The earliest bird lived about 150 million years ago, during the Jurassic Period. It inhabited the world dominated by cold-blooded reptiles. The first representative of the modern family of birds did not appear until the Eocene Period, between 60 and 40 million years ago, and species alive today emerged during the Pliocene, between 13 and 2 million years ago.
The key to the success of birds lay in their development of feathers and flight, and of warm-bloodedness. They have now conquered every habitat, from polar ice to tropical deserts. Some have also adapted to life on the water, conquering 3 environments, something no other vertebrates have been able to do. Several groups of birds have lost the power of flight and developed large size and strong legs to escape enemies.
The bird skeleton is built on the same basic plan as that of other vertebrates, but it is extensively modified for flight. The whole skeleton has become extremely light. The teeth have been replaed by lightweight horny bill. Many of the bones, like those of the skull, are very thin, whereas others, such as limb bones, have a honeycomb structure, being hollow with the internal struts fro strength with rigidiy and lightness.Masked Boobies
Although the basic structure of the limb bones is on the general vertebrate plan, they show significant modifications. The femur (thighbone) is normally hidden because it is held up close to the body beneath the feathers. What looks like the thigh of a bird is really the tibiotarsus, formed from the shinbones. The equivalent of our shins is provided by the elongated and fused bones of the ankles and feet, forming the tarso-metatarsus. Birds walk on their toes. There are usually 4 of these, each equipped with a claw. The forelimbs have been modified to form the wings.
A bird's lungs are connected by tubes to numerous thin-walled air sacs. These can form 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 high altitudes where oxygen is in short supply - some birds fly at 2,000 feet.
The digestive system features a thin-walled highly extensible crop at the base of the gullet, where food is stored and moistened. The food then starts to be broken down by enzymes in the first part of the stomach. The second part of the stomack is modified to form a thick-walled muscular gizzard, which grinds up the food, someitmes with the aid of swallowed grit (a bird has no teeth) before it is passed on to the rest of the digestive system. Both the digestive and reproductive systems open into one chamber, called the cloaca, with a single opening (the vent). Birds rid themselves of waste in the form of solid uric acid, liquid urine would involve losing too much water.