A large (33 inches) waterbird, the Double-crested Cormorant is most easily identified by its black body and wings, long hooked bill, and orange chin patch. This species may be separated from the related Great Cormorant ( Phalacrocorax carbo) by that species’ larger size and large white chin patch, from the related Neotropic Cormorant (Phalacrocorax brasilianus) by that species smaller size and small white chin patch, and from the similar-looking Anhinga (Anhinga anhinga) by that species’ longer neck and tail. Male and female Double-crested Cormorants are similar to one another in all seasons. The Double-crested Cormorant breeds in scattered locations along the Pacific coast of North America from Alaska to Baja California, along the Atlantic coast from Newfoundland to Florida, in the northern Great Plains, and in the West Indies. Southern breeding populations tend to be non-migratory, while northern breeding populations migrate south to the coasts, the interior southeastern U.S., and Mexico. Small numbers may breed or winter outside this species’ main range where habitat is appropriate. Double-crested Cormorants inhabit a variety of freshwater and saltwater wetland habitats, including rivers, lakes, marshes, and flooded grasslands. This species nests in trees surrounding bodies of water, on small islands, or on abandoned man-made structures near water. Double-crested Cormorants primarily eat small fish. On large bodies of water across the continent, Double-crested Cormorants may be seen floating low in the water, occasionally diving underwater for long periods while pursuing prey. Like many cormorants, this species may also be seen perched on rocks or snags with its wings outstretched and feathers ruffled. This species lacks the oily feather coating used by other water birds to keep dry and maintain buoyancy, and it has been suggested that this behavior allows the birds to dry their wings. Double-crested Cormorants are primarily active during the day.

Threat Status: Least concern

Double-crested cormorants breed across North America, as far north as southern Alaska. They winter in North America as far south as Sinaloa, Mexico, and are common on marine and inland waters throughout their range.

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

Double-crested cormorants breed across North America, as far north as southern Alaska. They winter in North America as far south as Sinaloa, Mexico, and are common on marine and inland waters throughout their range.

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

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

Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) BREEDING: southeastern Bering Sea and southern Alaska; southern British Columbia eastward through Manitoba to coastal Quebec and Newfoundland, south (in isolated colonies) to Baja California, coastal Sonora, central Chihuahua, central Durango, south-central Arizona, southern New Mexico, southern Texas, Gulf Coast, Florida, northern Bahamas, Cuba, Yucatan Peninsula, and Belize (Johnsgard 1993, AOU 1998). Breeding range in North America has expanded in recent years (Johnsgard 1993). Extirpated from Amchitka Island, Alaska, perhaps due to predation by arctic fox (Alopex lagopus; Siegel-Causey et al. 1991). Occurs throughout most of the coastal breeding range and beyond when not breeding. NON-BREEDING: Pacific coast from Aleutians and southern Alaska south to Baja California and Nayarit; inland from Washington and Montana south to California and northeastern Colorado, southern Minnesota, and the Great Lakes south to northwestern Mexico, Oklahoma, Texas, and the Gulf states; and along the Atlantic coast, from Lake Ontario and New England south to Florida, Bermuda, the Bahamas, Greater Antilles, Yucatan Peninsula, and northern Belize (AOU 1998).

Cormorants are large birds (70 to 90 cm in length, 1.2 to 2.5 kg) with dark brown or black plumage that has a dull greenish or bronze sheen. They have lean bodies with long necks and short wings. They have long, hooked beaks and bright orange-yellow skin that covers their face and throat. They have black webbed feet and short legs. Their tails are wedge-shaped. During the breeding season, double-crested cormorants have two curly black crests on their heads, blue eyelids, a dusky-colored bill and orange on their throat. In the winter, adults do not have the crests or blue on their eyelids, and they have a yellow bill.

Male double-crested cormorants are slightly bigger than females. Juveniles are much duller in color than adults. They are usually dark brown with grayish or whitish coloring on their chest and belly.

There are five subspecies of double-crested cormorants. These subspecies are different in size and the color and shape of their crests.

Range mass: 1200 to 2500 g.

Range length: 70 to 90 cm.

Sexual Dimorphism: male larger; ornamentation

Average basal metabolic rate: 5.6096 W.

