| Double-crested Cormorant | |
|---|---|
| Adult in non-breeding plumage | |
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Pelecaniformes |
| Family: | Phalacrocoracidae |
| Genus: | Phalacrocorax |
| Species: | P. auritus |
| Binomial name | |
| Phalacrocorax auritus (Lesson, 1831) |
|
| Synonyms | |
|
Carbo auritus Lesson, 1831 |
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The Double-crested Cormorant (Phalacrocorax auritus) is a member of the cormorant family of seabirds. It occurs along inland waterways as well as in coastal areas, and is widely distributed across North America, from the Aleutian Islands in Alaska down to Florida and Mexico. Measuring 70–90 cm (28–35 in) in length, it is an all-black bird which gains a small double crest of black and white feathers in breeding season. It has a bare patch of orange-yellow facial skin. Five subspecies are recognized.
The Double-crested Cormorant is found near rivers, lakes and along the coastline. It mainly eats fish and hunts by swimming and diving. Its feathers, like those of all cormorants, are not waterproof and it must spend time drying them out after spending time in the water. Once threatened by use of DDT, the numbers of this bird have increased markedly in recent years.
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The Double-crested Cormorant was described by Rene Primevere Lesson in 1831. Its scientific name is derived from the Greek words φαλακρος (phalakros), 'bald' and κοραξ (korax), 'crow' or 'raven,' and the Latin auritus, 'eared,' referring to its nuptial crests. Its common name refers to the same nuptial crests. Other common names of this species include Crow-duck, Lawyer, Nigger Goose, Shag and Taunton Turkey (in New England).[2]
Five subspecies are recognized:[3]
The Double-crested Cormorant is a large waterbird with a stocky body, long neck, medium-sized tail, webbed feet and a medium sized hooked bill. It has a body length of between 70–90 cm (28–35 in) long, with a wingspan of between 114–132 cm (45–52 in).[5][6] Double-crested Cormorants weigh between 1.2–2.5 kg (2.6–5.5 pounds) Males and females do not display sexual dimorphism.[6]
This species has dark-colored plumage with bare super-loral 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.[5]
The plumage of juvenile Double-crested Cormorants is more dark grey 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.[5]
A very common and widespread species, it winters anywhere that is ice-free along both coasts, as far north as southern Alaska (on the west coast) and southern New England (on the east coast). It can be found as far south as Mexico and the Bahamas. It migrates from the coldest parts of its breeding range, such as eastern Canada, and has occurred in Europe as a very rare vagrant, for example in Great Britain, Ireland, and the Azores.
The Double-crested Cormorant swims low in the water, often with just its neck and head visible, and dives from the surface. It uses its feet for propulsion and is able to dive to a depth of 1.5–7.5 m (5–25 feet) for 30–70 seconds. After diving, it spends long periods standing with its wings outstretched to allow them to dry, since they are not fully waterproofed. This species flies low over the water, with its bill tilted slightly upward, sometimes leaving the colony in long, single-file lines.
Food can be found in the sea, freshwater lakes, and rivers. Like all cormorants, the double-crested dives to find its prey. It mainly eats fish, but will sometimes also eat amphibians and crustaceans. Fish are caught by diving under water. Smaller fish may be eaten while the bird is still beneath the surface but bigger prey is often brought to the surface before it is eaten. Cormorants regurgitate pellets containing undigested parts of their meals such as bones. These pellets can be dissected by biologists in order to discover what they ate.
Breeding occurs in coastal areas as well as near inland rivers and lakes. They build stick nests in trees, on cliff edges, or on the ground on suitable islands. They are gregarious birds usually found in colonies, often with other aquatic birds, and have a deep, guttural grunt call.
The Double-crested Cormorant's numbers decreased in the 1960s due to the effects of DDT. Colonies have also been persecuted from time to time in areas where they are thought to compete with human fishing.
Recently the population of Double-crested Cormorants has increased. Some studies have concluded that the recovery was allowed by the decrease of contaminants, particularly the discontinued use of DDT.[7] The population may have also increased because of aquaculture ponds in its southern wintering grounds. The ponds favor good over-winter survival and growth.
In 1894, Thomas McIlwraith in his book, Birds of Ontario, concludes his section on Double-crested Cormorants by saying: “When the young are sufficiently grown, they gather into immense flocks in unfrequented sections, and remain until the ice-lid has closed over their food supply, when they go away, not to return till the cover is lifted up in the spring.”
For populations nesting in the Great Lakes region, it is believed that the colonization of the lakes by the non-native alewife (a small prey fish) has provided optimal feeding conditions and hence good breeding success. Double-crested cormorants eat other species of fish besides alewives and have been implicated in the decline of some sport-fish populations in the Great Lakes and other areas. Scientists are not in agreement about the exact extent of the role of cormorants in these declines, but some believe that Double-crested Cormorants may be a factor for some populations and in some locations.
In light of this belief, and because of calls for action by the public, the United States Fish and Wildlife Service (the U.S. Federal government agency charged with their protection) has recently extended control options to some other government entities. This includes culling of populations and measures to thwart reproduction, in an effort to control their growing numbers.
The U.S. Fish and Wildlife Service retains oversight and the control measures are not extended to the general public (no hunting season). Many government agencies at different levels in both the U.S. and Canada continue to wrestle with how best to respond to the situation.
In May 2008, the Canadian government reduced significantly the number of nests of the birds on Middle Island, a small island in Lake Erie and part of Point Pelee National Park.[8] This is an attempt to keep the small island in balance and preserve its vegetation[9] but opponents to he plan have pointed out that it is based on faulty information, provided in part by anglers who view cormorants as competitors.