Birds have unique colors and voices that appeal to our own senses, and many birds are also of economic importance. They are hunted as game, and provide food for our tables. There are around 8800 different species of birds, classified in 28 orders. To make the number of groups more manageable in this volume, we've grouped birds into 15 chapters, several of which contain more than one order. There are eight orders of waterbirds for instance, and here we discuss all of these in one chapter.
Birds have bodies covered with feathers.
They have two pairs of limbs - the anterior pair is usually modified as wings for flight, and the posterior pair is adapted as feet for swimming, walking or perching. Each foot typically has four toes, covered with scales of hardened skin. The bird skeleton is lightweight, and many bones are fused together, providing rigidity. The bird's mouth is a projecting beak or bill, and no living birds have teeth. Evolution of birds Birds are warm-blooded, backboned animals with feathers, and belong to the class Aves.