The Brown-headed Cowbird (Molothrus ater) is named for the male's distinctive brown head, which contrasts with his metallic green-black body. This is a common bird of woodlands, farmland, and suburbs across most of North America, breeding from British Columbia and southern Quebec in Canada south to southern Mexico, with a southward contraction of this range in the winter.

Brown-headed Cowbirds are well known as "brood parasites", sneaking their eggs into the nests of birds of other species to be reared by adoptive "host" parents (which are often much smaller than them). During the breeding season, Brown-headed Cowbirds tend to engage in courtship and nest parasitism mainly in the morning and to feed in the afternoon. A single female may travel more than 6 km through woodlands to lay as many as several dozen eggs in a breeding season. Providing bird food in spring around the edges of large woodlands unintentionally facilitates brood parasitism by cowbirds, which can greatly reproduce the reproductive output of their hosts.

In the breeding season, males display by fluffing up their body feathers, partly speading their wings and tail, and bowing deeply while singing. Groups of males sometimes perch together, singing and displaying. The male's song is a squeaky gurgle.

Brown-headed Cowbird eggs are whitish with brown and gray spots concentrated at the larger end and are often easily recognized because they look different and larger than the other eggs in a host nest. A female may lay nearly an egg per day for several weeks, up to 40 in a breeding season (rarely as many as 70 or more). A female will often remove a host egg before adding one of her own. Brown-headed Cowbird eggs have been found in the nests of more than 220 species and over 140 of these are known to have successfully reared young cowbirds. Fed by their host parents, cowbird nestlings develop rapidly, usually leaving the nest after about 10 or 11 days.

The diet of the Brown-headed Cowbird consists mainly of seeds and insects (seeds account for around half the diet during the breeding season, but more than 90% in the winter). Centuries ago, these birds probably followed bison herds on the Great Plains, as they often follow cattle and horses across North America today, feeding on insects flushed from the grass. This dramatic range expansion over the past century or two has negatively impacted a number of parasatized songbird species, some quite seriously.

(Kaufman 1996; AOU 1998; Dunn and Alderfer 2011)

Global Range: Breeding range extends from southeast Alaska, northern British Columbia, and southern Mackenzie to Newfoundland, and south to central Mexico, southern Texas, Gulf Coast, and southern Florida (AOU 1983, Lowther 1993). Winter range extends from northern California, southern New Mexico, Kansas, Great Lakes region, New England, and Nova Scotia south to southern Baja California, Oaxaca, central Veracruz, Gulf Coast, and southern Florida (AOU 1983, Lowther 1993). This species historically occurred in the Great Plains west of the Mississippi River and often was associated with bison that stirred up insects and uncovered seeds. In the 1700s and 1800s, European settlers fragmented the once unbroken expanse of eastern deciduous forest, leading to cowbird expansion throughout the eastern United States and Canada (Mayfield 1965, 1977). Fragmentation of forests and cowbird expansion occurred more recently in the western United States (Verner and Ritter 1983).

Cowbirds breed from SE Alaska, through lower Canada, through the entire continental U.S. to central Mexico. They winter throughout this range, and also in southern Mexico and the tip of Florida.

Biogeographic Regions: nearctic (Native )

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

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: Year-round

The cowbird is a small blackbird with a short conical bill and long, pointed wings. Males appear black with a unique brown head and neck. Females are either dullish gray or brown throughout. The bill is a dull grey, while the eyes are black.

Range mass: 40 to 50 g.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Average basal metabolic rate: 0.6315 W.

Length: 19 cm

Weight: 49 grams

Systems

• Terrestrial

Cowbirds prefer habitats with low or scattered trees among grassland vegetation, such as woodland edges, brushy thickets, prairies, fields, pastures, orchards, and residential areas. Fragmentation of forests has greatly increased the Brown-headed Cowbirds favorite habitat.

