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.
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. (Hatch and Weseloh, 1999; Pearson, 1936; Perrins, 1990)
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. (Hatch and Weseloh, 1999)
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. (Hatch and Weseloh, 1999; Landsborough, 1964; Pearson, 1936; Perrins, 1990)
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. (Hatch and Weseloh, 1999; Landsborough, 1964; Pearson, 1936; Perrins, 1990)
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. (Hatch and Weseloh, 1999)
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. (Hatch and Weseloh, 1999)
Double-crested cormorants are very social. They can be found in small and large groups while breeding and during the winter. They breed in colonies and often feed in large flocks. They also migrate in large groups.
Double-crested cormorants feed during the day by diving for fish. After diving, cormorants look for an elevated spot to perch with their wings outspread, probably to dry out their feathers. (Hatch and Weseloh, 1999)
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. (Hatch and Weseloh, 1999)
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. (Brooke and Birkhead, 1991; Hatch and Weseloh, 1999; Landsborough, 1964)
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. (Hatch and Weseloh, 1999)
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. (Hatch and Weseloh, 1999)
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. (Hatch and Weseloh, 1999)
Double-crested cormorants have no known positive impact on humans.
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. (Hatch and Weseloh, 1999)
Double-crested cormorants are also called crow ducks, shag, water-turkeys and lawyers.
Many double-crested cormorants spend the winter in the Florida Bay along with roseate spoonbills, great white herons, reddish egrets and many other wading birds. (Pearson, 1936; Rodgers, et al., 1996)
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