Double-crested cormorant

Double-crested cormorant
Adult in breeding plumage in California
Conservation status
Least Concern  (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Suliformes
Family:	Phalacrocoracidae
Genus:	Nannopterum
Species:	N. auritum
Binomial name
Nannopterum auritum (Lesson, 1831)
Range of the double-crested cormorant

The double-crested cormorant (Nannopterum auritum) is a large water bird in the cormorant family, Phalacrocoracidae. It is the most widespread cormorant in North America. It lives along sea coasts and around inland lakes, rivers, reservoirs, and other bodies of water.

Adults are dark brown to black and have yellow-orange bare skin around the base of the bill. During the breeding season, adults grow two small tufts of feathers on the sides of the head. These tufts are the "double crest" in the bird's common name. Double-crested cormorants mainly eat fish. They catch them by diving from the surface and swimming underwater with their webbed feet.

The species became much less common in parts of its range during the 19th and 20th centuries. People killed many cormorants, and the pesticide DDT caused eggshell thinning and poor breeding success. Numbers began to increase again in the late 1970s. It is now common in many areas and is listed as Least Concern by the IUCN.

Description

A non-breeding adult in Morro Bay, California

The double-crested cormorant is about 70 to 90 cm (28 to 35 in) long. It weighs about 1.2 to 2.5 kg (2.6 to 5.5 lb), and its wingspan is about 114 to 123 cm (45 to 48 in). Males and females look much alike.

The bird has a long neck and tail, a heavy body, and a thin bill with a strongly hooked tip. Its four toes are joined by webbing. When swimming, much of the body sits low in the water. Adults are brown-black with yellow or orange skin around the bill and throat. The eyes are green to blue-green, and the inside of the mouth is bright blue.

The two crests are easiest to see during courtship and may remain for part of the incubation period. Their color changes across the species' range. Birds in Alaska usually have straighter, whiter crests, while birds farther southeast usually have smaller, darker, curlier crests. Young birds are browner than adults. Their neck and breast are usually paler, becoming darker toward the belly.

Taxonomy

French naturalist René Lesson described the species in 1831 under the name Carbo auritus. The locality given in the original description was New Zealand, but later ornithologists treated this as an error and placed the type locality in North America.

For many years the species was placed in the genus Phalacrocorax and was called Phalacrocorax auritus. A genetic study published in 2014 found that the cormorants traditionally placed in Phalacrocorax belonged to several separate evolutionary groups. The International Ornithologists' Union and American Ornithological Society later moved the double-crested cormorant, the neotropic cormorant, and the flightless cormorant into the restored genus Nannopterum. The scientific name is therefore now Nannopterum auritum. The ending of some scientific names also changed because Nannopterum is treated as a neuter Latinized name.

Subspecies

Five subspecies are commonly recognized:

• N. a. auritum – central and northeastern North America
• N. a. cincinatum – Alaska
• N. a. albociliatum – the Pacific coast from southwestern British Columbia south into northwestern Mexico
• N. a. floridanum – the southeastern United States
• N. a. heuretum – the Bahamas

These subspecies were mainly separated by geography, body size, and the form and color of the crests. The boundaries are not clear in every case. A 2010 review found much overlap in their appearance and range. Genetic studies used in that review gave strong support for the Alaskan population being different from most continental birds, but gave much less support for several of the other traditional divisions. The five-subspecies classification is still used by major taxonomic databases.

Distribution and habitat

Breeding, migration, year-round, and non-breeding range

Double-crested cormorants occur across much of North America. They breed on both the Atlantic and Pacific coasts and around many large inland lakes. They are especially widespread in the Great Lakes and the prairie lake regions of Canada and the northern United States.

The species uses both salt water and fresh water. Habitats include coasts, bays, estuaries, lakes, reservoirs, rivers, ponds, and lagoons. Breeding colonies are usually close to water, but the birds can travel far to feed. Cornell Lab reports flights of up to about 40 mi (64 km) between a colony or roost and a feeding area.

Migration differs by region. Many birds from Canada and the northern interior move south for the winter to the southeastern United States, Mexico, or the Caribbean. Some populations on the Pacific coast and in warmer southern areas stay near their breeding region throughout the year.

In 2024, The State of Canada's Birds estimated about 320,000 double-crested cormorants in Canada. It described the Canadian population as having had a "large increase" since 1970.

Behavior and ecology

Feeding

The diet is almost entirely fish, although the birds also take small numbers of crustaceans, amphibians, and insects. More than 250 kinds of fish have been recorded in the diet. The U.S. Fish and Wildlife Service says an adult eats an average of about 1 lb (0.45 kg) of fish each day. Much of the diet is made up of small schooling or bottom-living fish, usually less than 6 in (15 cm) long.

A cormorant dives from the water's surface and chases fish underwater. It is mainly pushed forward by its large webbed feet. The hooked end of the bill helps it hold slippery prey. The species is an opportunistic feeder, so the fish it eats can change greatly between places and seasons.

After diving, double-crested cormorants often stand on a rock, branch, dock, or other exposed place with their wings held open. Their feathers become wetter than the feathers of ducks, which may reduce buoyancy and make underwater hunting easier. Holding the wings open helps dry the plumage after feeding.

Breeding

Double-crested cormorants in John James Audubon's The Birds of America

Double-crested cormorants breed in colonies. A colony may contain only a few pairs or several thousand pairs. Nests may be built in trees, on rocky islands, on cliffs or reefs, or directly on the ground. The male chooses a nest site and brings much of the nest material, while the female does most of the arranging. A nest is mainly made from sticks, with grass, seaweed, and other material.

