单选题
Eagles
Seen as symbols of strength, courage, determination and immortality since ancient times, eagles are found throughout the world with the exceptions of Antarctica and New Zealand. Eagles are powerful birds renowned for their keen vision. All eagles have powerful sharp claws called talons, as well as heavy hooked beaks or bills. This combination makes them feared predators as they swoop down from on high and attack small animals such as fish, rodents, birds, snakes, rabbits and even monkeys. The eagle uses its powerful talons and beak to tear apart prey that is too heavy to carry. Eagles also sometimes steal the catch of other predatory birds such as ospreys. Most of the 50 species of eagles range in length from 2 to 3 feet and have wingspans of approximately 6 feet.
Eagles live in aerial nests, which they build high in trees or on rocky ledges that are inaccessible to other animals. The nest must be kept secure because young eagles are helpless for a long period of time after hatching and remain in the nest for 50 to 100 days depending on the species. Made of sticks and lined with leaves and grass, the same nest may be used by a pair of birds for a number of years. The female eagle lays one to three brownish speckled eggs, and these are incubated by either parents or just the female. Young eagles are covered in down and do not develop adult coloration until they reach the age of two.
A particularly famous species of eagle is the Haliaeetus Leucocephalus or Bald Eagle, which is the national emblem of the United States. The Bald Eagle is not in fact bald, and the moniker was applied because of the contrast between its white head and brown body. The Bald Eagle is extremely rare and for many years it was feared that this magnificent creature would become extinct. However, due to conservation efforts this tragedy has been averted. The Bald Eagle and eagles in general have few natural enemies; it is human beings who are the greatest threat to their survival. The hunting and capture of eagles for sport, the use of agricultural pesticides and urban encroachment upon their natural habitats have all proved extremely dangerous to the survival of this majestic bird.
The exact causes for the slow but steady decline in the number of Bald Eagles around the world is still debatable. Early during the seventies, it has been speculated that the widespread use of DTT, a powerful pesticide, is the leading cause behind the demise of this majestic bird species. It seemed that DTT had rendered the Bald Eagle sterile or unable to hatch properly. Its eggshells were far too brittle to survive adverse weather conditions and cracked open before hatching. However, new studies prove that the thinning eggshells were rather the result of a calcium-deficient diet as eagles fed with the nutrient soon produced more robust eggs.
Currently, the number of Bald Eagles shows a steady improvement due mainly to the concerted efforts of conservationists worldwide. Whereas the Bald Eagle"s habitat of choice is the Pacific Northwest and Canada, the largest eagle colony in the world resides in Alaska. Bald Eagles are a favorite species to be observed by birdwatchers across the globe, and one of the choice areas for Bald Eagle watching is Squamish by the city of Vancouver. Large eagle colonies come to breed along the Cheakamus River by Squamish during the mild winter months from November to February, and it is not uncommon for some such birds to extend their stay over several seasons when the local salmon population provides enough feeding resources.
Glossary
Antarctica:
the continent surrounding the South Pole
moniker:
a person"s name, esp. a nick name or alias