughly equivalent to populations.

Estimated Number of Occurrences: 21 to >300

Comments: No exact figures.

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Global Abundance

10,000 to >1,000,000 individuals

Comments: Guesstimated number of breeding pairs in Canada in the early 1990s was 20,000-50,000 (Kirk et al. 1995). Total population may be 350,000-400,000 individuals.

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General Ecology

May form premigratory aggregations in summer. Nesting density in suitable habitat varies throughout range from 0.1-1.6 nests per 10 sq km (Bednarz and Hoffman 1988); nests average 1.4-2.4 km apart (see Rothfels and Lein 1983). At one site in California, five nests typically found along a 1 km riparian strip, the nearest nests only 60 meters apart (England et al. 1997). Home ranges during breeding season vary greatly--from 69 to 8718 hectares (reviewed in England et al. 1997). Interspecific territoriality with Red-tailed Hawk in some areas; in other areas may compete with Ferruginous Hawk or be limited by presence of and predation by Great Horned Owl (Palmer 1988).

In California, dispersal distances from natal sites to subsequent breeding sites ranged from 0 to 18 kilometers, mean 8.8 kilometers (Woodbridge et al. 1995). In contrast, none of 697 banded nestlings in Saskatchewan returned to the study area; three were found 190, 200 and 310 kilometers away (Houston and Schmutz 1995).

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Habitat-related Fire Effects

More info for the term: fire suppression

The Swainson's hawk occurs in the following four major fire-dependent
plant associations in the western United States:  grassland, semidesert
grass-shrub, sagebrush grass, and pinyon-juniper [26].

Although fire may reduce potential nest trees, it may enhance the
foraging habitat of Swainson's hawks.  Fires that reduce vegetation
height and create open areas probably increase hunting efficiency by
Swainson's hawks.  Open-habitat raptors such as the Swainson's hawk use
scattered patches of woody vegetation near open foraging areas for
nesting and perching.  However, where extensive invasion of woody
species has occurred, Swainson's hawk foraging habitat may be reduced.
The Swainson's hawk is favored by fires that reduce pinyon-juniper
woodlands [26].  Raptors associated with pinyon-juniper woodlands depend
upon edges of openings created by fire and scattered islands of unburned
woodlands [13].  Fire suppression in pinyon-juniper habitats of the
Great Basin of California may have reduced suitable Swainson's hawk
habitat in this area [26].

Regular burning helps to maintain habitat for many prey species of
Swainson's hawk [13,25].  Several studies indicate that many prey
populations increase rapidly subsequent to burning in response to
increased food availability [13].  Fire suppression in grasslands was
detrimental to small bird and mammal populations due to organic matter
accumulation and reduced plant vigor [36].  The Swainson's hawk has been
observed hunting on recently burned areas in Colorado county, Texas [2].
On the Bridger Teton National Forest, Swainson's hawks were more
commonly observed using a high-severity fall burn than a low-severity
spring burn in the same area [27].
  • 13.  Dodd, Norris L. 1988. Fire management and southwestern raptors. In:        Gliski, R. L.; Pendleton, Beth Giron; Moss, Mary Beth; [and others]
  • 2.  Baker, R. H. 1940. Effects of burning and grazing on rodent populations.        Journal of Mammalogy. 21: 223.  [2849]
  • 25.  Landers, J. Larry. 1987. Prescribed burning for managing wildlife in        southeastern pine forests. In: Dickson, James G.; Maughan, O. Eugene,        eds. Managing southern forests for wildlife and fish: a proceedings;        [Date of conference unknown]
  • 26.  Lehman, Robert N.; Allendorf, John W. 1989. The effects of fire, fire        exclusion and fire management on raptor habitats in the western United        States. In: Proceedings of the western raptor management symposium and        workshop; 1987 October 26-28; Boise, ID. Scientific and Technical Series        No. 12. Washington, DC: National Wildlife Federation: 236-244.  [22324]
  • 27.  McGee, John Michael. 1976. Some effects of fire suppression and        prescribed burning on birds and small mammals in sagebrush. Laramie, WY:        University of Wyoming. 114 p. Dissertation.  [16998]
  • 36.  Wagle, R. F. 1981. Fire: its effects on plant succession and wildlife in        the Southwest. Tucson, AZ: University of Arizona. 82 p.  [4031]

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Timing of Major Life History Events

More info for the term: tree

Age at sexual maturity - Swainson's hawk are generally sexually mature
at 2 years of age [31].

Nesting season - The Swainson's hawk arrives on its breeding grounds
later than most raptors [31].  The nesting season generally occurs from
March to October depending on geographic area [14,21,35].  In
California, the Swainson's hawk breeds from late March to mid-August,
with peak activity from late May to late July.  In Nevada, it breeds
from April to October [21].  In Montana, the breeding season is from May
to September [14].
 
Clutch size and incubation - The Swainson's hawk lays two to four eggs,
with two most common [14,21,31,35].  The eggs are incubated for 28 to 35
days [14,21,31].  The Swainson's hawk may lay a replacement clutch if
the first clutch is destroyed [31].

Fledging - Nestlings fledge in 35 to 44 days [21,31].  Fledglings
continue to be fed by the adults and remain within the nesting territory
for 14 to 21 days after fledging; they often return to the nest tree to
roost [21].

Migration - The Swainson's hawk travels in large flocks (sometimes
containing over 100 individuals) from the nesting areas south to their
winter grounds in South America [6,21,31].

Peak fall migration clears the southern plains states and southern Texas
by early October.  The Swainson's hawk arrives in Central America the
last 3 weeks of October to early November; arrival in Argentina is
reported as late November.  Average dates for spring migration of the
Swainson's hawk are mid-March in Panama, the last 3 week of March in
Costa Rica, the last half of March and first week of April in the state
of Veracruz, Mexico, and early April in southern Texas [31].

Longevity - The Swainson's hawk probably seldom lives longer than 16
years [31].
  • 14.  DuBois, Kristi; Becker, Dale; Thornbrugh, Joe. 1987. Identification of        Montana's birds of prey. Montana Outdoors. 18(6): 11-31.  [3606]
  • 21.  Herron, Gary B.; Mortimore, Craig A.; Rawlings, Marcus S. 1985.        Red-tailed hawk. In: Nevada raptors: Their biology and management.        Biological Bulletin No. 8. Reno, NV: Nevada Department of Wildlife:        29-30.  [22694]
  • 31.  Palmer, Ralph S., editor. 1988. Handbook of North American birds. Volume        5. New Haven, CT: Yale University Press. 463 p.  [22303]
  • 35.  Verner, Jared; Boss, Allan S., tech. coords. 1980. California wildlife        and their habitats: western Sierra Nevada. Gen. Tech. Rep. PSW-37.        Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific        Southwest Forest and Range Experiment Station. 439 p.  [10237]
  • 6.  Bednarz, James C. 1988. Swainson's hawk. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]

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Life History and Behavior

Behavior

Perception Channels: visual ; tactile ; acoustic ; chemical

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Life Expectancy

Range lifespan

Status: wild:
19.5 (high) years.

Average lifespan

Status: wild:
235 months.

