In response to a range of ecological conditions, descendents of maniraptorian theropods have diversified in form and function. Early in the evolutionary process, birds split into two diverse groups. The first, and much smaller, group comprises ratites, waterfowl, guans and pheasants. The second group include all the other avian orders. Adaptive radiation, which is the process of evolving different species, resulted in great variations in bird features. Diet determined the nature of most bills; diet and habitat influenced the evolution of legs and claws; foraging and flying needs dictated wing forms.
All these factors contributed to body shapes. Fossil discoveries suggest that both birds and mammals underwent a period of explosive adaptive radiation in the early Tertiary Period (65–2 mya) producing all the modern lineages within about 10 million years. Fossils show that before the Jurassic there were no birds. Birds began evolving from bipedal reptiles more than 150 million years ago. Many species appeared and went extinct over the next 100 million years. From the fossil records, it becomes clear that birds evolved rapidly from those early reptilian beginnings.
Recognizable birds had evolved by the end of the Jurassic period, 145 million years ago, but it was not until the Paleocene era (65-55mya) that modern birds emerged. The Cretaceous period (130-65 mya) saw the peak of development and extinction of the dinosaurs and rise of flowering plants as well as the rise and extinction of many toothed birds. Because of the conditions needed for fossilization scientists have only come to know of aquatic species such as the flightless, diver-like Hesperornis or the tern-like Ichthyornis, but there must have been many terrestrial forms also.
Birds of this epoch show much reduction of the tail. Hesperornis must have lumbered along like a seal on land, but its streamlined shape and webbed feet made it fast and maneuverable underwater like today’s grebes. It had tiny wings and legs placed far back along its body, near its tail. Ichtyornis was similar to some of the seabirds alive today, although it still had one primitive feature — a beak lined with sharp teeth.
Internally it had two other features that are found in all modern flying birds: many of its bones contained large air spaces, which helped reduce its overall weight, and it also had a keel that protruded from its breastbone. Such a well-developed sternal keel is indicative of strong flight. Unlike archaeopteryx, its wings did not have claws. However, the evolution of toothless, beaked birds must also have occurred rapidly during the Cretaceous because by the start of Eocene Period, 55 million years ago, many modern groups such as divers, grebes, cormorants, pelicans, flamingos, ibises, rails and sandpipers had already appeared.
Again, aquatic forms yield evidence more readily in fossil record than their terrestrial counterparts. When dinosaurs became extinct, the birds occupied some of the niches of the bipedal dinosaurs. The study of bird evolution after the Cretaceous Period has centered largely on the use of biochemical techniques to compare skeletal and other morphological traits. Modern birds diverged from one another about 60 mya.
Loons, auks, gulls, ducks, cranes and petrels occupied aquatic habitats between 55 and 35 million years ago, and about 25 million years ago, insect and fruit-eating species evolved. By the end of the Eocene, 36 million years ago, at least 20 modern orders had appeared. By 10 and 5 million years ago, birds occupied most of their present-day habitat. As continents shifted, closely related birds, separated from each other, evolved differently. Climate changes also caused many species to seek new habitats.