A clutch can contain from one to seven pale blue eggs, although a typical successful nest has two or three chicks. Both parents care for the nest. Incubation usually lasts about 25 to 30 days. Young birds can usually fly at about five or six weeks old. They continue to feed with adults for several more weeks and are usually independent at about ten weeks. Most birds do not breed successfully until they are about three or four years old.

In ground colonies, young birds sometimes leave their nests and gather in groups called crèches, but return to their own nests to be fed. Adults nesting in direct sun may shade their young and bring them water in their mouths.

Effects of large colonies

Cormorants may use the same nesting area for many years. Large amounts of droppings, or guano, can change the soil and kill nest trees. Birds also break branches and remove leaves for nesting material. When trees die and fall, a colony may move or begin nesting on the ground.

The effects can be strong on small islands with dense colonies. A 2019 study of reservoir islands in Alabama found lower tree and plant diversity and lower diversity of other birds on islands occupied by cormorants. Some changes could still be measured as long as ten years after a colony had been abandoned.

Population history and conservation

Early written records show that double-crested cormorants were very numerous in parts of North America before 1900. Their numbers fell sharply during the late 1800s, and the birds disappeared from some areas. Populations partly recovered during the first half of the 20th century, then declined again from the 1950s through the 1970s.

One cause of the second decline was chemical pollution. Double-crested cormorants ate fish that contained DDE, a breakdown product of DDT. DDE caused their eggshells to become thinner and easier to break. A 1979 study of seabird eggs in eastern Canada found DDE levels in cormorant eggs high enough to cause local population declines through eggshell thinning.

Conditions changed during the 1970s. The United States restricted DDT in 1972, and double-crested cormorants received federal protection under the Migratory Bird Treaty Act that year. Shooting and other killing also became less common. Populations then increased across much of the continent. Wires and Cuthbert estimated that the large Interior and Atlantic breeding populations increased from about 32,000 pairs in the early 1970s to more than 226,000 pairs by the late 1990s.

A U.S. Fish and Wildlife Service environmental review estimated 871,001 to 1,031,757 birds in the continental United States and Canada. The species is not considered globally threatened and is listed as Least Concern by the IUCN.

Relationship with people

Fisheries and aquaculture

Because double-crested cormorants eat fish and sometimes gather in large numbers, they can come into conflict with fish farmers, anglers, and fisheries managers. Large flocks may cause important local losses where fish are concentrated, such as aquaculture ponds, hatcheries, fish-stocking sites, and spawning areas.

The effect on wild fisheries is not the same everywhere. A nine-year study at Little Galloo Island in eastern Lake Ontario found that 58 percent of prey were forage fish, 37 percent were panfish, and about 5 percent were game fish. Even though game fish were a small part of the diet, the researchers estimated that cormorants there ate more biomass of smallmouth bass and yellow perch than sport and commercial fishers removed.

A different study in western Lake Erie found that cormorants mainly ate young gizzard shad, emerald shiners, and freshwater drum. The researchers concluded that the cormorant population there at the time was not harming sport or commercial fishing enough to justify control for that purpose. These studies show why the effects of cormorants on fisheries are usually studied at a local or regional level rather than assumed to be the same throughout the species' range.

Management

In the United States, the double-crested cormorant is protected by the Migratory Bird Treaty Act. Killing, capturing, or destroying birds, eggs, or active nests normally requires federal authorization.

Federal policy has changed several times. Broad "depredation orders" created in 2003 allowed some control without individual permits. In 2016, the United States District Court for the District of Columbia set those orders aside after ruling that the U.S. Fish and Wildlife Service's environmental review had not adequately considered alternatives and effects on cormorants and other resources.

After a new environmental review, the Fish and Wildlife Service published a final rule on December 29, 2020. The rule created a special permit for state and federally recognized Tribal wildlife agencies in the lower 48 states. It took effect on February 12, 2021. The current permit is listed at 50 CFR § 21.123. It may be used for certain conflicts involving aquaculture, human health or safety, protected wildlife, agency property, and wild or publicly stocked fish.

The federal population model allows a maximum combined take of 121,504 double-crested cormorants per year across four lower-48 management populations. The Fish and Wildlife Service says this is a biological ceiling, not a target number that managers are expected to kill.

Management rules are different in Canada and are mainly set by provinces. Ontario created a fall hunting season for double-crested cormorants in 2020. The current season runs from September 15 through December 31 in most of the province, with a daily limit of 15 birds.

Further reading

• Kennedy, Martyn; Spencer, Hamish G. (2014). "Classification of the cormorants of the world". Molecular Phylogenetics and Evolution. 79: 249–257. doi:10.1016/j.ympev.2014.06.020.
• Wires, Linda R.; Cuthbert, Francesca J. (2006). "Historic Populations of the Double-crested Cormorant (Phalacrocorax auritus): Implications for Conservation and Management in the 21st Century". Waterbirds. 29 (1): 9–37.
• Adkins, Jessica Y.; Roby, Daniel D. (March 31, 2010). "A Status Assessment of the Double-crested Cormorant (Phalacrocorax auritus) in Western North America: 1998–2009" (PDF). U.S. Geological Survey – Oregon Cooperative Fish and Wildlife Research Unit.

External links

• Double-crested cormorant at the Cornell Lab of Ornithology
• Double-crested cormorant management at the U.S. Fish and Wildlife Service