Thursday, Oct 23, 2014

Double-crested Cormorants (Phalacrocorax auritus)

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Double-crested Cormorant (Phalacrocorax auritus)


Double-crested Cormorant 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. Once threatened by use of DDT, the numbers of this bird have increased markedly in recent years.

Alternate common names of this bird include crow-duck, Farallon cormorant, Florida cormorant, lawyer, shag, devil bird, nigger goose and Taunton turkey.


Taxonomy

The double-crested cormorant was described by Rene Primevere Lesson in 1831. Its scientific name is derived from the Greek words phalakros/φαλακρος "bald" and korax/κοραξ "raven", and the Latin auritus "eared", referring to its crests.

Four subspecies are recognised:

  • P. a. albociliatus
  • P. a. auritus
  • P. a. cincinatus
  • P. a. floridanus

Description

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.


Distribution and habitat

Double-crested CormorantsA 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.


Behaviour

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.


Diet

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.


Reproduction

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.


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 population of double-crested cormorants has increased. Some believe that the recovery was allowed by the decrease of contaminants, particularly the discontinued use of DDT. 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.” Linda Wires and Francesca J. Cuthbert state that there is solid indication that cormorants were once at least as common in North America as they are now, and most likely much more so, but were killed off physically, like so many other predatory and game species, in the 19th century, although beginning earlier than that, and continuing to the present.

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 lethal 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.


Copyright: Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia.org ... Additional information and photos added by Avianweb.

Double-crested Cormorants




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