Cormorants are large birds (70 to 90 cm in length, 1.2 to 2.5 kg) with dark brown or black plumage that has a dull greenish or bronze sheen. They have lean bodies, long necks and relatively short wings. They have long beaks with a hooked upper mandible and bright orange-yellow skin that covers the face, throat and base of the bill. Their black feet are webbed feet and found on short legs, and their tails are wedge-shaped. During the breeding season, double-crested cormorants have two curly black crests on their heads, blue eyelids, a dusky bill and orange on the throat sac and lores. In the winter, adults lack the crests, show no blue on eyelids, have a yellow bill with red on gular sac, and yellow behind the ocher.

Males are slightly bigger than females. Juveniles are much duller in color than adults. They are usually dark brown with grayish or whitish coloring underneath. (Pearson 1936)

There are five subspecies of double-crested cormorants. These subspecies are differentiated by size and the color and shape of their crests.

Range mass: 1200 to 2500 g.

Range length: 70 to 90 cm.

Sexual Dimorphism: male larger; ornamentation

Average basal metabolic rate: 5.6096 W.

Length: 81 cm

Weight: 1818 grams

Double-crested cormorants are found in a variety of marine and inland aquatic habitats. They require water for feeding and nearby perches, such as rocks, sandbars, pilings, shipwrecks, wires, trees or docks for resting on and drying out during the day.

Habitat Regions: temperate ; saltwater or marine ; freshwater

Aquatic Biomes: lakes and ponds; rivers and streams; coastal

• Terrestrial
• Freshwater
• Marine

Double-crested cormorants are found in a variety of marine and inland aquatic habitats. They require water for feeding and nearby perches, such as rocks, sandbars, pilings, shipwrecks, wires, trees or docks for resting on and drying out during the day.

Habitat Regions: temperate ; saltwater or marine ; freshwater

Aquatic Biomes: lakes and ponds; rivers and streams; coastal

Comments: Lakes, ponds, rivers, lagoons, swamps, coastal bays, marine islands, and seacoasts; usually within sight of land. Nests on the ground or in trees in freshwater situations, and on coastal cliffs (usually high sloping areas with good visibility). See Spendelow and Patton (1988) for further details on nesting sites in different geographic areas.

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 coastal and especially interior populations migrate southward for nonbreeding season; migratory tendency is stronger on east coast than on west coast. Usually follows river valleys, coastlines, and water courses. Migrates day or night (Palmer 1962). East of the Rockies, migrates southward from northern latitudes in October-November, northward in April-May; breeders from the central and eastern parts of Canada and the northern U.S. winter mainly in the southern U.S. between Texas and Florida, with considerable overlap of different breeding populations on the wintering grounds; there is little intermixing of birds from east and west of the Rockies (Dolbeer 1991).

Double-crested cormorants feed primarily on fish, but also eat insects, crustaceans and amphibians. They usually feed in shallow water (less than 8 m deep) within 5 km of shore. They catch prey by diving underwater to chase it. They may swallow small fish while underwater, but they bring larger prey up to the surface to shake it, clean it or hammer it on the water before eating.

When hunting schooling fish, cormorants may feed together in large flocks. They have a hook-like tip on their bill and specialized muscles that allow them to grasp their slippery prey.

Double-crested cormorants drink by dipping their bill into water, and then raising their head to let the water fall into their throat.

Animal Foods: amphibians; fish; insects; aquatic crustaceans

Double-crested cormorants feed primarily on fish, but also eat insects, crustaceans and amphibians. They generally feed in shallow water (less than 8 m deep) within 5 km of shore, diving underwater to catch their prey. They may swallow small fish while underwater, but bring larger prey up to the surface to shake, clean or hammer on the water before consuming them.

When feeding on schooling fish, cormorants may feed together in flocks. They have a hook-like tip on the upper maxilla of their bill and specialized muscles that aid them in grasping their slippery prey.

Animal Foods: amphibians; fish; insects; aquatic crustaceans

Primary Diet: carnivore (Piscivore )

Comments: Feeds opportunistically on fishes (usually less than 13 cm long); dives from surface of water; usually feeds in water < 15 m deep. Accused of reducing sport fish populations in New York, but this contention has not been documented (Carroll 1988). Eats mostly schooling fishes (in marine waters, mainly slow-moving species of bottom and mid-water), sometimes aquatic invertebrates and rarely small vertebrates other than fishes. Sometimes forages in compact flocks.

Double-crested cormorants nest in colonies with up to thirteen other species of birds. Within these mixed colonies, they may take nest sites away from other species. Other species, such as gulls may also benefit from the cormorants by eating their eggs and chicks, the pellets that they cast, and fish that they regurgitate. Other species may also steal food from the cormorants. Cormorants also hunt in flocks with other species. This may help the cormorants and the other species to find food more quickly.

