Birds are relative newcomers to the animal kingdom. The earliest fossil recognizable as a bird, Archaeopteryx, lived about 150 million years ago during the Jurassic Period in a world dominated by reptiles. After Archaeopteryx, of which a few specimens and some feathers have been found, the next fossil birds—Hes-perornis and Ichthyornis—come from the Cretaceous Period, between 138 million and 63 million years ago. Hesperornis was a water bird that lost the power of flight. Ichthyornis was a flying sea bird.
The first representatives of the modern families of birds appeared as early as the Eocene Period, between 55 million and 38 million years ago, and species alive today emerged during the Pliocene, between 5 million and 2 million years ago. The key to the success of modern birds lay in their development of feathers, flight, and warm-bloodedness. Birds now live in every habitat on earth, from polar regions to tropical deserts. Some have also adapted to life on water, thus conquering three environments air, land, and water-something no other vertebrates have been able to do. Several groups of birds have lost the power of flight and have developed large size and strong legs to escape enemies.
The lightest skeleton
The bird skeleton has the same basic parts as that of other vertebrates, but it is extensively adapted for flight. The whole skeleton has become extremely light. The teeth have been replaced by a lightweight, horny bill. Many of the bones, like those of the skull, are very thin, and others, such as the limb bones, have a honeycomb structure—hollow, with thin internal struts that provide strength and rigidity without adding a lot of weight. The vertebrae in the backbone near the pelvic girdle are fused together to form the synsacrum, which provides firm support for the legs and cushions the bird against the shock of landing. To make up for this rigidity, the neck is extremely flexible, with up to 28 vertebrae—many more than in the neck of mammals, which have only 7 vertebrae.
Although the basic structure of a bird’s limb bones is like that of most other vertebrates, there are some significant differences. For example, the femur (thighbone) of a bird is normally hidden because it is held up close to the body beneath the feathers. And what looks like the thigh of a bird is really a structure made up of fused shinbones, called the tibiotarsus. The equivalent to human shins are the elongated and fused bones of the ankle and feet, which form the tarsometatarsus. A bird foot typically has four clawed toes, upon which it walks. The arrangement of tendons and muscles in a perching bird ensures that as it bends its legs to perch, the toes are automatically pulled inward and the foot grasps the perch tightly. This means that a sleeping bird can remain attached to a perch without effort.
The collarbones (clavicles) are fused into a V-shape, forming the furcula, or wishbone. Together with the coracoids (shoulder blades) they form the pectoral girdle. This structure prevents the ribs from being crushed by the powerful wing muscles, which may make up as much as 30 per cent of a bird’s weight. The pectoral girdle is attached firmly to the large breastbone, or sternum, which is shaped like the keel of a ship and which provides a strong anchorage for the wing muscles.
The forelimbs have been modified to form the wings, which are joined to the pectoral girdle. The humerus (bone of the upper arm) is short and stout and has a large surface area for the attachment of the flight muscles from the sternum. The ulna of the forearm is flattened to accommodate the secondary flight feathers. There are two wrist bones, three hand bones—two of which are fused together—and three finger bones. Attached to the first of the finger bones are the primary wing feathers. A flat membrane of skin is on each side of the wing bones. Together with long flight feathers, the broad wing surface helps create lift and power in flight.
Unique breathing system
A bird’s lungs are connected by tubes to numerous thin-walled air sacs. These air sacs can take up one-tenth of the volume of the body, spreading into the spaces between the muscles, body organs, and even the hollow bones. The single-direction airflow system allows the bird to extract oxygen from the air even at such high flying altitudes as 25,000 feet (7,620 meters), where oxygen is in short supply.
Unlike almost all mammals, birds have no teeth. So instead of chewing food, birds break it up with their bill or swallow it whole. Their digestive system features a thin-walled, baglike swelling called a crop at the base of the gullet, where food is stored and moistened before it passes into the two-part stomach. In the first part of the stomach, food is broken down by enzymes. In the second part of the stomach, which is called a gizzard, thick muscular walls grind up the food, sometimes with the aid of swallowed gravel or other course material. Food then passes into the intestines, where the nutritious matter and most of the water from the food is absorbed.
Wastes are further processed and then passed into a chamber called the cloaca. Both the digestive system and the reproductive system connect to the cloaca. Wastes pass out of the bird through a single opening called the vent. Birds conserve water by eliminating waste in the form of solid uric acid.