Bird

I

Introduction

Bird, common name for any member of the class of vertebrates that contains animals with feathers. All adult birds have feathers, although some species, such as pelicans, kingfishers, woodpeckers, and jays, are completely naked when hatched.

Birds share certain features with mammals, such as warm-bloodedness and a four-chambered heart. Nevertheless, birds are distinct, having evolved from dinosaurs long after the mammalian and reptilian groups diverged. All birds—like most reptiles and a few primitive mammals—develop from embryos in eggs outside the mother's body. Unlike most reptile eggs, those of birds have hard shells, which are very strong in large birds and rather brittle in small birds.

II

Anatomy and Physiology

Most birds can fly, and all are ultimately descended from ancestors that could fly, although there are many extinct flightless species. The bodies of birds are therefore highly modified for efficiency in flight. The digital and wrist bones of the forelimb are extensively fused to form a rigid support for the large flight feathers of the wing. Fusing of bones for strength and lightness is also found in the skull and pelvic girdle. Many bones of adult birds are hollow rather than filled with marrow, and are connected to a system of air sacs dispersed through the body. The sternum, or breast bone, of most birds is relatively large and bears a central ridge, or keel, known as the carina. The sternum and carina support some of the major muscles used in flying. Flightless (ratite) birds—including the ostrich, the kiwi, and their relatives—do not have a carina and the sternum is reduced in size. The wing structure of most flying birds is the same but there are differences in flying styles. Larger flying birds, such as buzzards, vultures, albatrosses and eagles, spend much of their time in the air gliding or soaring. They use their broad wings with splayed feathers to glide on strong winds or “rest” on rising warm air. Most smaller birds have to flap their wings to sustain their flight.

The jaws of modern birds are extended as toothless bills or beaks, covered with a layer called the ramphotheca, which can be hard, as in most birds, or leathery, as in ducks. The lack of teeth lessens the weight of the skull.

Birds have no sweat glands and cannot cool the body by perspiring. When birds are in flight, excess heat is dispersed by the passage of air through the air sacs. When they are at rest, they rid themselves of heat by panting. The digestive and excretory systems of birds are similar to those of reptiles. The cloaca at the base of a bird's body is used for the excretion of waste products and as the genital opening. Birds produce almost solid urine, uric acid. In sea birds excess salt is excreted in a salt-rich fluid from the nostrils or mouth. It is processed not by the kidney but by a modified tear gland. Other significant internal organs important to birds include the gizzard, which breaks down food, and the crop, which is a chamber that stores food.

A winter survival technique well known in mammals but rare in birds is a slowing of the physiological processes, including reduction of body temperature, resulting in extreme cases in hibernation. Hibernation in birds was long thought to be a myth. Recent research has shown, however, that several species of nightjars, swifts, and hummingbirds that live in deserts or high mountain areas, where winter nights are very cold, can enter a hibernation-like condition of torpor to conserve energy.

III

Evolutionary History

The earliest known fossils identified as birds link their ancestry to reptiles, possibly small dinosaurs of the Triassic period (245 million to 208 million years ago). The earliest known fossil bird is Archaeopteryx, about the size of a small pigeon, of which six complete or partial specimens—and an isolated feather—have been found in the Solnhofen limestone beds in Germany; all date from the Late Jurassic period (157 million to 145 million years ago). The species is a mixture of bird-like and dinosaurian anatomical characteristics. If the original skeletons had not clearly shown imprints of feathers exactly like those of modern birds, the fossils might have been classified as small, somewhat peculiar dinosaurs. Archaeopteryx had teeth, which are lacking in all modern birds. Feathers undoubtedly evolved from reptilian scales, but lacking fossil evidence, the mode of transition from scale to feather remains a subject for conjecture. Some skeletal features of this earliest known bird are typical of modern birds, however, and are not known from reptiles.

Fossil bones discovered in Triassic rocks in Texas, United States, in the mid-1980s and named Protoavis texensis in 1991, have been described as a species of bird, 75 million years older than Archaeopteryx, but anatomically closer to modern birds than Archaeopteryx, which lived 150 million years ago. However, the fossil remains consist of isolated bones, not skeletons, and most experts feel that they do not provide sufficient evidence to show that Protoavis was a bird.

Several intermediates between the earliest known bird fossils and modern birds are known. In 1988, fossils in Spain were reported with bird-like tail and shoulder bones but a primitive pelvis and hind limbs. The fossils date from the Early Cretaceous period, about 130 to 120 million years ago. The discovery of another anatomically intermediate fossil was announced in 1990. It is a sparrow-size bird from Liaoning province, China, and is probably only 10 to 15 million years younger than the earliest known bird. Its wings and tail were more like those of modern birds, but its ribs, pelvis, and hind limbs were still primitive. The next good fossils date from about 88 million years ago. These were true birds, little different skeletally from modern birds, except that at least some still bore teeth and longer tails. Most of the species discovered were water birds, largely because conditions for preserving fossils are best in deposits of sand or silt under shallow water. They included a giant diver-like diving bird and one species that perhaps resembled the surviving tern in habits.

Some fossil birds from the end of the Cretaceous period, about 65 million years ago, resembled some living water birds closely enough to be classified with them, but the resemblances are only superficial. The most rapid and diverse evolution of birds took place during the next period, the Tertiary, by the end of which (1.6 million years ago) all groups of living birds had evolved, while some evolutionary lines had died out without leaving descendants.

The Quaternary period, which began about 1.8 million years ago, is divided into two epochs, the Pleistocene and the Holocene (including the present), with the transition usually estimated at about 11,500 years ago. Most living species of birds, or species very much like them, evolved during the Pleistocene. Some Pleistocene species died out completely, possibly because of the severe climatic fluctuations created by the advance and retreat of the great glaciers that have given the Pleistocene epoch the popular name Ice Age.

Extinction is a natural process of evolution, so some species were undoubtedly disappearing when modern human beings appeared. Of the nearly 10,000 species of birds known since historical records began, at least 75 have become extinct. Most of these, such as the dodo and the great auk, were killed off directly by human beings or by animals that people introduced around the world, or else they disappeared because people had altered the environment too severely for the birds to survive. Clearing of forests, draining of swamps and marshes, and other habitat destruction have been so extensive (especially in the tropics) since the mid-20th century, that it is impossible to estimate how many species of birds have been lost.

IV

Classification

The classification of birds is disputed even among experts. Decisions as to which species are related to one another are usually easy, but at higher levels relationships become more and more uncertain. The relationships of living orders of birds to one another and to orders of birds known from fossils are constantly being argued, especially when new fossils are found or new techniques are discovered for studying modern birds. Early classifications depended entirely on gross anatomy, but these are being re-evaluated with the help of new evidence from such fields as biochemistry, genetics, and comparative behaviour. Anatomical characteristics are being looked at again in an effort to determine which are more primitive and which more advanced. The table of bird orders accompanying this article is only one of several arrangements that have been proposed.