Double-crested cormorants also affect the populations of the fish they consume.

Larus, Corvidae and Quiscalus are probably significant predators of cormorant eggs and chicks. Canis latrans, foxes and Procyon lotor may also prey on cormorant chicks. Adult cormorants and chicks are susceptible to predation by Haliaeetus leucocephalus, and occasionally by Bubo virginianus, Caiman crocodylus and Pelecanus occidentalis.

When threatened by a predator, gulls may threaten the predator or vomit fish at them. If the predator is large, the adults usually leave the nest, and circle overhead.

Known Predators:

• crows and jays (Corvidae)
• grackles (Quiscalus)
• coyotes (Canis_latrans)
• foxes (Canidae)
• raccoons (Procyon_lotor)
• bald eagles (Haliaeetus_leucocephalus)
• great horned owls (Bubo_virginianus)
• caimans (Caiman_crocodylus)
• brown pelicans (Pelecanus_occidentalis)
• gulls (Larus)

Double-crested cormorants may nest with up to thirteen other species of colony-nesting birds. Within these mixed colonies, they may affect nest-site availability to other species, and provide food for the other species by means of chicks, eggs, pellets, regurgitated fish and stolen food. Cormorants also hunt in mixed flocks, benefiting others and benefiting others through their combined effort to find prey.

Double-crested cormorants impact the populations of the fish they consume.

Gulls, crows and jays and grackles are probably significant predators of cormorant eggs and chicks. Coyotes, foxes and raccoons may also prey on cormorant chicks. Adult cormorants and chicks are susceptible to predation by bald eagles, and occasionally by great horned owls, caiman and brown pelicans.

When threatened by a predator, gulls may threaten the predator or vomit fish at them. If the predator is large, the adults usually leave the nest, and circle overhead.

Known Predators:

• crows and jays (Corvidae)
• grackles (Quiscalus)
• coyotes (Canis latrans)
• foxes (Canidae)
• raccoons (Procyon lotor)
• bald eagles (Haliaeetus leucocephalus)
• great horned owls (Bubo virginianus)
• caimans (Caiman crocodilus)
• brown pelicans (Pelecanus occidentalis)
• gulls (Larus)

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

Estimated Number of Occurrences: > 300

Comments: In 1994, 852 colonies throughout North America (Tyson et al. 1997); number of defined occurrences may be somewhat lower.

100,000 to >1,000,000 individuals

Comments: Widespread and increasing. In 1994, estimated at least 372,000 nesting pairs throughout North America (Tyson et al. 1997). Relative abundance recorded on North American Breeding Bird Survey (BBS) survey-wide 1966-1996 was 0.76 birds per route. Highest relative abundance recorded in Minnesota (2.64 birds per route; Sauer et al. 1997). In winter, survey-wide Christmas Bird Count (CBC) shows 11.96 birds per 100 survey hours, 1959-1988. Highest CBC relative abundance recorded in North Carolina during the same period (170.64 birds per 100 survey hours; Sauer et al. 1996). High winter densities also occur in southern Florida, northern South Carolina, and along the lower Colorado River valley in the southwestern U.S. (Root 1988).

Typically forages within about 20 km of roost site (Johnsgard 1993). No available information on interannual fidelity to colony; median distance of breeding birds to their natal site was < 25 kilometres (Dolbeer 1991). Increased sea surface temperatures, such as those associated with El Nino events, were correlated with decreases in nesting populations in Washington (Wilson 1991). Vigorously defends eggs and young against avian predators (Ehrlich et al. 1992), though large gulls, crows, and ravens are significant predators on eggs and young in some areas.

Double-crested cormorants use calls and physical displays to communicate with one another. While cormorants use their small range of calls to communicate some things, they are usually silent. One example of the physical displays that cormorants use is the "wing wave display" that males use to attract a mate.

Communication Channels: visual ; acoustic

Double-crested cormorants use calls and physical displays to communicate with one another. While cormorants use their small range of calls in certain social situations, they are largely silent. One example of the physical displays used to communicate between cormorants is the "wing wave display" used by males to attract a mate.

Communication Channels: visual ; acoustic

The oldest known wild double-crested cormorant lived to be 17 years and 9 months old. The average life expectancy for wild birds is 6.1 years.

Range lifespan

Status: wild: 17.75 (high) years.

Average lifespan

Status: wild: 6.1 years.

