Class Aves

The bird group, also known as "Aves," is unique in that it is the only Chordata phylum in which powered flight is found, although flight is not a prerequisite to be a bird. Some of birds' distinguishing features are their feathers, hollow bones to make them lighter, and a one-way breathing system. Birds also lay amniotic eggs, just like reptiles. Almost all birds share similar flight adaptations, such as a prominent keel where flight muscles attach.

Birds have backbones and feathers. Like mammals, birds are warm-blooded, or endothermic. They can live in warm and cold climates. Most birds fly, but there are some species, like the penguins, that swim. Others, like the ostrich, are ground-dwelling.

Birds have bills and no teeth, and they lay eggs. They have four parts to their bodies: a head, a neck, a torso, and a tail. They have light, hollow bones that make flight easier, and they have two legs and two wings.

There are about 9,600 species. Scientists believe that birds are descended from dinosaurs. There are two subclasses of birds: Archaeornithes and Neornithes. Archaeornithes are, or were, primitive birds from the Jurassic or early Cretaceous periods. The Neornithes subclass includes all of the birds alive today.