For many years, it was unquestioned that birds evolved from two-legged, lizard-like reptiles which lived over 150 million years ago. Some modern paleontologists say that they are descendents of the dinosaurs. Regardless of the hypotheses, birds still retain some reptilian characteristics, including egg laying, scales and arrangement of internal organs. Although bird remains are not well represented among fossils because of their delicate skeletal structure, several birds which lived during the Jurassic period (130 mya) have been recovered as fossils. The “first” bird, Archaeopteryx, combined lizard-like and modern bird characteristics: a long feathered tail, feathered wings with claws and a beak with teeth. There are nearly 10,000 living species of birds classified into 27 orders. Birds vary in size from 2 grams to over 300 pounds, from a tiny wingspan to one over 12 feet wide. They also have varying color patterns, courtship rituals and social habits.

Birds have many common characteristics, most notable of these being feathers. There is much discussion about the origin of feathers, but most experts agree they derived from reptilian scales that may have gradually developed fringed margins. They continued to evolve into the feathers we see on modern birds as bird-like reptiles developed warm-bloodedness and the need for covering to retain the heat.

Other avian characteristics include the development of wings usually, but not always, used for flight. A feathered tail for balancing, steering, and lift is present in all birds. They also have a toothless, horny beak. A bird's skeleton shows many adaptations for flight. These include pneumatic bones (bones filled with air spaces), a rigid skeleton (achieved by the elimination of of many bones found in other vertebrates and the fusion of others), and a skeletal sternum (for the attachment of flying muscles in all but flightless birds). The penguin, though flightless, has a keeled sternum to which its “flipper” muscles are attached.

A bird's large four-chambered heart permits the complete separation of arterial and venous blood, as in mammals. A rapid heart rate and high body temperature provide for the rapid metabolism which gives birds the high energy needed to sustain flight. Birds have compact lungs pressed closely against the rib cage. Almost all birds have a special voice box called the syrinx. Some birds lack the functional muscles to create any sounds other than a grunt, hiss or boom. However, some birds, such as song birds, possess tympaniform membranes whose tension is under delicate muscular control and which produce beautiful songs when air is passed between them as it is expelled from the lungs.

Birds have a rapid digestive system to fuel their high energy needs. Because they lack teeth, many birds will swallow pieces of sand or gravel to help grind up their food. Many birds have a crop for storing food and a muscular gizzard for grinding hard material. Sight and hearing are highly developed in birds, while taste and smell are comparatively underdeveloped. Birds must find their food by sight, therefore their sight is the most highly developed of any animal. Also, they see in color. A bird's retina is nearly twice as thick as that in the human eye, and densely packed with visual cells. Most birds have monocular vision which allows them to use each eye independently for lateral views.

Nesting, whether on a specially built nest, or a hole in a tree or the ground, is another characteristic of birds. All birds lay eggs, but their number, size, shape and color vary. One of the interesting feature of most, but not all, birds is migration. This regular seasonal movement to and from a particular breeding ground is believed to be triggered by a decreasing food supply and photoperiodism, the increase or decrease of daily light. However, the method by which birds find their way from on area to another is still under study. Some theories include magnetic attraction, landmark recognition, and using the sun as a navigational tool.