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Lifespan, longevity, and ageing

Maximum longevity: 19.6 years (wild) Observations: Maximum longevity in banded birds was 19.6 years (Blumstein and Moller 2008).
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Reproduction

The Swainson's Hawk starts the breeding season by building nests in March and April. The nest are usually found in trees, shrubs, on the ground, or on top of utility poles. These hawks are mostly mongamous, so a breeding pair may return to a previous nesting site. These birds become highly territorial towards their nest and their mate during this time of the year. When the nest is complete, the female lays 2 to 4 whitish-colored eggs with brown flecks. The male usually helps the female with the incubation, which lasts for about 30 days. The young hatch between March and July, and stay in the nest for another 30 days. While most juveniles migrate the following winter with their parents, there are some groups that do not migrate their first winter (Brown 1996, TPWD 1997).

Key Reproductive Features: iteroparous ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; oviparous

Average time to hatching: 31 days.

Average eggs per season: 3.

Average age at sexual or reproductive maturity (male)

Sex: male:
730 days.

Average age at sexual or reproductive maturity (female)

Sex: female:
730 days.

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Egg dates: mainly April-May in southwestern U.S., California, and Oregon; mainly May-June in central plains states and Canada. Clutch size usually is 2-3. Incubation lasts 34-35 days per egg, almost exclusively by female (male provides food). Young are tended by both adults, leave nest in about 30 days, attain flight at 42-44 days (around 3rd week in July in southwestern U.S.), dependent on parents for 4-4.5 weeks after fledging. First breeds at 2 years. Usually 0.1-0.2 pairs per sq km; average of 1.4-2.4 km between nests. See Bednarz (1988) for information on reproduction in New Mexico. Reported nest density throughout range varies from 0.08-1.61 nests per sq km.

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Molecular Biology and Genetics

Molecular Biology

Barcode data: Buteo swainsoni

The following is a representative barcode sequence, the centroid of all available sequences for this species.


There are 4 barcode sequences available from BOLD and GenBank.

Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.

See the BOLD taxonomy browser for more complete information about this specimen and other sequences.

CCTATACCTAATCTTCGGTGCCTGAGCCGGTATAGTCGGCACCGCCCTCAGCCTACTTATTCGTGCAGAACTCGGCCAACCAGGCACACTCCTAGGTGACGACCAGATCTACAACGTAATCGTTACCGCACATGCCTTCGTAATAATCTTCTTCATAGTTATACCAATTATGATTGGAGGCTTCGGAAACTGACTTGTCCCACTCATAATCGGCGCCCCCGACATAGCCTTCCCACGCATAAACAACATAAGCTTCTGACTACTTCCTCCATCCTTCCTCCTCCTCCTAGCCTCCTCAACAGTAGAAGCAGGAGCCGGCACTGGATGAACTGTCTACCCCCCACTAGCTGGCAATATAGCCCATGCCGGAGCTTCAGTAGACCTAGCCATCTTCTCCCTACACTTAGCCGGAGTCTCGTCCATTCTAGGAGCAATCAACTTTATCACAACCGCCATCAACATAAAACCCCCAGCCCTCTCCCAATACCAAACACCCCTATTCGTATGATCTGTCCTCATTACCGCTGTCCTTCTACTACTCTCACTCCCAGTCCTAGCCGCCGGCATTACTATACTGCTTACAGACCGAAACCTAAACACAACATTCTTTGACCCCGCTGGCGGAGGTGATCCCATCCTATACCAACATCTCTTTTGATTCTTTGGCCACCCANNNNNNNNNNNNNNNNNNNNN
-- end --

Download FASTA File

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Statistics of barcoding coverage: Buteo swainsoni

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 6
Specimens with Barcodes: 6
Species With Barcodes: 1
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Conservation

Conservation Status

This species of hawk is on the list of Federal Species of Concern, and is also considered threatened by the state of California. The primary cause of this concern is the massive killing of more than 20,000 Swainson's Hawks by pesticides used in the Argentina agricultural areas. In order to help these hawks recover, the use of deadly pesticides by Argentinian farmers must be stopped. Although the farmers are in support of saving the birds, this recovery effort is proving to be a daunting task (Brown 1996, Line 1996).

US Migratory Bird Act: protected

US Federal List: no special status

CITES: appendix ii

State of Michigan List: no special status

IUCN Red List of Threatened Species: least concern

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IUCN Red List Assessment


Red List Category
LC
Least Concern

Red List Criteria

Version
3.1

Year Assessed
2012

Assessor/s
BirdLife International

Reviewer/s
Butchart, S. & Symes, A.

Contributor/s

Justification
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
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National NatureServe Conservation Status

Canada

Rounded National Status Rank: N4B - Apparently Secure

United States

Rounded National Status Rank: N5B - Secure

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NatureServe Conservation Status

Rounded Global Status Rank: G5 - Secure

Reasons: Large breeding range in western and central North America; winters mainly in southern South America; relatively common in some areas, but pesticide use and habitat loss in breeding and nonbreeding range have resulted in declines; recently experienced severe mortality associated with pesticide use in Argentina.

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Information on state-level protected status of animals in the
United States and Canada is available at NatureServe.

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U.S. Federal Legal Status

Not listed [43]
  • 43.  U.S. Fish and Wildlife Service. 2013. Listed animals. In: Environmental Conservation Online System, [Online]

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Population

Population Trend
Stable
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Threats

Degree of Threat: B : Moderately threatened throughout its range, communities provide natural resources that when exploited alter the composition and structure of the community over the long-term, but are apparently recoverable

Comments: In California, threats include expansion of cropland unsuitable for foraging (see GHABCOM) and residential and commercial development in former agricultural and grassland areas. In California, Risebrough et al. (1989) concluded that organochlorine contamination of eggs, mortality during migration, and toxic contamination and habitat loss on wintering grounds did not account for a decline of more than 90%. However, widespread use of pesticides and rodenticides throughout the range is cause for concern (California Department of Fish and Game 1990, Kirk and Houston 1995). For example, in early 1996, thousands (perhaps well over 20,000) died as a result of pesticide (Monocrotophos and others) spraying for grasshopper control in croplands in Argentina; those dead were predominantly adults (subadult winter range is unknown); among the dead were hawks banded in Alberta, Saskatchewan, Colorado, Idaho, and California (Geoff Holroyd, Canadian Wildlife Service). Reduced prey populations and shooting possibly are significant threats in Central and South America (Kirk and Houston 1995). See Bednarz and Hoffman (1988) for threats in New Mexico.
Easily disturbed during nesting; often abandons nest if disturbed before the eggs hatch (Biosystems Analysis 1989).

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Management

Biological Research Needs: Determine degree of competition with B. jamaicensis.

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Global Protection: Few to several (1-12) occurrences appropriately protected and managed

Comments: Probably protected in several federal, state, and provincial parks/refuges, but this may account for relatively few pairs.

Needs: Ensure availability of nesting sites in conjunction with suitable foraging habitat.

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Use of Fire in Population Management

Prescribed fire can be beneficial to Swainson's hawk populations by
enhancing habitat and increasing the prey base [13,25].  Burning in
grasslands where scattered trees are retained benefits Swainson's hawk
populations, particularly in areas where nesting sites are limited.
Prescribed burning plans should strive for creation of maximum
interspersion of openings and edge, with high vegetative diversity.
Habitats should be maintained in a random mosaic.  In most cases,
burning plans must be integrated with proper range management.
Reseeding of perennial grasses as well as rest from livestock grazing
may be necessary to achieve desired goals.  Burning should be deferred
until nesting is completed in areas where impact to breeding Swainson's
hawk may occur [13].

