Phalacrocorax auritus - Double-Crested Cormorant
Habitat Double-crested cormorants are the most widespread cormorant in North America, and the one most frequently seen in freshwater. They breed on the coast as well as on large inland lakes. They form colonies of stick nests built high in trees on islands or in patches of flooded timber.
Range Year-round, double-crested cormorants are found in Florida and the Aleutian Islands, off of Alaska. During their breeding season (summer), double-crested cormorants are found throughout the Great Plains of the United States and into the provinces of Canada (Alberta, Saskatchewan and Manitoba). In the winter, they are found in the Deep South of the United States and the Outer Banks of North Carolina.
Description 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. The crests are white in cormorants from Alaska, and black in other regions.
Ecological Notes The double-crested cormorant's diet is almost all fish, with just a few insects, crustaceans, or amphibians. They eat a wide variety of fish (more than 250 species have been reported), and they have impressive fishing technique: diving and chasing fish underwater with powerful propulsion from webbed feet. The tip of a cormorant’s upper bill is shaped like a hook, which is helpful for catching prey. Double-crested cormorant's have 1-2 broods a year, with 1-7 eggs per clutch. Nests can be on the ground, on rocks or reefs with no vegetation, or atop trees.
Personal Information The double-crested cormorant was frequently flying above freshwater in formation while we were driving. We saw this bird the most on the first day of the trip, while visiting the Oregon Inlet.
Wires, L. R., & Cuthbert, F. J. (2010). Characteristics of double-crested cormorant colonies in the U.S. Great Lakes island landscape. Journal of Great Lakes Research, 36, 232-241.
This study used data from the U.S. Great Lakes Colonial Waterbird Database and The Nature Conservancy's Great Lakes Island GIS database to identify important features of breeding sites in the U.S. Great Lakes and broaden understanding of cormorant presence at the island-landscape scale. Certain characteristics of breeding sites (e.g., small islands, proximity to mainland) may increase negative human attitudes about cormorants, but to understand cormorant impacts to island resources (e.g., vegetation; other colonial waterbird species), the authors suggest reviewing cormorant presence in the Great Lakes to be considered in the broader context of island science, conservation and known threats, and at a landscape scale.
Robinson, S. A., Forbes, M. R., & Hebert, C. E. (2010). Mercury in parasitic nematodes and trematodes and their double-crested cormorant hosts: Bioaccumulation in the face of sequestration by nematodes. Science of the Total Environment, 408, 5439-5444.
This study looked at the endoparasite Contracaecum spp. (a nematode), and how it bioaccumulated mercury from the double-crested cormorants. Concentrations in the endoparasite were 1.5 times greater than cormorant tissue concentrations, which suggests that nematodes were consuming host tissues and accumulating the cormorants body burden of methyl mercury.
Contributed by Cassie Nix - 2014