Average lifespan

Status: wild: 270 months.

The oldest known wild double-crested cormorant lived to be 17 years and 9 months old. The average life expectancy for wild birds is 6.1 years.

Range lifespan

Status: wild: 17.75 (high) years.

Average lifespan

Status: wild: 6.1 years.

Average lifespan

Status: wild: 270 months.

Cormorants are monogamous and breed in colonies of up to three thousand pairs. The male chooses a nest site and then advertises for a female by standing in a “wing-waving display” that shows off the brightly-colored skin on his head and neck. Males also perform elaborate courtship dances, including dances in the water where they present the female with nest material. After forming a pair, double-crested cormorants lose their crests.

Double-crested cormorants do not defend a large territory around the nest. They defend a small area immediately around the nest that is less than one meter in diameter.

Mating System: monogamous

Double-crested cormorants breed between April and August. The males arrive at the breeding colony first and chose a nest site. Then they advertise for a mate. The male and female work together to repair an old nest or to build a new one. Nests are built of sticks, twigs, vegetation and whatever else the cormorants can find lying around. This can include rope, fishnet, buoys and deflated balloons. The male brings most of the material to the female who builds the nest. She also guards it from neighboring cormorants who try to steal the nest materials. Nests are usually built on the ground, but are sometimes built in trees.

After the nest is finished, the female lays 1 to 7 (usually 4) pale bluish-white eggs with a chalky coating. The eggs are laid 1 to 3 days apart. Both parents incubate the eggs, which hatch after 25 to 28 days. The newly hatched young are altricial, and are cared for by both parents. Both parents feed the chicks regurgitated food. The young begin to leave the nest when they are 3 to 4 weeks old. They can fly at about 6 weeks and dive at 6 to 7 weeks. The chicks become completely independent of their parents by 10 weeks of age. Double-crested cormorants do not breed until they are at least 2 years old.

Breeding interval: Double-crested cormorants breed once yearly.

Breeding season: Double-crested cormorants breed between April and August, with peak activity occurring in May through July.

Range eggs per season: 1 to 7.

Range time to hatching: 25 to 28 days.

Range fledging age: 3 to 4 weeks.

Average time to independence: 10 weeks.

Range age at sexual or reproductive maturity (female): 2 (low) years.

Range age at sexual or reproductive maturity (male): 2 (low) years.

Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)

Average eggs per season: 3.

Both parents incubate the eggs and care for the altricial chicks. The parents feed the chicks regurgitated food 2 to 6 times per day. On hot days, parents fetch water and pour it directly from their beak into the open mouths of the chicks.

Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female)

Cormorants are monogamous and breed in colonies of up to three thousand pairs. The male chooses a nest site and then advertises for a female by standing in a “wing-waving display” that shows off the brightly-colored skin on his head and neck. Males also perform elaborate courtship dances, including dances in the water where they present the female with nest material. After forming a pair, double-crested cormorants lose their crests.

Double-crested cormorants do not defend a large territory around the nest. They defend a small area immediately around the nest that is less than one meter in diameter.

Mating System: monogamous

Double-crested cormorants breed between April and August, with peak activity occurring in May through July. The males arrive at the breeding colony first and chose a nest site. They then advertise for a mate. The male and female work together to repair an old nest or to build a new one of sticks, twigs, vegetation and flotsam and jetsam found nearby, including rope, fishnet, buoys and deflated balloons. The male brings most of the material to the female who builds the nest and guards it from other colony members who would otherwise steal the nest materials. The nests typically built on the ground, but are occasionally built in trees. After nest construction is complete, the female lays 1 to 7 (usually 4) pale bluish-white eggs with a chalky coating. The eggs are laid 1 to 3 days apart. Both parents incubate the eggs, which hatch asynchronously after 25 to 28 days. The newly hatched young are altricial, and are cared for by both parents. Both parents feed the chicks regurgitated food. The young begin to leave the nest when they are 3 to 4 weeks old. They can fly at about 6 weeks and dive at 6 to 7 weeks. The chicks become completely independent of their parents by 10 weeks of age. Double-crested cormorants do not breed until they are at least 2 years old.

Breeding interval: Double-crested cormorants breed once yearly.

Breeding season: Double-crested cormorants breed between April and August, with peak activity occurring in May through July.

Range eggs per season: 1 to 7.

Range time to hatching: 25 to 28 days.

Range fledging age: 3 to 4 weeks.

Average time to independence: 10 weeks.