An extensive body of research has been published on fire effects on animals
in semidesert grassland, oak savanna, and Madrean oak woodlands of southeastern
Arizona, including the response of Swainson's hawk to fire. See the Research Project Summary of this work for more information on
Swainson's hawk and more than 100 additional species of birds, small
mammals, grasshoppers, and herbaceous and woody plant species.
  • 13.  Dodd, Norris L. 1988. Fire management and southwestern raptors. In:        Gliski, R. L.; Pendleton, Beth Giron; Moss, Mary Beth; [and others]
  • 25.  Landers, J. Larry. 1987. Prescribed burning for managing wildlife in        southeastern pine forests. In: Dickson, James G.; Maughan, O. Eugene,        eds. Managing southern forests for wildlife and fish: a proceedings;        [Date of conference unknown]

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Management Considerations

More info for the term: density

Swainson's hawk populations have declined over much of their breeding
range due to habitat loss from cultivation, removal of riparian areas,
and removal of shelterbelts [14,31].  Nest site availability may limit
occurrence and breeding density of Swainson's hawk [6].  Schmutz and
others [34] reported that the nesting density of the Swainson's hawk
increased significantly following the erection of 98 artificial nest
platforms in an experimental study plot.  When roads, pipelines, or
other surface facilities are constructed, trees taller than 10 feet (3
m) with a d.b.h. of 2 inches (5 cm) or more should be conserved.  If
destruction of potential nest trees cannot be avoided, they should be
replaced with artificial nest platforms [7].  Within treeless expanses,
constructing artificial nest platforms or planting trees may also benefit
Swainson's hawk populations [6].  Additionally, establishing and
enhancing small wooded areas in the nesting habitat, and protecting
habitat used by the prey base, may benefit the Swainson's hawk [10,39].

The Swainson's hawk is more tolerant of human disturbance than other
hawks and will often nest close to occupied houses [7,14].  However,
intensive human activity in a small area near an active Swainson's hawk
nest would likely result in nest abandonment and breeding failure at
that site [7].

Swainson's hawk is sometimes eaten by people in South America.  Large
numbers are taken from communal roosting areas and killed.  This
activity could have a significant effect on populations that nest in
North America [21].  Additionally, the use of biocides in North,
Central, and South America may have an effect on Swainson's hawk
populations.  The Swainson's hawk may accumulate high pesticide levels
via food-chain concentration [31].
  • 10.  Call, Mayo. 1979. Habitat management guides for birds of prey. Techical        Note 338. Denver, CO: U.S. Department of the Interior, Bureau of Land        Management, Denver Service Center. 70 p.  [22451]
  • 14.  DuBois, Kristi; Becker, Dale; Thornbrugh, Joe. 1987. Identification of        Montana's birds of prey. Montana Outdoors. 18(6): 11-31.  [3606]
  • 21.  Herron, Gary B.; Mortimore, Craig A.; Rawlings, Marcus S. 1985.        Red-tailed hawk. In: Nevada raptors: Their biology and management.        Biological Bulletin No. 8. Reno, NV: Nevada Department of Wildlife:        29-30.  [22694]
  • 31.  Palmer, Ralph S., editor. 1988. Handbook of North American birds. Volume        5. New Haven, CT: Yale University Press. 463 p.  [22303]
  • 34.  Schmutz, Josef K.; Fyfe, Richard W.; Moore, David A.; Smith, Alan R.        1984. Artificial nests for ferruginous and Swainson's hawks. Journal of        Wildlife Management. 48(3): 1009-1013.  [23122]
  • 39.  Young, Leonard S. 1989. Effects of agriculture on raptors in the western        United States: an overview. In: Proceedings of the western raptor        symposium and workshop; 1987 October 26-28; Boise, ID. NWF Scientific        and Technical Series No. 12. Washington, DC: National Wildlife        Federation: 209-218.  [22649]
  • 6.  Bednarz, James C. 1988. Swainson's hawk. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]
  • 7.  Bednarz, James C.; Hoffman, Stephen W. 1988. The status of breeding        Swainson's hawks in southeastern New Mexico. In: Glinski, Richard L.;        Pendleton, Beth Giron; Moss, Mary Beth; [and others]

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Relevance to Humans and Ecosystems

Benefits

The Swainson's Hawk is of special importance to farmers, both in North and South America. Some Swainson's Hawks will live entirely on insects and rodents that it catches in crop fields, thus alleviating some crop destruction for farmers. This species is also important to scientists as they can study the ecological details of its massive migration of over 5,000 miles (Brown 1996, AID 1997, Line 1996).

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Wikipedia

Swainson's hawk

Swainson's hawk (Buteo swainsoni), is a large Buteo hawk of the Falconiformes, sometimes separated in the Accipitriformes like its relatives. This species was named after William Swainson, a British naturalist. It is colloquially known as the grasshopper hawk or locust hawk, as it is very fond of Acrididae (locusts and grasshoppers) and will voraciously eat these insects whenever they are available.

Their breeding habitat is prairie and dry grasslands in western North America. They build a stick nest in a tree or shrub or on a cliff edge. This species is a long-distance migrant, wintering in Argentina; it has been recorded as a vagrant in neighboring Chile, in the island countries of the Dominican Republic, and Trinidad and Tobago, and in Norway.[1]

This species or its immediate predecessor is the ancestor of the Galápagos hawk, as demonstrated by recent research. The latter diverged from the mainland birds perhaps 300,000 years ago, a very short time in evolution.[2]

Description[edit]

Rufous-morph bird in Hereford, Arizona, on its way to the pampas

Swainson's hawk is a raptor and a medium-sized member of the Buteo genus. It broadly overlaps in size with the red-tailed hawk (B. jamaicensis), a related species found as a breeding resident almost throughout North America. Swainson's hawk is on average a little shorter in length, 43–56 cm (17–22 in) long, and weighs a bit less, 0.5–1.7 kg (1.1–3.7 lb).[3][4][5] However, Swainson's hawk has a slightly longer wingspan at 117–137 cm (46–54 in), with more slender, elongated wings, than the red-tailed hawk.[3] Female Swainson's hawks, at an average weight of 1.15 kg (2.5 lb), are somewhat larger and heavier than males, at an average of 0.81 kg (1.8 lb).[3] Among standard measurements, the wing chord is 36.2–42.7 cm (14.3–16.8 in), the tail is 18.5–23.4 cm (7.3–9.2 in), the tarsus is 6.2–8 cm (2.4–3.1 in) and the bill (from the gape) is 3–3.5 cm (1.2–1.4 in).[3] In flight, Swainson's hawk holds its wings in a slight dihedral; it tips back and forth slightly while soaring.

There are two main color variations. Over 90% of individuals are light-morph; the dark morph is most common in the far west of the range:[6]

  • Light-morph adults are white on the underparts with a dark, reddish "bib" on the chest and a noticeable white throat and face patch. The underwings, seen as the bird soars, have light linings (leading edge) and dark flight feathers (trailing edge), a pattern unique among North American raptors. The tail is gray-brown with about six narrow dark bands and one wider subterminal band. The upperparts are brown. Juveniles are similar but dark areas have pale mottling and light areas, especially the flanks, have dark mottling. The chest is pale with some darker marks. The subterminal band of the tail is less obvious. Birds in their first spring may have pale heads because of feather wear.
  • Dark-morph birds are dark brown except for a light patch under the tail. There is a rufous variant that is lighter on the underparts with reddish bars. The tails of both these forms resemble those of the light morph.