Terrestrial Biomes: forest

Comments: Breeding habitat includes woodland, forest (primarily deciduous), forest edge, city parks, suburban gardens, farms, and ranches. Cowbirds often are associated with forest-field edge habitat and clearings in forests. Feedlots, pastures, and fields with livestock also attract cowbirds, especially in predominately forested areas. In the Sierra Nevada of California, cowbirds feed in horse corrals, meadows with herds of cattle, and at bird feeders in towns, and few cowbirds are found more than 10 km from these food sources (Verner and Ritter 1983). In this same area, the number of cowbirds in meadows decreases as the distance of the meadow from pack stations and horse corrals increases. In migration and winter, cowbirds often occur in open situations, cultivated lands, fields, pastures, and scrub.

Before European settlement of the eastern U.S., Brown-headed Cowbirds were apparently limited to the open grasslands of the United States, mostly west of the Mississippi River. However, because the cowbirds need suitable perches for searching for nests to parasitize, the species was probably limited in the breeding season to prairie riparian corridors (Mayfield 1965).

The main reason for cowbird expansion outside of the Great Plains appears to be the fragmentation of once unbroken forest tracts into small pieces of forest interspersed with fields, pastures, roads, and towns (Mayfield 1977b, Kerlinger and Doremus 1981).

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: Yes. At least some 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.

Northern breeding populations are long-distance migrants. Most migrant cowbirds arrive in northern breeding areas in late March-April. Adult males and females arrive first, followed two weeks later by yearling males, who are followed one week later by yearling females (Darley 1982). Southward migration begins in August, peaks in September, and extends into October. Specific timing varies with latitude.

Cowbirds often feed on the ground, away from vegetation. Their main food items are seeds and arthropods. They sometimes hawk, looking for slow flying insects. In a quantitative anaylsis of the cowbird diet, it was found that nearly 75% of the diet was 'weed' seed, with most of the remaining 25% made up of grasshoppers and beetles.

Comments: Feeds almost entirely in open habitats such as pastures, old fields, and prairies (Whitcomb et al. 1981). Diet includes insects (wasps, ants, beetles, grasshoppers and caterpillars), waste grain (corn, wheat, oats, sunflowers, rice), and seeds from noncommercial plants such as panic grass, ragweed (Ambrosia artemisifolia), barnyard grass (Echinochloa crusgalla) and yellow foxtail (Choetochloa glauca) (Friedmann 1929, Bent 1958). Females eat some of the eggs removed from nests (Condor 94:579-584). Nestlings are fed the typical insect diet of their host (Friedmann 1929).

Females hold and defend territories, whereas males have home ranges (0.4-2.5 ha in Ontario; Darley 1982) but do not defend a territory (Ankney and Scott 1982). Breeding and feeding areas of breeding females may be up to several kilometers apart: mean of 1.2 km in Illinois-Missouri; mean of 4.0 km in one area in California (Thompson 1994), and up to 6.7 km in the Sierra Nevada (Rothstein et al. 1980, 1984). NON-BREEDING: Roosts and forages in mixed flocks with red-wing blackbirds (Agelaius phoeniceus) and grackles (Quiscalus quiscula).

Average lifespan

Status: wild: 202 months.

Maximum longevity: 16.9 years (wild) Observations: IMR was calculated from demographic data in the wild. Females may lay up to 40 eggs in a season (http://www.fs.fed.us/database/feis/).

The Brown-headed Cowbird is a brood parasite; the female cowbird selects a host nest of another species of bird to lay her eggs in. She wanders about, parasitizing many nests and laying up to 40 eggs per season. The young are then cared for by the host. 144 different host species have been documented to rear cowbird young, so the incubation and parental care of cowbirds varies greatly depending on the nest in which they are deposited. During the various care processes for the cowbirds, however, they are always fed more often than the young of the host species. The smallest recorded host of cowbird young is the 10 g creeper, while the largest host is the 150 g meadowlark.

Average time to hatching: 11 days.

Average age at sexual or reproductive maturity (male)

Sex: male: 365 days.

