Birds make a natural group of animals, all having feathers and scaly legs, all being warm-blooded and egg-laying. On the ground, they move on two legs.
Aside from their physical similarities, birds have many similar behaviours: most of them make nests for their eggs, care for their young, and tend to flock together.
Birds of a species all nest in the same way, because nest-building behaviour is driven by instinct, although the materials used for the nest may vary.
Birds select a mate with care, and most birds provide detailed support for their young, except for cuckoos, hatching the eggs and feeding the young birds.
Cuckoo behaviour is a trade-off that works for the cuckoos, which can spend more of their energy in producing extra eggs, rather than feeding their young.
If enough birds began to behave like cuckoos, the strategy would no longer pay off, and the birds would either need to change their behaviour or go extinct.
As a general rule, species which are under threat of extinction have no way of perceiving the threat, so it is improbable that they will change their behaviour.
Birds are adapted in many ways for how they live: they are able to live in all of the environments where humans can live, and a few where humans cannot.
Wild birds can be identified in a variety of ways: from traces and tracks, by their calls, their plumage and beaks, their size, location, and the way they fly.
All birds show behaviour, and birds of the same species show similar behaviour, but most birds are also able to learn new behavioural patterns.
Flightless birds have evolved from flying birds, which was a change that allowed them to grow larger or to function better in the absence of predators.
Flight feathers can be recognized even in a fossil, because the feathers are asymmetrical, with the narrower edge to the front (leading edge) of the wing.
Fossils of Archaeopteryx have feathers on their wings which are narrower on the leading edge (front side), showing that Archaeopteryx could fly.