Range and migration[edit]

Swainson's hawk inhabits North America mainly in the spring and summer and winters in South America. Breeding areas include south-central Alberta, central Saskatchewan, southwestern Manitoba, and west and southern Minnesota. They will breed as far north as east-central Alaska, and southwestern Yukon. Breeding continues south through the eastern parts of Washington and Oregon, locally to the central valley of California, Arizona, New Mexico, Colorado, and most of Texas. The eastern part of its range includes Minnesota, northwestern Iowa, most of Nebraska, Kansas, and Oklahoma, and all but eastern Texas. It periodically occurs in Iowa and rarely in northwestern Missouri, northern Illinois, and southwestern Wisconsin.

Swainson's hawk migration route.
30 birds were fitted with satellite tracking devices to produce this map

Small populations winter in southeastern Florida and along the Texas coast, probably having failed to find the way south around the Gulf of Mexico. Individuals reported north of these areas in winter (for example, on Christmas Bird Counts) are almost invariably misidentified buteos of other species. Immature Swainson's hawks winter on the pampas of South America in Argentina, Uruguay, and southern Brazil. It is not known with certainty where most of the adults spend the winter.

Swainson's hawk is probably the longest migrant of any North American raptor. The flight from breeding ground to South American pampas in southern Brazil or Argentina can be as long as 14,000 mi (23,000 km). Each migration can last at least two months.

They leave the breeding grounds from August to October. Fall migration begins each clear day on which a wind blows in the general direction of travel. Birds gain altitude by soaring in circles on a rising thermal and then set their wings and close their tails as they glide, slowly losing altitude until they find another thermal and rise with it. Thus, waves and small groups are strung out across the sky.

The birds gradually head southwards toward Central America where virtually the entire population funnels through the Isthmus of Panama. Concentrations over locations like Ancon Hill, Balboa, and Panama City are spectacular. In the Andes, it migrates along a narrow corridor and rarely strays off course; for example, it was only recorded in the Serranía de las Quinchas of Colombia – just 100 km (62 mi) or so off its usual migration route – in 2000/2001.[7]

In Brazil, migrating birds pass through the western states of Acre and Mato Grosso, while wintering birds may stray to the southern states of Paraná, Rio Grande do Sul and São Paulo. But surprisingly, the occasional Swainson's hawk — including birds one or two years of age — has also been recorded in the eastern states of Maranhão, Pará, Pernambuco, Piauí and Tocantins, thousands of kilometers away from their usual migration route and wintering grounds and sometimes in mid-summer. This suggests that individuals occasionally become lost during migration, and/or that they may spend a whole year in the tropical regions and range about, rather than just overwintering at one site.[8]

In Uruguay, the first dedicated studies show it to be not uncommon but patchily distributed across the country in winter. Notably, it had been underreported in Flores and Paysandú Departments, where it seems in fact to be a regular visitor. In recent years, the first birds were seen in early November, and some stayed until late February. Numbers increase throughout November and peak in December, when flocks of many dozen roam the open lands. But many stay only for a scant few weeks before leaving again.[9]

Spring migration broadens once the birds have passed through Mexico as they disperse through the breeding range. Migrant groups are noted in the southern U.S. states in March. The earliest Swainson's hawks arrive in southern Canada in late March, with migration peaking from mid April onwards.

Ecology[edit]

Soaring light-morph adult

The habitat of Swainson's hawk consists of open and semi-open country – deserts, grasslands and prairies – in both its breeding and wintering ranges. It favors wild prairie, hayfields, and pastures over wheat fields and alfalfa fields, which may offer its prey too much cover. It requires elevated perches for hunting and a supply of small mammals such as young ground squirrels as prey for its nestlings. The breeding distribution of Swainson's hawk is tied very closely to the distribution of various small mammals for this reason. In Saskatchewan, for example, the distribution of Richardson's ground squirrel and Swainson's hawk are precisely the same.

Swainson's hawk will defend its breeding territory from other buteos. Breeding densities may vary from one area to the next but averages one pair per 2.5 sq mi (6.5 km2). The average home range estimate for this hawk is 1 to 2 sq mi (2.6 to 5.2 km2). It gathers in groups for feeding and migrating. However, in each case, such gathering is not social, but motivated by good feeding or migrating conditions.

Swainson's hawk, the red-tailed hawk (B. jamaicensis) and the ferruginous hawk (B. regalis) compete for territory, and defend territories against each other. In many parts of the plains these three species nest in the same general area and exploit much the same prey base. Although diets overlap greatly, habitats may not overlap as much. In Oregon, Swainson's hawk selects nesting trees having a different configuration than those used by red-tailed or ferruginous hawks. In southern Alberta, different nesting habitats help reduce food competition, with Swainson's hawk favoring areas with scattered trees or riparian borders, while red-tailed hawks nest in stands of tall trees, and ferruginous hawks nest on the open plains.

Reduced reproductive success may result from Swainson's hawk's nesting proximity to these two other buteos. Swainson's hawk is generally tolerant of people. The bird is attracted to haying, mowing, and plowing operations. House sparrows, European starlings, and other small birds may nest in or near a Swainson's hawk's nest.

In winter quarters, they are far more tolerant, though many birds will still fend for themselves. In Uruguay, the species likes largely open but broken (with rocks or woods) plains or low hills, where it can be seen to gather in larger groups. Groups of a few dozen birds are not uncommon. Flocks of over one hundred birds have been recorded several times, e.g. one that roamed the Cuchilla Marincho region south of Andresito (Flores Department) in mid-late December 2005.[9]

Hunting and food[edit]

Swainson's hawks hunt using various methods. Many still-hunt, watching for prey activity from a perch such as a tree, bush, pylon, telephone pole, hummock or other high object.[3] Others hunt by soaring over open ground with wings held in a dihedral, using their stellar vision to watch for prey activity below. It occasionally courses low over the ground like a northern harrier (Circus cyaneus) or hovers like a rough-legged hawk (B. lagopus) while hunting. They frequently engage in transect-glides while actively hunting in flight.[3] It commonly perches on the ground both during migration and on the breeding grounds. While hunting on the ground, almost entirely for large insects, their gait can appear awkward but they are often successful in pinning down several insects per day.[3] During migration, it typically roosts for the night on bare ground with scattered trees, a habit that distinguishes it from fellow long-distance migrants such as the broad-winged hawk (B. platypterus), which roosts in closed-canopy woodlands.

These birds patrol open areas or scan for prey from a perch; they may also catch insects in flight. They take advantage of insects turned up by farm equipment or driven out by fire. A hunting Swainson's hawk will use several strategies. It hunts insects such as dragonflies or dobsonflies while in flight, flapping little as it rides a wind current and stoops upon a fly, grabbing it with its foot and immediately transferring the prey to its bill. It uses a similar strategy to grab individual free-tailed bats from flying streams of bats. Also, when dragonfly hordes are grounded by weather, Swainson's hawk will stand near groups sheltering from the wind and pluck at individual insects. Swainson's hawk closely follows both tractors and wild fires for injured or fleeing food. It will also run down insect prey on the ground. Occasionally a hawk will stand still on a dirt bank or elevated mound waiting for prey to appear. It commonly hunts from elevated perches such as telephone poles, stooping on prey when it is sighted.

