• Birds are winged, warm-blooded, vertebrate animals that lay eggs. There are around 10,000 living species, making them the most

numerous tetra pod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from

5 cm (2 in) to 2.7 m (8 ft 10 in). The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around

150–200 million years ago, and the earliest known bird is the Late Jurassic Archaeopteryx.

Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-

chambered heart, and a lightweight but strong skeleton. All birds have forelimbs modified as wings and most can fly, with some

exceptions including ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and

respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent

animal species.

• Birds' diets are varied and often include nectar, fruit, plants, seeds, carrion, and various small animals, including other birds. Because birds have no teeth, their digestive system is adapted to process unmasticated food items that are swallowed whole.

• Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands. Some desert birds can obtain their water needs entirely from moisture in their food. They may also have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting. Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils.

• Most birds scoop water in their beaks and raise their head to let water run down the throat. Some species, especially of arid zones, belonging to the pigeon, finch, mousebird, button-quail and bustard families are capable of sucking up water without the need to tilt back their heads.

• Birds communicate using primarily

visual and auditory signals.• Birds sometimes use plumage to

assess and assert social dominance, to display breeding condition in sexually selected species, or to make threatening displays, as in the Sun bittern's mimicry of a large predator to ward off hawks and protect young chicks. Variation in plumage also allows for the identification of birds, particularly between species. Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression.

• A bird nest is the spot in which a bird lays and incubates its eggs and raises its young. While the term popularly refers to a specific structure made by the bird itself—such as the grassy cup nest, that is too restrictive a definition. For some species, a nest is simply a shallow depression made in sand; for others, it is the knot-hole left by a broken branch, a burrow dug into the ground, a chamber drilled into a tree, an enormous rotting pile of vegetation and earth, or a mud dome with an entrance tunnel. The smallest bird nests are those of some tiny cups which can be a mere 2 cm (less than one inch) across and 2–3 cm (about one inch) high. At the other extreme, some nest measure more than 11 m (34 ft) in diameter and stand nearly 5 m (15 ft) tall.

• In most species, the female does all or most of the nest construction, though the male often helps. however, the male may do most or all of the nest building.

• Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man. Birds have benefited from human activities. Human activities can also be detrimental, and have threatened numerous bird species with extinction.

• Birds can act as vectors for spreading diseases such as psittacosis, salmonellosis, campylobacteriosis, mycobacteriosis, avian influenza (bird flu), giardiasis, and cryptosporidiosis over long distances.