Double-crested Cormorant
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| Phalacrocorax auritus (Lesson, 1831) |
The Double-crested Cormorant (Phalacrocorax auritus) is a North American member of the cormorant family of seabirds. Its name is derived from the Greek words phalakros (bald) and kora (raven), and the Latin auritus (eared). Folk names of this bird include Crow-duck, Farallon Cormorant, Florida Cormorant, lawyer, shag, and Taunton turkey.
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[edit] Breeding
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.
[edit] Feeding
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 brough 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.
[edit] Appearance
The Double-crested Cormorant is a large black bird 74–91 cm (29"–36") long, with a wingspan up to 132 cm (52"). It has a long tail and a yellow throat-patch, and can appear to have a green sheen in certain lighting. The white double head crest is seen for a short period during the breeding season in western birds; it is duller in eastern birds. Juveniles are brown with a white face, foreneck, and breast.
[edit] Range and Migration
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.
[edit] Behavior
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.
[edit] Recovery
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 Double-crested Cormorant has greatly increased in abundance beyond what was known historically. It is believed that the recovery was allowed by the decrease of contaminants, particularly the discontinued use of DDT. It is believed that it has increased to such great numbers because of aquaculture ponds in its southern wintering grounds. The ponds favor good over-winter survival and growth.
Also, 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 important sport-fish populations in the Great Lakes and other areas. Scientists are not in universal agreement about the exact extent of the role of cormorants in these declines, but the growing consensus is that Double-crested Cormorants are a real factor for some populations and in some locations.
In light of this, 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 some lethal culling of populations and measures to thwart reproduction, all 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.
[edit] References
- BirdLife International (2004). Phalacrocorax auritus. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 12 May 2006. Database entry includes justification for why this species is of least concern
- Alsop, Fred J. III; Birds of Texas. Smithsonian Handbooks: DK Publishing, Inc., New York, NY (2002).
- Phalacrocorax auritus (TSN 174717). Integrated Taxonomic Information System. Accessed on 9 February 2006.
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