Grasshopper, a favorite food of Swainson's hawk

Swainson's hawks may be largely insectivorous except when nesting. Insect prey commonly taken includes grasshoppers, crickets, and locusts. Other Buteo hawks in this species range, including smaller-bodied species, do not normally prefer insects in their diet but instead focus on rodents and other small vertebrates.[3] However, breeding birds switch mainly to capturing vertebrate prey, which pairs then bring to their nestlings. Breeding Swainson's hawks rely heavily upon small mammals such as young ground squirrels, young cottontails, pocket gophers, mice, young jackrabbits, and, at least locally, small birds and other vertebrates including reptiles and amphibians. Birds taken include large birds such as Mallards, and Sage Grouse which may have been injured initially.

Other unusual bird species taken include American kestrel, and young short-eared owls. More typical in size are young lark buntings taken at their fledging time. Reptiles, which can comprise large parts of a diet, include snakes such as racers, gopher snakes and striped whipsnakes, and lizards. Amphibians may include tiger salamanders and toads. Swainson's hawk is an opportunistic feeder which responds quickly to local concentrations of food.

In Argentina, flocks of immature Swainson's hawks feed on flocks of the migratory darner dragonfly Rhionaeschna bonariensis, following the hordes of insects and feeding mostly on the wing. Local outbreaks of locusts may also be exploited for food by one or more age-classes of birds. The immatures wintering in southern Florida apparently feed upon either insects, mice, or both, when turned up from field plowing. They move from one freshly ploughed field to the next.

There is also some evidence that road-killed birds and animals are also consumed both on the wintering grounds and on the breeding grounds. The species commonly follows tractors and other agricultural equipment during haying or ploughing, where rodents are exposed for the hawks to capture, or insects are uncovered after crop cutting. Wildfires often attract foraging Swainson's hawks, especially grass fires in their South American wintering range. In South American grass fires, the hawks frequently wait around the edges of the fire, picking off not only insects but also vertebrates including nothuras, lizards and snakes.[3]

Reproduction and life span[edit]

A Swainson's hawk chick

When Swainson's hawks arrive at their nesting sites in March or April, they may return to their original nests as these hawks are noted to be monogamous. Research indicates that they have a high degree of mate and territorial fidelity. This is unusual in a long-distance migrant. Seven to fifteen days after the birds arrive, the males begin constructing nests on the ground, ledges or in a trees. The nest consists of twigs and grasses and can take up to two weeks to complete. New nests may be constructed, old nests refurbished, or abandoned nests of other species — namely corvids (e.g. Common Raven, Black-billed Magpie, and American Crow) — are refurbished.

The courtship displays of Swainson's hawk are not well known. One activity involves circling and diving above a potential nest site. The underwings and rump are flashed and the birds call. The display may end with one bird diving to land on the edge of the nest. Copulation occurs mainly in the morning and evening on the dead limbs of trees. The female may assume the receptive position without a prior display. During treading one of the birds calls.

Swainson's hawks typically nest in isolated trees or bushes, shelterbelts, riparian groves, or around abandoned homesteads. Occasionally, a pair will nest on the ground or on a bank or ledge. Nest trees and bushes include ponderosa pine, Douglas-fir, spruce, cottonwood, domestic poplar, aspen, elm, mesquite, willow, saguaro cactus, and soaptree yuccas. Nests are located from 9 to 15 ft (2.7 to 4.6 m) above the ground, often in the shaded canopy but near the top of the tree. Nests are flimsy structures, usually smaller than the nests of the red-tailed hawk, and often blow down after nesting season.

Juvenile Swainson's hawk

Clutch size ranges from one to four eggs, but averages two to three. Each egg is elliptical in shape, about 2.25 in (57 mm) long and 1.8 in (46 mm) wide. The egg is smooth with fine granulations and the ground color is white, often tinted bluish or greenish. During incubation the shell color quickly wears to dull white. Some eggs are plain; others are lightly marked with spots and blotches of light brown. The incubation period is 34 to 35 days, with the female incubating while the male brings food.

Young Swainson's hawks are fed small, young mammals. Flight feathers begin to emerge on the young at 9 to 11 days. High nestling mortality often occurs when the young are 15 to 30 days old and may be a result of fratricide. The young begin to leave the nest for surrounding branches at 33 to 37 days, fledging occurs at about 38 to 46 days. The fledglings are dependent upon their parents for 4 to 5 weeks. This species has one brood a year and apparently does not lay replacement clutches.

The oldest wild Swainson's hawk on record is 24 years. Swainson's hawks die because of collisions with traffic, illegal shooting, electrocution, and even during severe prairie weather such as hailstorms. Wind storms and hail caused 30% nest failure in one study. When sharing a grove with nesting great horned owls, the hawks suffer much egg loss due to owl predation. The species also suffers from frequent, unexplained egg infertility.

Status and conservation[edit]

Injured light-morph Swainson's hawk recuperating in Zoo Boise

Swainson's hawk has suffered population declines since the first half of the twentieth century and was Blue-listed in the United States from 1972 to 1982. It has since been placed on the National Audubon's List of Special Concern in 1986. It is now listed by the United States Fish and Wildlife Service as a Category 3C candidate. It should be noted that Swainson's hawk was removed from the active federal list because it was found to be more abundant than previously thought; it is not considered a threatened species by the IUCN.[1] It remains listed as a threatened species by the California Department of Fish and Game as it has been since 1983.

A major cause of Swainson's hawk population decline was pesticide use in its wintering grounds of Argentina. Farmers there were using pesticides (DDT and monocrotophos) to control grasshopper and locust infestations, and Swainson's hawks were ingesting these pesticides in several different ways, but mainly by gorging themselves on the insects as they lay dying. The U.S. has worked with Argentine farmers to resolve this problem.[10]

Swainson's hawk has adapted well to grazing and pastureland and seems to be holding its own over much of its breeding range, from northern Mexico to the southern parts of the prairie provinces. However, far western populations, like that of Oregon, and southern California, have drastically declined, often due to habitat loss or incompatible agricultural practices. A possible reason for declines in parts of its range may be agriculturally motivated reductions in populations of both ground squirrels and grasshoppers, major seasonal foods.

Although often nesting close to human activity, some Swainson's hawks are very easily disturbed at the nest and often desert, especially early in the season. The bird is often quite tame and an easy target for shooters traveling isolated prairie roads. The species may also be affected in ways yet to be understood by some insecticides and herbicides, including those used on its wintering grounds.[10]

Observing Swainson's hawk[edit]

One of the best places to view the hawk is in the Snake River Birds of Prey National Conservation Area (NCA) in Idaho. Birds in the NCA are most frequently sighted in mid March, May, and June, in the early morning and evening when they are actively hunting. In April, Swanson's hawks engage in more sedentary breeding and egg-guarding, and are thus more difficult to spot. In July, rising canyon temperatures make prey scarce, so many birds of prey migrate away.