Range age at sexual or reproductive maturity (female): 2 (low) years.

Range age at sexual or reproductive maturity (male): 2 (low) years.

Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)

Average eggs per season: 3.

Both parents incubate the eggs and care for the altricial chicks. The parents feed the chicks regurgitated food 2 to 6 times per day. On hot days, parents fetch water and pour it directly from their beak into the open mouths of the chicks.

Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female)

Time of nesting varies geographically, with local variations, and among different years a particular colony. Nesting begins in winter in Florida, as late as early June in southern Alaska. Clutch size usually one to seven (average typically three or four). Incubation 24-33 days (average around 28-30), by both sexes in turn. Hatching success was 54-75% in three studies. Survival from hatching to fledging was 72-95% in two studies. First flight to water at about 35-42 days. Independent at about 9-10 weeks. Usually first breeds at three years, sometimes at two years, rarely at one year. Renesting following loss of clutch is fairly common. Nest in relatively dense colonies; nests only 0.6 - 2.0 meters apart (Hatch and Weseloh 1999). New colonies may be abandoned within a few years, but once well established, likely to persist (Hatch and Weseloh 1999). See Johnsgard (1993) for further information.

Populations of double-crested cormorants have increased dramatically over the last thirty years. This species has no special protection under CITES or the Endangered Species Act. It is, however, protected under the U.S. Migratory Bird Act.

Cormorants are susceptible to poisoning from pesticides and other contaminants, and to oil spills. They are sometimes killed or injured when they are caught on fishhooks and in gill-nets, lobster traps, and trawls. They are also very susceptible to disturbance at their nest sites. Adult cormorants leave the nest unguarded when they are disturbed, leaving the chicks and eggs vulnerable to predation by gulls and other predators, and to overheating in the hot sun.

IUCN Red List of Threatened Species: least concern

US Migratory Bird Act: protected

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

Populations of double-crested cormorants have increased dramatically over the last thirty years. This species has no special protection under CITES or the Endangered Species Act. It is, however, protected under the U.S. Migratory Bird Act.

Cormorants are susceptible to poisoning from pesticides and other contaminants, and to oil spills. They are sometimes killed or injured when they are caught on fishhooks and in gill-nets, lobster traps, and trawls. They are also very susceptible to disturbance at their nest sites. Adult cormorants leave the nest unguarded when they are disturbed, leaving the chicks and eggs vulnerable to predation by gulls and other predators, and to overheating in the hot sun.

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: N5B,N5N : N5B: Secure - Breeding, N5N: Secure - Nonbreeding

Rounded Global Status Rank: G5 - Secure

Reasons: Large breeding range in North America; rapidly increasing populations.

Other Considerations: Considered a nuisance species by fisherman. Can feed heavily on small fish being raised commercially on minnow farms for bait, or for human consumption at fish farms or aquacultural sites. Game fish, however, appear to be minor components of the diet in Great Lakes region (Belyea et al. 1997, Bur et al. 1997, Ross and Johnson 1997). Also, in some locations within the Great Lakes basin have propensity for killing the trees in which they nest and roost and are competing with other colonial nesting water and wading birds for the same island nesting sites. There is heightened concern when this competition jeopardizes the reproductive success of rare, threatened, or endangered plant and animal species (USFWS 1999a).

Global Short Term Trend: Increase of 10 to >25%

Comments: From 1973 to 1993, Great Lakes population increased over 300-fold to over 38,000 pairs; a historic high (Weseloh and Collier 1999). Along St. Lawrence River, have increased from 12,000 pairs in 1979 to 22,000 pairs in 1990 (QBW 1999). Now increasing almost everywhere. Although BBS and other roadside surveys are not the most appropriate census method for colonial-nesting water birds trend data still provides useful information. Survey-wide increase according to BBS 1966-1996 (6.8 percent annual change; P = 0.00; n = 327). Greatest percent increase in central region (22.7 percent annual change; P = 0.00; n = 77) for same period. No significant decreases for periods 1966-1999, 1966-1979, or 1980-1996 (Sauer et al. 1997). In winter, CBC 1959-1988 indicates survey-wide increase (7.3 percent annual change; P less than 0.01; n = 790; Sauer et al. 1996).

Global Long Term Trend: Increase of >25%

Comments: Declined in the 1960s and subsequently recovered. Largest decline in the Great Lakes population largely due to effects of DDT. Great Lakes population decreased by 86 percent from approximately 900 in the early 1950s to a mere 125 in 1973. Disappeared as a nesting bird on Lakes Michigan and Superior and only about ten pairs remained on Lake Ontario (Weseloh and Collier 1999).