Average age at sexual or reproductive maturity (female)

Sex: female: 365 days.

This is an obligate brood parasite; females always deposit their eggs in the nests of other bird species,usually resulting in the death of some or all of the host species eggs or nestlings. Females never build a nest, incubate eggs, or tend young. Eggs have been found in the nests of more than 200 species, and young cowbirds are known to have been raised by at least 139 host species (Eastzer et al. 1980).

Females usually locate nests to parasitize by watching nest-building activities from a perch (Thompson and Gottfried 1976, 1981), especially in semi-open or open habitats. Females have also been observed walking on the ground in dense woods, quietly searching for activity or nests and noisily flapping through dense shrubbery, supposedly to flush incubating females (Norman and Robertson 1975).

Egg laying, in the nests of other bird species, generally extends from April to July, with most eggs laid in May-June. Individual females lay up to a few dozen eggs each season, but usually much fewer (Jackson and Roby 1992, Holford and Roby 1993). Females may lay eggs in different nests or lay multiple eggs in a single nest. They often, but not always, remove and eat one of the host eggs so that the clutch size is the same when the host female returns. The incubation period (11-12 days, sometimes 10) is often shorter than that of the host species. The host birds feed and tend the nestling and recently fledged cowbirds. A nestling cowbird may weigh up to 10 g when the host nestlings, weighing 1-3 g, hatch and the smaller nestlings are often crushed, crowded out of the nest, or starved (Friedmann 1929, 1963; Mayfield 1965). Cowbird nestlings stay in the nest 10-12 days, begin feeding themselves around day 20-22, and become independent when 25-39 days old (Woodward 1983). Young cowbirds just out of the nest are fed more by the host parent than an equivalent mass of host young. During this period, cowbird young perch at the same height and have the same home ranges as their host (Woodward 1983). Hatching success is not necessarily higher than that of their host species. In Pennsylvania, 57.3% of the eggs of several passerine hosts in unparasitized nests hatched but only 42.6% of the cowbird eggs hatched (Friedmann 1963). Approximately 15% of all cowbird eggs laid in Kansas grasslands resulted in a cowbird young leaving the nest (Zimmerman 1983). Cowbird survival in the nest is similar to host nestling survival but, in Vancouver, cowbird survival was much lower after leaving the nest, possibly because the young cowbird's loud begging calls may attract predators (Smith 1981).

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

 
There are 10 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.
 

Download FASTA File

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 10
Species: 10
Species With Barcodes: 1

The cowbird is common. It does, however, pose a threat to the populations of other birds, such as the Kirtland's Warbler, Black-capped Vireo, and Least Bell's Vireo, whose small populations are frequently parasitized by the Brown-headed Cowbird. This has led to programs that involve trapping of the cowbirds at specific breeding areas where other species are threatened.

US Migratory Bird Act: protected

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

IUCN Red List of Threatened Species: least concern

Canada

Rounded National Status Rank: N5B - Secure

United States

Rounded National Status Rank: N5 - Secure

Rounded Global Status Rank: G5 - Secure

Global Short Term Trend: Decline of 10-30%

Comments: Increased range as forested areas were cleared during human settlement; increases were related to more abundant forest edge habitat and to increased food supplies (e.g., in pastures, feedlots). Overall, population trend is downward; breeding populations throughout the west coast ranges, northern Rockies, and much of the eastrn U.S. (except coastal areas from New England to the Deep South) are declining; populations within interior basins, ranges, deserts, and most of the northern prairie are increasing; perhaps declines are related to reduced availability of farm grain (data analysis by S. Droege, North American Breeding Bird Survey 1966-1993, Christmas Bird Count 1959-1988).

Preserve Selection and Design Considerations: The main reason for the recent spread of the cowbird's breeding distribution appears to be a dramatic increase in the amount of field-forest edge habitat and increased availability of feeding areas. Therefore, the best long-term control of cowbirds probably is to eliminate as much field-forest edge habitat as possible and to reduce the number of nearby food sources. With endangered populations with very limited distributions, removing cowbirds is probably the optimal method for immediately reducing the incidence of cowbird parasitism.