References[edit]

This article incorporates text from the Bureau of Land Management which is in the public domain.
  1. ^ a b c BirdLife International (2012). "Buteo swainsoni". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013. 
  2. ^ Bollmer, Jennifer L.; Kimball, Rebecca T.; Whiteman, Noah Kerness; Sarasola, José Hernán; Parker, Patricia G. (2005). "Phylogeography of the Galápagos hawk (Buteo galapagoensis): A recent arrival to the Galápagos Islands". Molecular Phylogenetics and Evolution 39 (1): 237–247. doi:10.1016/j.ympev.2005.11.014. PMID 16376110. 
  3. ^ a b c d e f g h i Ferguson-Lees, J.; Christie, D. (2001). Raptors of the World. London: Christopher Helm. pp. 717–719. ISBN 0-7136-8026-1. 
  4. ^ Goldstein, Michael I.; Bloom, Peter H.; Sarasola, Jose H.; Lacher, Thomas E. (1999). "Post-Migration Weight Gain of Swainson's Hawks in Argentina". Wilson Bulletin 111 (3): 428–432. JSTOR 4164111. 
  5. ^ "Swainson's Hawk". All About Birds. Cornell Lab of Ornithology. Retrieved 3 March 2013. 
  6. ^ Sibley, David Allen (2000). The Sibley Guide to Birds. New York: Alfred A. Knopf. p. 120. ISBN 0-679-45122-6. 
  7. ^ Laverde-R., Oscar; Stiles, F. Gary; Múnera-R., Claudia (2005). "Nuevos registros e inventario de la avifauna de la Serranía de las Quinchas, un área importante para la conservación de las aves (AICA) en Colombia" [New records and updated inventory of the avifauna of the Serranía de las Quinchas, an important bird area (IBA) in Colombia]. Caldasia (in Spanish) 27 (2): 247–265. 
  8. ^ Olmos, Fábio; Pacheco, José Fernando; Silveira, Luís Fábio (2006). "Notas sobre aves de rapina (Cathartidae, Acciptridae e Falconidae) brasileiras" [Notes on Brazilian birds of prey]. Revista Brasileira de Ornitologia (in Portuguese) 14 (4): 401–404. 
  9. ^ a b Azpiroz, Adrián B.; Menéndez, José L. (2008). "Three new species and novel distributional data for birds in Uruguay". Bulletin of the British Ornithologists' Club 128 (1): 38–56. 
  10. ^ a b Goldstein, M.I.; Woodbridge, B.; Zaccagnini, M.E.; Canavelli, S.B.; Lanusse, A. (1996). "An assessment of mortality of Swainson's hawks on wintering grounds in Argentina". Journal of Raptor Research 30 (2): 106–107. 
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Swainson's Hawk

Swainson's Hawk (Buteo swainsoni), is a large Buteo hawk of the Falconiformes, sometimes separated in the Accipitriformes like its relatives. This species was named after William Swainson, a British naturalist. It is colloquially known as the Grasshopper Hawk or Locust Hawk, as it is very fond of Acrididae (locusts and grasshoppers) and will voraciously eat these insects whenever they are available.

Their breeding habitat is prairie and dry grasslands in western North America. They build a stick nest in a tree or shrub or on a cliff edge. This species is a long-distance migrant, wintering in Argentina; there is a single record of a vagrant from Norway[clarification needed].

This species or its immediate predecessor is the ancestor of the Galápagos Hawk, as demonstrated by recent research. The latter diverged from the mainland birds perhaps 300,000 years ago, a very short time in evolution.[2]

Description[edit]

Rufous-morph bird in Hereford, Arizona, on its way to the pampas

Swainson's Hawk is a raptor and a medium-sized member of the Buteo genus. It broadly overlaps in size with the Red-tailed Hawk (B. jamaicensis), a related species found as a breeding resident almost throughout North America. Swainson's Hawk is on average a little shorter in length, 43–56 cm (17–22 in) long, and weighs a bit less, 0.5–1.7 kg (1.1–3.7 lb).[3][4][5] However, Swainson's Hawk has a slightly longer wingspan at 117–137 cm (46–54 in), with more slender, elongated wings, than the Red-tailed Hawk.[3] Female Swainson's Hawks, at an average weight of 1.15 kg (2.5 lb), are somewhat larger and heavier than males, at an average of 0.81 kg (1.8 lb).[3] Among standard measurements, the wing chord is 36.2–42.7 cm (14.3–16.8 in), the tail is 18.5–23.4 cm (7.3–9.2 in), the tarsus is 6.2–8 cm (2.4–3.1 in) and the bill (from the gape) is 3–3.5 cm (1.2–1.4 in).[3] In flight, Swainson's Hawk holds its wings in a slight dihedral; it tips back and forth slightly while soaring.

There are two main color variations. Over 90% of individuals are light-morph; the dark morph is most common in the far west of the range:[6]

  • Light-morph adults are white on the underparts with a dark, reddish "bib" on the chest and a noticeable white throat and face patch. The underwings, seen as the bird soars, have light linings (leading edge) and dark flight feathers (trailing edge), a pattern unique among North American raptors. The tail is gray-brown with about six narrow dark bands and one wider subterminal band. The upperparts are brown. Juveniles are similar but dark areas have pale mottling and light areas, especially the flanks, have dark mottling. The chest is pale with some darker marks. The subterminal band of the tail is less obvious. Birds in their first spring may have pale heads because of feather wear.
  • Dark-morph birds are dark brown except for a light patch under the tail. There is a rufous variant that is lighter on the underparts with reddish bars. The tails of both these forms resemble those of the light morph.

Range and migration[edit]

Swainson's Hawk inhabits North America mainly in the spring and summer and winters in South America. Breeding areas include south-central Alberta, central Saskatchewan, southwestern Manitoba, and west and southern Minnesota. They will breed as far north as east-central Alaska, and southwestern Yukon. Breeding continues south through the eastern parts of Washington and Oregon, locally to the central valley of California, Arizona, New Mexico, Colorado, and most of Texas. The eastern part of its range includes Minnesota, northwestern Iowa, most of Nebraska, Kansas, and Oklahoma, and all but eastern Texas. It periodically occurs in Iowa and rarely in northwestern Missouri, northern Illinois, and southwestern Wisconsin.

Swainson's hawk migration route.
30 birds were fitted with satellite tracking devices to produce this map

Small populations winter in southeastern Florida and along the Texas coast, probably having failed to find the way south around the Gulf of Mexico. Individuals reported north of these areas in winter (for example, on Christmas Bird Counts) are almost invariably misidentified buteos of other species. Immature Swainson's hawks winter on the pampas of South America in Argentina, Uruguay, and southern Brazil. It is not known with certainty where most of the adults spend the winter.

Swainson's Hawk is probably the longest migrant of any North American raptor. The flight from breeding ground to South American pampas in southern Brazil or Argentina can be as long as 14,000 miles (22,400 km). Each migration can last at least two months.

They leave the breeding grounds from August to October. Fall migration begins each clear day on which a wind blows in the general direction of travel. Birds gain altitude by soaring in circles on a rising thermal and then set their wings and close their tails as they glide, slowly losing altitude until they find another thermal and rise with it. Thus, waves and small groups are strung out across the sky.

The birds gradually head southwards toward Central America where virtually the entire population funnels through the Isthmus of Panama. Concentrations over locations like Ancon Hill, Balboa, and Panama City are spectacular. In the Andes, it migrates along a narrow corridor and rarely strays off course; for example, it was only recorded in the Serranía de las Quinchas of Colombia – just 100 km or so off its usual migration route – in 2000/2001.[7]

In Brazil, migrating birds pass through the western states of Acre and Mato Grosso, while wintering birds may stray to the southern states of Paraná, Rio Grande do Sul and São Paulo. But surprisingly, the occasional Swainson's Hawk – including birds one or two years of age – has also been recorded in the eastern states of Maranhão, Pará, Pernambuco, Piauí and Tocantins, thousands of kilometers away from their usual migration route and wintering grounds and sometimes in mid-summer. This suggests that individuals occasionally become lost during migration, and/or that they may spend a whole year in the tropical regions and range about, rather than just overwintering at one site.[8]

In Uruguay, the first dedicated studies show it to be not uncommon but patchily distributed across the country in winter. Notably, it had been underreported in Flores and Paysandú Departments, where it seems in fact to be a regular visitor. In recent years, the first birds were seen in early November, and some stayed til late February. Numbers increase throughout November and peak in December, when flocks of many dozen roam the open lands. But many stay only for a scant few weeks before leaving again.[9]

Spring migration broadens once the birds have passed through Mexico as they disperse through the breeding range. Migrant groups are noted in the southern U.S. states in March. The earliest Swainson's Hawks arrive in southern Canada in late March, with migration peaking from mid April onwards.

