History of Birds
Birds are warm-blooded, oviparous vertebrate animals characterized primarily by feathers, forelimbs modified as wings, and (in most) hollow bones.
Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu. Depending on the taxonomic viewpoint, there are about 8,800-10,200 living bird species (and about 120-130 that have become extinct in the span of human history) in the world, making them the most diverse class of terrestrial vertebrates.
Birds feed on nectar, plants, seeds, insects, fish, mammals, and carrion.
Most birds are diurnal, or active during the day, but some birds, such as the owls and nightjars, are nocturnal or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night, or other birds. Many birds migrate long distances to utilise optimum habitats (e.g., Arctic Tern) while others spend almost all their time at sea (e.g. the Wandering Albatross). Some, such as Common Swifts, stay aloft for days at a time, even sleeping on the wing.
Common characteristics of birds include a bony beak with no teeth, the laying of hard-shelled eggs, high metabolic rate, a 4-chambered heart, and a light but strong skeleton.
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Most birds are characterised by flight, though the ratites are flightless, and several other species, particularly on islands, have also lost this ability. Flightless birds include the penguins, ostrich, kiwi, and the extinct Dodo. Flightless species are vulnerable to extinction when humans or the mammals they introduce arrive in their habitat. The Great Auk, flightless rails, and the moa of New Zealand, for example, all became extinct due to human influence.
Birds are among the most extensively studied of all animal groups. Hundreds of academic journals and thousands of scientists are devoted to bird research, while amateur enthusiasts (called birdwatchers, twitchers or, more commonly, birders) probably number in the millions.
Birds have a body plan that shows so many unusual adaptations (mostly aiding flight) that birds have earned their own unique class in the vertebrate phylum.
Unlike mammals, birds don't urinate. Their kidneys extract nitrogenous wastes from the bloodstream, but instead of excreting it as urea dissolved in urine as we do, they excrete it in the form of uric acid. Uric acid has a very low solubility in water, so it emerges as a white paste. This material, as well as the output of the intestines, emerges from the bird's cloaca. The cloaca is a multi-purpose hole for birds: their wastes come out of it, they have sex by putting their cloacas together, and females lay eggs out of it.
Birds have one of the most complex lung system of all organisms. Air enters the bird and immediately 75% of the
air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. When the bird exhales the air from the posterior air sac is forced into the lungs thus birds receive a
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