Degree of Threat: C : Not very threatened throughout its range, communities often provide natural resources that when exploited alter the composition and structure over the short-term, or communities are self-protecting because they are unsuitable for other uses

Comments: PERSECUTION: Has long been hunted for both eggs and meat. Persecuted because thought to compete with fishermen. Hunted illegally; in June of 1998, over 800 birds were shot in eastern Lake Ontario and 20 were shot in 1994 on Four Brothers Island, New York. Also hunted legally; in Quebec, Canada, a government-sponsored control program involves shooting adults, destroying nests, and spraying eggs with an oily substance that asphyxiates the embryos (QBW 1999). U.S. Fish and Wildlife Service permits the lethal take of cormorants, without a permit, on catfish and bait fish farms in 12 southeastern states and Minnesota, where economic impacts have been well-documented and non-lethal control has proven ineffective (USFWS 1999a). PESTICIDES: Contamination by organochlorine pesticides greatly reduced reproductive success during the 1950s and 1960s significantly reducing populations along the Great Lakes and in other areas (Anderson et al. 1969, cited in INRIN 1999; Weseloh et al. 1983, cited in USFWS 1999b). Eggshell thinning due to exposure to DDE, DDT, and PCB's widely reported during this period (INRIN 1999, USFWS 1999a). By the early 1970s, eggshells from the Great Lakes region were nearly 30% thinner than normal (Weseloh and Collier 1999). Contamination caused reproductive failure, and chicks that hatched sometimes had crossed bills, club feet, and eye and skeletal deformities (USFWS 1999a). Deformities resulting from the ingestion of PCBs have been noted in the vicinity of Green Bay, Wisconsin (Ehrlich et al. 1992). Populations increased in New England beginning 1970 after the use of these pesticides was banned; did not increase in other areas until the 1980s (USFWS 1999b). PREDATION: Predation of eggs and young by crows and ravens (CORVUS spp.) and gulls (Larus spp.; INRIN 1999). Verbeek (1982) reports that crows were responsible for the destruction of 22 percent of eggs (first clutch) in British Columbia. Also, temporary food shortages may be a possible limiting factors. HABITAT: Nesting habitat may become an important limiting factor (USFWS 1999a). See also Spendelow and Patton 1988; Carroll 1988; Johnsgard 1993; Markham, 1978 COSEWIC report; Hyslop and Kennedy 1992; Chapdelaine and Brousseau 1992; Vermeer and Sealy 1984; Lensink 1984; Buckley and Buckley 1984).

Management Requirements: In Illinois, a public viewing area used once a week by humans 229 m from a rookery did not cause any overt responses from nesting birds (DeMauro 1993). See Conniff (1991) for an informal discussion of some methods to keep cormorants out of catfish ponds.

Global Protection: Many to very many (13 to >40) occurrences appropriately protected and managed

Comments: Likely protected by numerous managed areas given the widespread range. Placed on the state lists of threatened and endangered wildlife in Wisconsin and Illinois. In 1972, added to the list of species protected by the 1918 Migratory Bird Treaty Act (USFWS 1999a).

Double-crested cormorants and other fish-eating birds may compete with people for valuable fish. However, it is difficult to tell how many fish they really eat. It is also possible that double-crested cormorants change vegetation where they nest and rest, making the habitat less valuable to people and other animals. They may also spread fish diseases and parasites between different bodies of water.

Double-crested cormorants have no known positive impact on humans.

Double-crested cormorants and other fish-eating birds are considered by some to be detrimental to commercial fisheries and fish farms. However, the extent of their impact on fish populations is difficult to quantify, and has been demonstrated by some studies to be very small. Some landowners also complain of lowering of property values as a result of the impact of double-crested cormorants on vegetation. Finally, double-crested cormorants have been implicated as vectors for fish diseases and parasites.

Double-crested cormorants have no known positive impact on humans.

Comments: Regarded as a nuisance when eating catish from catfish farm ponds (Conniff 1991).

Comments: See Siegel-Causey (1988) for analysis of relationships within family. Siegel-Causey (1988) proposed removing this species from the genus Phalacrocorax and placing it in the genus Hypoleucus; DeBenedictus (1989) concluded that the taxonomic ranks of many groups recognized by Siegel-Causey (1988) are inflated and inconsistent with other taxonomic data.