Management Requirements: TRAPPING: An effective but labor-intensive method of rapidly decreasing cowbird parasitism in a small region is to live-trap the birds in decoy traps baited with grain or seeds (Shake and Mattsson 1975). The cowbirds are then removed and destroyed. In the Kirtland's warbler project in Michigan, blackbird traps operating between May 1 and July 15 successfully removed cowbirds within a 1 km radius area of the trap and during 1972-1981, 33,536 cowbirds were removed (Mayfield 1977). The parasitism rates of warbler nests dropped from 74.6% during 1957-1971 to 6.1% during 1972-1977 (Walkinshaw 1983). Cowbird removal substantially increased warbler reproductive success. During 1966-1971, an average of 0.8 warblers survived to leave the nest, and this number increased to 2.7 warblers per nest after cowbird removal (Walkinshaw 1983).

ELIMINATION OF EDGE: Because parasitism rates are highest near forest-field edges (Gates and Gysel 1978, Johnson pers. comm.), one method of cowbird control may be to eliminate as much edge habitat as possible. In grasslands, nest parasitism by cowbirds can be reduced by removing any structures that provide an observation perch, such as fences, posts, telephone lines, or fencerows. In all habitats, preserves can be designed with a high ratio of non-edge, "interior" habitat to edge habitat. Efforts should be made to avoid fragmenting large tracts of forest or grasslands into smaller pieces with more field-forest edge. Finally, the nearby presence of livestock or supplemental food (e.g., pastures, feedlots, grain fields) may attract cowbirds and should be reduced or eliminated, if possible.

See Glahn et al. (1991) for information on the impact of ground-based surfactant roost control treatments on local urban and agricultural blackbird/starling problems.

Fragmentation of forest habitats in North America has resulted in a great increase in the edge habitats favored by cowbirds, and a reduction (and in many places an elimination) of forest-interior habitats that cowbirds do not penetrate. As a result, a number of species of forest birds are now subject to increasing parasitism by cowbirds. Cowbird parasitism may be one cause in the general decrease in numbers of songbirds in North America.

The cowbird eats many insects which are pests to humans.

Stewardship Overview: In areas where cowbirds are detrimentally impacting the reproduction of native birds, the following actions may be appropriate:

Reduce or eliminate the amount of forest-field edge habitat by removing fencerows and other perches in grasslands and designing preserves with proportionally larger amounts of "interior" habitat and less forest-field edge habitat.

Reduce or eliminate livestock herds and supplementary feeding areas, such as agricultural fields, feedlots, and bird feeders.

Trap and remove cowbirds.

Species Impact: Cowbird parasitism apparently is contributing to the decline of some songbird populations by reducing the reproductive success of the host species. In the eastern U.S., birds have only recently been exposed to brood parasitism (Mayfield 1977, Brittingham and Temple 1983) and many species lack appropriate responses to minimize the impact of cowbird parasitism. Cowbird parasitism has contributed to the decline in numbers of endangered species such as Kirtland's warbler (Dendroica kirtlandii), least Bell's vireo (VIREO BELLI PUSILLUS), southwestern willow flycatcher (Empidonax traillii EXTIMUS), and black-capped vireo (Vireo atricapillus) (Robinson et al. 1995). Because cowbirds do not rely on just one host, parasitism pressure usually does not relax even when a single host's numbers decrease.