Ecology[edit]

Soaring light-morph adult

The habitat of Swainson's Hawk consists of open and semi-open country – deserts, grasslands and prairies – in both its breeding and wintering ranges. It favors wild prairie, hayfields, and pastures over wheat fields and alfalfa fields, which may offer its prey too much cover. It requires elevated perches for hunting and a supply of small mammals such as young ground squirrels as prey for its nestlings. The breeding distribution of Swainson's Hawk is tied very closely to the distribution of various small mammals for this reason. In Saskatchewan, for example, the distribution of Richardson's Ground Squirrel and Swainson's Hawk are precisely the same.

Swainson's Hawk will defend its breeding territory from other buteos. Breeding densities may vary from one area to the next but averages one pair per 2.5 square miles (6.5 km²). The average home range estimate for this hawk is 1 to 2 square miles (2.5 to 5 km²). It gathers in groups for feeding and migrating. However, in each case, such gathering is not social, but motivated by good feeding or migrating conditions.

Swainson's Hawk, the Red-tailed Hawk (B. jamaicensis) and the Ferruginous Hawk (B. regalis) compete for territory, and defend territories against each other. In many parts of the plains these three species nest in the same general area and exploit much the same prey base. Although diets overlap greatly, habitats may not overlap as much. In Oregon, Swainson's Hawk selects nesting trees having a different configuration than those used by Red-tailed or Ferruginous Hawks. In southern Alberta, different nesting habitats help reduce food competition, with Swainson's Hawk favoring areas with scattered trees or riparian borders, while Red-tailed Hawks nest in stands of tall trees, and Ferruginous Hawks nest on the open plains.

Reduced reproductive success may result from Swainson's Hawk's nesting proximity to these two other buteos. Swainson's Hawk is generally tolerant of people. The bird is attracted to haying, mowing, and plowing operations. House Sparrows, European Starlings, and other small birds may nest in or near a Swainson's Hawk's nest.

In winter quarters, they are far more tolerant, though many birds will still fend for themselves. In Uruguay, the species likes largely open but broken (with rocks or woods) plains or low hills, where it can be seen to gather in larger groups. Groups of a few dozen birds are not uncommon. Flocks of over one hundred birds have been recorded several times, e.g. one that roamed the Cuchilla Marincho region south of Andresito (Flores Department) in mid-late December 2005.[9]

Hunting and food[edit]

Swainson's Hawks hunt using various methods. Many still-hunt, watching for prey activity from a perch such as a tree, bush, pylon, telephone pole, hummock or other high object.[3] Others hunt by soaring over open ground with wings held in a dihedral, using their stellar vision to watch for prey activity below. It occasionally courses low over the ground like a Northern Harrier (Circus cyaneus) or hovers like a Rough-legged Hawk (B. lagopus) while hunting. They frequently engage in transect-glides while actively hunting in flight.[3] It commonly perches on the ground both during migration and on the breeding grounds. While hunting on the ground, almost entirely for large insects, their gait can appear awkward but they are often successful in pinning down several insects per day.[3] During migration, it typically roosts for the night on bare ground with scattered trees, a habit that distinguishes it from fellow long-distance migrants such as the Broad-winged Hawk (B. platypterus), which roosts in closed-canopy woodlands.

These birds patrol open areas or scan for prey from a perch; they may also catch insects in flight. They take advantage of insects turned up by farm equipment or driven out by fire. A hunting Swainson's Hawk will use several strategies. It hunts insects such as dragonflies or dobsonflies while in flight, flapping little as it rides a wind current and stoops upon a fly, grabbing it with its foot and immediately transferring the prey to its bill. It uses a similar strategy to grab individual free-tailed bats from flying streams of bats. Also, when dragonfly hordes are grounded by weather, Swainson's Hawk will stand near groups sheltering from the wind and pluck at individual insects. Swainson's Hawk closely follows both tractors and wild fires for injured or fleeing food. It will also run down insect prey on the ground. Occasionally a hawk will stand still on a dirt bank or elevated mound waiting for prey to appear. It commonly hunts from elevated perches such as telephone poles, stooping on prey when it is sighted.

Grasshopper, a favorite food of Swainson's Hawk

Swainson's Hawks may be largely insectivorous except when nesting. Insect prey commonly taken includes grasshoppers, crickets, and locusts. Other Buteo hawks in this species range, including smaller-bodied species, do not normally prefer insects in their diet but instead focus on rodents and other small vertebrates.[3] However, breeding birds switch mainly to capturing vertebrate prey, which pairs then bring to their nestlings. Breeding Swainson's Hawks rely heavily upon small mammals such as young ground squirrels, young cottontails, pocket gophers, mice, young jackrabbits, and, at least locally, small birds and other vertebrates including reptiles and amphibians. Birds taken include large birds such as Mallards, and Sage Grouse which may have been injured initially.

Other unusual bird species taken include American Kestrel, and young Short-eared Owls. More typical in size are young Lark Buntings taken at their fledging time. Reptiles, which can comprise large parts of a diet, include snakes such as racers, gopher snakes and striped whipsnakes, and lizards. Amphibians may include tiger salamanders and toads. Swainson's Hawk is an opportunistic feeder which responds quickly to local concentrations of food.

In Argentina, flocks of immature Swainson's Hawks feed on flocks of the migratory darner dragonfly Rhionaeschna bonariensis, following the hordes of insects and feeding mostly on the wing. Local outbreaks of locusts may also be exploited for food by one or more age-classes of birds. The immatures wintering in southern Florida apparently feed upon either insects, mice, or both, when turned up from field plowing. They move from one freshly ploughed field to the next.

There is also some evidence that road-killed birds and animals are also consumed both on the wintering grounds and on the breeding grounds. The species commonly follows tractors and other agricultural equipment during haying or ploughing, where rodents are exposed for the hawks to capture, or insects are uncovered after crop cutting. Wildfires often attract foraging Swainson's Hawks, especially grass fires in their South American wintering range. In South American grass fires, the hawks frequently wait around the edges of the fire, picking off not only insects but also vertebrates including nothuras, lizards and snakes.[3]

Reproduction and life span[edit]

A Swainson's Hawk chick

When Swainson's Hawks arrive at their nesting sites in March or April, they may return to their original nests as these hawks are noted to be monogamous. Research indicates that they have a high degree of mate and territorial fidelity. This is unusual in a long-distance migrant. Seven to 15 days after the birds arrive, the males begin constructing nests on the ground, ledges or in a trees. The nest consists of twigs and grasses and can take up to two weeks to complete. New nests may be constructed, old nests refurbished, or abandoned nests of other species – namely corvids[10] – are refurbished.

