The Double-crested cormorant (Nannopterum auritum) is a water bird widely distributed across North America. It is a relative of the frigate bird and is skilled in diving for small fish. Its feathers, like those of all cormorants, are not waterproof and it must spend time drying them out after spending time in the water.
The Double-crested cormorant is large with a stocky body, long neck, medium-sized tail, webbed feet, and a medium-sized hooked bill. It has dark-colored plumage with bare supra-loreal skin and gular skin that is yellow or orange. An adult in breeding plumage will be mostly black with the back and coverts being a dark grayish towards the center. Nuptial crests, for which the species is named, are either white, black or a mix of the two. These are located just above the eyes with the bare skin on the face of a breeding adult being orange. A non-breeding adult will lack the crests and have more yellowish skin around the face. The bill of the adult is dark-colored. The Double-crested cormorant is very similar in appearance to the larger Great cormorant but it can be separated by having more yellow on the throat and the bill. The plumage of juvenile Double-crested cormorants is more dark gray or brownish. The underparts of a juvenile are lighter than the back with a pale throat and breast that darkens towards the belly. As a bird ages, its plumage will grow darker. The bill of a juvenile will be mostly orange or yellowish
Double-crested cormorants occur widely across North America, breeding locally along all coasts as well as extensively in Florida, in the center of the continent, and along the St. Lawrence Seaway, the Great Lakes, and in Mexico, the Bahamas, Belize, and Cuba. Cormorants usually winter along the coast of the Pacific Ocean from Alaska to Mexico, and along the Atlantic and the Gulf coasts from Belize to North Carolina, as well as inland in ice-free areas along lakes and large rivers. They inhabit a range of marine and aquatic inland habitats. They need water for feeding and perches nearby, such as rocks, sandbars, trees, pilings, shipwrecks, wires, or docks where they can rest and dry out during the day.
Double-crested cormorants are diurnal birds. They are very gregarious and can be found in large and small groups at the breeding grounds, and in winter, often feeding in large flocks. They breed in colonies and migrate in large groups. This species feeds during the day, diving for fish for 30-70 seconds, as deep as 1.5-7.5 m (4 ft 11 in-24 ft 7 in). After diving, the birds stand with their wings outstretched for a long time for them to dry, as they are not waterproofed fully. Double-crested cormorants swim low down in the water, often just their head and neck showing, and dive from the water’s surface. Their feet are used to propel them along. They fly low over the water, their bill tilted upward slightly, sometimes flying out from the colony in a long, single-file line. They use physical displays and calls to communicate with each other, using their limited range of calls for certain social situations, but usually being silent.
Double-crested cormorants are carnivores (piscivores). They primarily eat fish, but also insects, crustaceans, and amphibians.
Double-crested cormorants are monogamous. This means that males mate with only one female and females mate with only one male. The colonies they breed in can number as many as three thousand pairs. Males choose a nest site, then advertise for a female, standing to perform a “wing-waving display” to show off the brightly-colored skin on the head and neck. A male will also engage in elaborate courtship dances, including a dance in the water where he presents a female with nest material. These birds defend a small space immediately around their nest which is smaller than one meter across. Breeding takes place from April to August, peaking from May through July. A pair works together to repair an old nest or to build a new one on the ground or sometimes in a tree. The female lays 4-7 pale bluish white chalky eggs and both parents incubate them for 25 to 28 days. Chicks are altricial when hatched, and both parents care for them. They begin to leave their nest at 3 to 4 weeks of age. They can fly starting about 6 weeks of age and are able to dive when they are 6 to 7 weeks old. Chicks are completely independent at 10 weeks of age and breed once they are 2 years old.
These birds are susceptible to pesticide poisoning and other contaminants, as well as to oil spills. Sometimes they are killed or injured when caught by fishhooks, trawls, gill nets, and lobster traps. Also, they are very susceptible to disturbance while nesting. Adults leave their nest unguarded when disturbed, leaving chicks and eggs open to being eaten by gulls and further predators, and in danger of overheating in the sun.
According to the All About Birds resource, the continental population is more than 630,000 breeding birds. Overall, currently, Double-crested cormorants are classified as Least Concern (LC) on the IUCN Red List and their numbers today are increasing.
