Bird
From Encyclopædia
[edit] CLASSIFICATION
Birds are formally classified as members of the class Aves, subphylum Vertebrata, phylum Chordata, kingdom Animalia. They are characterized as being generally small vertebrates, with feathers, scaly legs, and no teeth (except in a few early fossil forms). They have well-developed air-breathing lungs and a four-chambered heart and maintain a constant body temperature of about 41 deg C (106 deg F). Birds reproduce by laying comparatively large, hard-shelled eggs.The class Aves is generally divided into 32 orders, of which three are known only from fossil forms. By far the largest order is passeriformes (see PERCHING BIRDS), which comprises more than 5,000 species, a large majority of the birds of the world. These passerines are the most highly evolved members of the class and are characterized by a specialized "perching" foot structure. They are generally divided into six suborders, based largely on the structure of the syrinx (vocal apparatus).One of the most lasting controversies in bird taxonomy has been the classification of the passerines into families. Some systems divide the passerines into as many as 60 or more families.
[edit] EXTINCT AND ENDANGERED BIRDS
All forms of life follow the same pattern. Species develop, then flourish, and eventually become extinct. The rate of this natural process varies greatly and is strongly tied to geological, climatic, and biological changes in the environment. During the Pleistocene Period the dramatic changes in temperature and rainfall, and the advancing and melting back of ice sheets, resulted in an especially high rate of extinction, perhaps 25 percent for birds during the approximately 2.5 million years of this epoch.The life expectancy of an avian species is estimated to have been 1.5 million years at the beginning of the Pleistocene but only 0.04 million years at the end, approximately 10,000 years ago. Since 1680 (about when bird records were begun) that expectancy has dropped to 16,000 years, demonstrating the adverse effects humans have had on the natural environment. Approximately 85 species and 50 distinct subspecies have become extinct in the last 300 years, over half of which vanished during the 1800s and another 30 percent since 1900. More than 90 percent of these historic extinctions have been of island forms, which are particularly vulnerable to human interference.Extinction, of course, is still partly caused by natural circumstances such as changes in climate and volcanic eruptions, but the most important modern causes are those of humans. These include destruction of habitat (the single greatest); introduction of predaceous human symbionts such as cats, rats, dogs, pigs, and goats; introduction of diseases, for example, avian malaria in the Hawaiian islands; introduction of competitors, for example, foreign GAME BIRDS; environmental poisoning, for example, pesticides and other chemicals; and direct hunting. Although concern about extinctions has increased, and although worldwide efforts are being made to save as many species as possible, human self-interest and local apathy about endangered species will probably continue to cause many birds to become extinct.
[edit] STRUCTURE AND FUNCTION
Feathers: Essentially aerial, warm-blooded creatures, birds have been limited in body size and weight by the physical laws governing heat conservation and flight. Thus all flying species are small to medium-sized, compared to mammals.Feathers serve as a versatile covering for the avian body. They form a smooth surface that reduces friction with the air; furnish flexible but strong structures for powered flight (wings) and steering (tail); act as superb insulation to conserve body heat; and are relatively waterproof, to protect the body. Feathers grow from pits in the skin called follicles. Feather follicles have muscles and nerves that serve to Control the feather's position, and it is through the follicles that the feathers receive the blood necessary to growth. The follicles of most feathers are not randomly positioned in the skin but rather are in organized groups (tracts or pterylae) with bare spaces (apteria) between. Fully grown feathers are lifeless structures composed of keratin (a protein also found in scales and hair). All feathers are replaced periodically, usually once or twice a year, by molting, or the loss of old feathers and growth of new ones.Typical feathers are called contour feathers; they consist of a base (calamus), a Central tapering shaft (rachis), and a paired series of thin barbs on either side of the shaft that form the vane. Barbs have a system of interlocking hooks that give flexibility, strength, and substance to the feather. If a vane becomes disarranged, it can be "zipped up" again by being drawn through the bird's bill.Most of the feathers visible on an adult bird are contour feathers, ranging in size from the large flight feathers on the wing (remiges) and tail (rectrices) to the tiny feathers covering the face.Other types of feathers include Downs (with little or no shaft) for insulation; filoplumes (hairlike feathers) that may be sensory in function; and bristles (with stiff shafts but little or no vane) with several functions, including serving as eyelashes or as filters over the nostrils.Feathers may be dull or brightly colored, and plain or with complex patterns. colors, sometimes bright, are also present in unfeathered parts such as bills, legs, wattles, and bare areas of the skin (usually on the head). Birds manufacture their own melanins (brown and black pigments) and porphyrins (some red and green pigments), but most of the carotenoids (red, orange, and yellow pigments) are derived from their food. The blues in birds do not come from pigments but from the internal structure of their feathers, which reflects the blue component of light while the other colors in the spectrum are absorbed by melanins. Greens are usually the result of yellow pigment and structural blue. The many iridescent colors of birds are also largely structural.