The courtship displays of Swainson's Hawk are not well known. One activity involves circling and diving above a potential nest site. The underwings and rump are flashed and the birds call. The display may end with one bird diving to land on the edge of the nest. Copulation occurs mainly in the morning and evening on the dead limbs of trees. The female may assume the receptive position without a prior display. During treading one of the birds calls.

Swainson's Hawks typically nest in isolated trees or bushes, shelterbelts, riparian groves, or around abandoned homesteads. Occasionally, a pair will nest on the ground or on a bank or ledge. Nest trees and bushes include ponderosa pine, Douglas-fir, spruce, cottonwood, domestic poplar, aspen, elm, mesquite, willow, saguaro cactus, and soaptree yuccas. Nests are located from nine to 15 feet above the ground, often in the shaded canopy but near the top of the tree. Nests are flimsy structures, usually smaller than the nests of the Red-tailed Hawk, and often blow down after nesting season.

Juvenile Swainson's Hawk

Clutch size ranges from one to four eggs, but averages two to three. Each egg is elliptical in shape, about 2.25 inches (57 mm) long and 1.8 inches (46 mm) wide. The egg is smooth with fine granulations and the ground color is white, often tinted bluish or greenish. During incubation the shell color quickly wears to dull white. Some eggs are plain; others are lightly marked with spots and blotches of light brown. The incubation period is 34 to 35 days, with the female incubating while the male brings food.

Young Swainson's Hawks are fed small, young mammals. Flight feathers begin to emerge on the young at nine to 11 days. High nestling mortality often occurs when the young are 15 to 30 days old and may be a result of fratricide. The young begin to leave the nest for surrounding branches at 33 to 37 days, fledging occurs at about 38 to 46 days. The fledglings are dependent upon their parents for four to 5 weeks. This species has one brood a year and apparently does not lay replacement clutches.

The oldest wild Swainson's Hawk on record is 24 years. Swainson's Hawks die because of collisions with traffic, illegal shooting, electrocution, and even during severe prairie weather such as hailstorms. Wind storms and hail caused 30 percent nest failure in one study. When sharing a grove with nesting Great Horned Owls, the hawks suffer much egg loss due to owl predation. The species also suffers from frequent, unexplained egg infertility.

Status and conservation[edit]

Injured light-morph Swainson's Hawk recuperating in Zoo Boise

Swainson's Hawk has suffered population declines since the first half of the twentieth century and was Blue-listed in the United States from 1972 to 1982. It has since been placed on the National Audubon's List of Special Concern in 1986. It is now listed by the United States Fish and Wildlife Service as a Category 3C candidate. It should be noted that Swainson's Hawk was removed from the active Federal list because it was found to be more abundant than previously thought; it is not considered a threatened species by the IUCN.[1] It remains listed as a threatened species by the California Department of Fish and Game as it has been since 1983.

A major cause of Swainson's Hawk population decline was pesticide use in its wintering grounds of Argentina. Farmers there were using pesticides (DDT and monocrotophos) to control grasshopper and locust infestations, and Swainson's Hawks were ingesting these pesticides in several different ways, but mainly by gorging themselves on the insects as they lay dying. The U.S. has worked with Argentine farmers to resolve this problem.[11]

Swainson's Hawk has adapted well to grazing and pastureland and seems to be holding its own over much of its breeding range, from northern Mexico to the southern parts of the prairie provinces. However, far western populations, like that of Oregon, and southern California, have drastically declined, often due to habitat loss or incompatible agricultural practices. A possible reason for declines in parts of its range may be agriculturally motivated reductions in populations of both ground squirrels and grasshoppers, major seasonal foods.

Although often nesting close to human activity, some Swainson's Hawks are very easily disturbed at the nest and often desert, especially early in the season. The bird is often quite tame and an easy target for shooters traveling isolated prairie roads. The species may also be affected in ways yet to be understood by some insecticides and herbicides, including those used on its wintering grounds.[11]

Observing Swainson's Hawk[edit]

One of the best places to view the hawk is in the Snake River Birds of Prey National Conservation Area (NCA) in Idaho. Birds in the NCA are most frequently sighted in mid March, May, and June, in the early morning and evening when they are actively hunting. In April, Swanson's Hawks engage in more sedentary breeding and egg-guarding, and are thus more difficult to spot. In July, rising canyon temperatures make prey scarce, so many birds of prey migrate away.

References[edit]

This article incorporates text from the Bureau of Land Management which is in the public domain.
  1. ^ a b BirdLife International (2012). "Buteo swainsoni". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013. 
  2. ^ Bollmer, Jennifer L.; Kimball, Rebecca T.; Whiteman, Noah Kerness; Sarasola, José Hernán & Parker, Patricia G. (2005). "Phylogeography of the Galápagos hawk (Buteo galapagoensis): A recent arrival to the Galápagos Islands". Molecular Phylogenetics and Evolution 39 (1): 237–247. doi:10.1016/j.ympev.2005.11.014. PMID 16376110. 
  3. ^ a b c d e f g h i Ferguson-Lees, J.; Christie, D. (2001). Raptors of the World. London: Christopher Helm. pp. 717–19. ISBN 0-7136-8026-1. 
  4. ^ Goldstein, Michael I.; Bloom, Peter H.; Sarasola, Jose H.; Lacher, Thomas E. (1999). "Post-Migration Weight Gain of Swainson's Hawks in Argentina". Wilson Bulletin 111 (3): 428–432. JSTOR 4164111. 
  5. ^ Swainson's Hawk, Life History, All About Birds – Cornell Lab of Ornithology. Allaboutbirds.org. Retrieved on 2013-03-03.
  6. ^ Sibley, David Allen (2000): The Sibley Guide to Birds. Alfred A. Knopf, New York. ISBN 0-679-45122-6, p. 120
  7. ^ Laverde-R., Oscar; Stiles, F. Gary & Múnera-R., Claudia (2005). "Nuevos registros e inventario de la avifauna de la Serranía de las Quinchas, un área importante para la conservación de las aves (AICA) en Colombia [New records and updated inventory of the avifauna of the Serranía de las Quinchas, an important bird area (IBA) in Colombia]". Caldasia 27 (2): 247–265. 
  8. ^ Olmos, Fábio; Pacheco, José Fernando & Silveira, Luís Fábio (2006). "Notas sobre aves de rapina (Cathartidae, Acciptridae e Falconidae) brasileiras [Notes on Brazilian birds of prey]". Revista Brasileira de Ornitologia 14 (4): 401–404. 
  9. ^ a b Azpiroz, Adrián B. & Menéndez, José L. (2008). "Three new species and novel distributional data for birds in Uruguay". Bulletin of the British Ornithologists' Club 128 (1): 38–56. 
  10. ^ E.g. Common Raven, Black-billed Magpie, and American Crow
  11. ^ a b Goldstein, M. I.; Woodbridge, B.; Zaccagnini, M. E.; Canavelli, S. B. & Lanusse, A. (1996). "An assessment of mortality of Swainson's hawks on wintering grounds in Argentina". Journal of Raptor Research 30 (2): 106–107. 
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Names and Taxonomy

Taxonomy

The currently accepted scientific name for the Swainson's hawk is Buteo
swainsoni Bonaparte [1]. There are no recognized subspecies or races.
  • 1.  American Ornithologists' Union. 1983. Checklist of North American birds.        6th ed. Lawrence, KS: Allen Press, Inc. 877 p.  [21234]

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Common Names

Swainson's hawk

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