Birds:

Birds are warm-blooded bipedal vertebrates that lay hard-shelled eggs. They are characterized by bony beaks, hollow bones, feathers and forelimbs modified as wings. In short, birds can be called Feathered Bipeds. Since birds are warm-blooded, their body is covered with insulating feathers to maintain an even temperature. Size of the birds can vary from the tiny flowerpeckers & hummingbirds, to the huge Ostrich and the Sarus Crane. In India, other than the Sarus Crane, the Himalayan Bearded Vulture or Lammergeier is the biggest bird and the tiny Tickell's Flowerpecker is hardly bigger than a human thumb. Most birds are capable of flight, but some larger birds like Penguins, Ostrich and Kiwi are flightless.

Depending on different scientific classifications, as of today there are over 9000 bird species (1250 in India), with almost 150 having become extinct after the arrival of Humans. The Sibley-Ahlquist taxonomy, based on extensive DNA-DNA hybridization, has resulted in the Sibley-Monroe checklist which has revolutionized the whole system of bird classification.

Birds have a keen eyesight and good hearing but their sense of taste and smell is poor. Birds are generally diurnal (active during the day), some are nocturnal (active during the night), some crepuscular (active during twilight hours) and some nocturnal & crepuscular both. Many birds migrate long distances with change in seasons to maximize feeding hours and/or to breed in suitable habitats.

Arctic Terns migrate twice every year from Arctic to Antarctica and back (over 15,000 km one way!). Some birds, like the Wandering Albatross, spend most of their time at sea. Some, such as frigatebirds, stay aloft for days at a time, even sleeping on the wing. The large variety of bird food includes honey, nectar, seeds, grains, vegetable matter, insects, larva, spiders, fish, molluscs, rodents, reptiles, small mammals, carrion or other birds.