Changes

== Physical characteristics ==

== Physical characteristics ==

All birds have two legs and two wings (although in some flightless species such as the [[kiwi]], the wings are very small and serve little purpose). Birds also have a beak, although its shape, size and use varies enormously between species. Uniquely among vertabrates, birds have hollow [[bone]]s; this makes them very light for their size, facilitating flight. Living birds do not have teeth, although some extinct species had teeth. Birds' feathers can be any color, and the colors and pattern often vary between males and females of the same species, with males typically having the more garish and elaborate plumage; in some species such as [[peacock]]s and [[birds of paradise]] this is taken to striking extremes.

All birds have two legs and two wings (although in some flightless species such as the [[kiwi]], the wings are very small and serve little purpose). Birds also have a beak, although its shape, size and use varies enormously between species. Uniquely among vertebrates, birds have hollow [[bone]]s; this makes them very light for their size, facilitating flight. Living birds do not have teeth, although some extinct species had teeth. Birds' feathers can be any color, and the colors and pattern often vary between males and females of the same species, with males typically having the more garish and elaborate plumage; in some species such as [[peacock]]s and [[birds of paradise]] this is taken to striking extremes.

== Birds as dinosaurs ==

== Birds as dinosaurs ==