Friedmann (1963) summarized the reactions of hosts to cowbird eggs. Female hosts may 1) continue laying and incubating eggs without apparently noticing the cowbird eggs, 2) throw out the strange eggs, 3) bury the eggs by building a new floor to the nest, or 4) desert the nest. Most of the lower reproductive success experienced by hosts is from egg mortality caused by nest desertion or removal of host eggs by the female cowbird (Gates and Gysel 1978, Smith 1981). In Kirtland's warbler, unparasitized clutches averaged 4.4 eggs but parasitized nests contained an average of only 2.2 warbler eggs (Walkinshaw 1983). Nonparasitized dickcissel (Spiza americana) nests averaged 4.0 eggs while parasitized nests contained an average of 2.4 dickcissel eggs (Zimmerman 1983). In most species, fewer host young fledge in parasitized nests than in unparasitized nests (Friedmann 1963). For instance, Smith (1981) estimated that cowbird parasitism reduced the average number of song sparrows (Melospiza melodia) alive at age six days by 0.38 young.

Parasitism rates vary according to host species (Friedmann 1929, 1963) and habitat (Gates and Gysel 1978, Brittingham and Temple 1983) and vary within a species' range (Mayfield 1965, Zimmerman 1983). On a community-wide basis, some reported parasitism rates include 30.8% of the nests in central Pennsylvania, 13.2% in eastern Washington, 22.4% in Michigan and 1% and 21% in the Sierra Nevada (Ricklefs 1969, Verner and Ritter 1983). The birds most prone to parasitism are open-nesting species that are smaller than cowbirds, have a longer incubation period, feed insects to their young, and do not desert or destroy cowbird eggs. Most of the host species are flycatchers, finches, vireos, and warblers (Friedmann 1929). Within a species, the rate of parasitism can vary greatly. For instance, in a study in Ontario, there was no parasitism of red-eyed vireo (Vireo olivaceus) nests but 72.2% of nests in Michigan were parasitized (Friedmann 1963). In the center of the dickcissel's range, parasitism rates vary from 31% to 95% (Zimmerman 1983).

Timing of breeding can also determine how seriously a host is affected by cowbird parasitism. Cowbirds generally breed from early May through early July; any single-brooded species that breeds outside of this period, and double-brooded species, are less affected by cowbird parasitism (Friedmann 1963, Zimmerman 1983). In a Michigan study, only 3% of the nests failed because of parasitism in May, July, and August, while 11% of the June nests were parasitized (Gates and Gysel 1978). Cowbird parasitism rates can differ among habitat types. The nesting substrate of the host may be important. In riparian habitats of central Iowa, cowbirds did not parasitize nests in deciduous or evergreen trees but did parasitize nests (at a rate of 6-8%) in deciduous saplings, shrubs, forbs, and grass (Best and Stauffer 1980). In a study on the effect of fire on field sparrows (Spizella pusilla) in Iowa, Best (1979) found that two years before a burn 11% of the nests were parasitized by cowbirds. The first season after the late April burn, in which almost all of the herbaceous vegetation and litter was completely burned, no parasitized sparrow nests were found.

In Michigan, Gates and Gysel (1978) found that as distance from the forest-field edge increased, the rate of parasitism decreased. Johnson (pers. comm.) obtained similar results in Minnesota prairies. Nests less than 45 m from the prairie-forest edge had a higher probability of being parasitized than did prairie nests farther than 45 m from an edge (Johnson pers. comm.). One possible reason for the increased rate of parasitism near forest-field edges is because there is a higher density of host nests in edge habitats (Gates and Gysel 1978) and this may attract more cowbirds. In addition, edge provides observation perches nest-searching females. Brittingham and Temple (1983) studied the incidence of cowbird parasitism in Wisconsin deciduous forests in areas with openings such as campgrounds, fields, and logged areas. Forest nests within 100 m of a forest opening >0.2 ha had a parasitism rate of 65% while only 18% of the nests more than 300 m from an opening were parasitized. Cowbirds fed in open areas and parasitized nests that were the shortest distances from feeding areas.

Comments: Range-wide mtDNA data reveal no evidence of long-standing population separations (Ball and Avise 1992). See Fleischer et al. (1991) for information on gene flow between subspecies OBSCURUS and ARTEMISIAE in the Sierra Nevada, California.