All birds, of which there are perhaps 15,000 species, have feathers. This one conspicuous characteristic suffices to identify a bird, even to a child, for no other animals have feathers. The vertebrate type probably reaches its highest differentiation, in certain directions at least, in birds, and for this reason it is not at all difficult to find many other distinguishing characteristics, aside from feathers, in this familiar and much studied class of animals.
The secret of the anatomical peculiarities of birds lies in their adaptation to flight. Speed of animal locomotion culminates in birds. The same combination of organs which converts a fish into a living submarine and adapts a reptile to a life of continuous contact with the earth, transforms a bird into a flying machine heavier than air.
The skeletal framework of a bird, comparable bone by bone with that of other vertebrates, is compacted together, thus affording the smallest possible bulk to pass through the air, although the surfaces of individual bones remain relatively expansive for the attachment of greatly developed muscles of flight. The surfaces of the breastbone, the humeral heads, and the sacropelvic complex particularly, are increased beyond those of other vertebrates. Every possible part of a bird is transferred from the anatomical suburbs into the compact urban district of the body. The heaviest parts hang beneath the line of support joining the wing-sockets where the power is applied in flight. Considerable weight is shifted from the periphery to the center by means of the replacement of heavy dense teeth, commonly found in the head of other vertebrates, by the light horny beak, while a tough muscular centralized gizzard, containing powerful grinding stones, does the work which teeth once did in ancestral birds.
The cumbersome trailing reptilian tail is telescoped into a degenerate skeletal stub, thus centralizing weight. In place of it a secondary tail of light air-resisting feathers is added as a rudder in flight. The presence in birds of a bony tail, composed of several foreshortened vertebrae instead of a single bone which might better have served as a support for the tail feathers, is one of the numerous evidences of reptilian ancestry. Indeed someone has happily described birds as “glorified reptiles.”
The bones of a bird are not only compact but are also lightened and adapted as parts of a flying machine, by being hollowed out to the limit of mechanical safety. Furthermore, bodily weight is particularly counterbalanced by the development of numerous air sacs that grow out from the lungs, occupying all available spaces between the internal organs and extending even to the cavities of the hollow bones. Feathers, which clothe a bird, hold a blanket of enveloping air next to the body, that, since it is warmed by the body and is consequently lighter than the surrounding air, adds somewhat to the bird’s buoyancy.
In addition, the large intestine, particularly the rectum where the feces are carried, is very much reduced in length. Since flying animals can ill afford to be weighted down with any excess fecal baggage, birds, having no suitable provision for its storage, promptly get rid of it.
The entire support of a bird’s body devolves upon the hind legs alone, leaving the fore legs free to serve as wings. The wing is composed largely of feathers attached to an arm terminating in three reduced and partially fused fingers. As a consequence the forelegs, or arms, which are modified and entirely given over to flight, cannot be used for the capture and manipulation of food. The head, therefore, is necessarily not only periscopic but prehensile, being mounted upon an extremely flexible neck and equipped with a forceps-like beak for picking up food. As a result the remarkably developed eyes, located on either side of a bird’s beak, are much nearer their objective than the eyes of any other vertebrate which reaches for its food.
The excessive activity involved in flight is provided for in birds by a relatively larger heart than other animals possess, as well as by a particularly effective respiratory apparatus, which so increases the warmth of the body as to render it constant, regardless of the surrounding temperature. Birds are active, therefore, the year around, in cold as in warm weather, never becoming sluggish or obliged to hibernate as “cold-blooded” animals do. Enabled ordinarily to rise above the handicaps of temperature and climate, when occasion demands they resort by migration to distant and more congenial localities.
There are two subclasses of the Aves: (a) archaeornithes (archae, old, primitive; ornithos, bird); (b) neornithes (neo, new).
Subclass Archaeornithes (Primitive Birds)
On account of their light bones and the rapid disintegration of their bodies after death, birds are not subject to fossilization except under the most favorable conditions, and do not, therefore, present so extensively recorded a story of past achievements as reptiles.
The earliest known trace of bird life is the imprint of a single tail feather, discovered in the Jurassic slate quarries of Bavaria. This unmistakable fragment dates back to the middle of the long Age of Reptiles, eons before mammals had arisen to become a power upon the earth. In splitting up the fine-grained lithographic stone of the Solenhofen deposit in Bavaria from which this priceless feather came, there were found also at different times later two entire skeletons, crushed flat and embedded in the slate, of the same land of birds that doubtless produced this famous tail feather. From these slight but convincing remains, the species of this oldest of all known birds, was named Archaeopteryx lithographica (Figs. 47 and 48). It was about as large as a crow, had lizard-like teeth set in sockets in the elongate jaws, a long uncentralized bony tail bearing two oblique rows of feathers, a flat sternum, three fingers with claws terminating each wing instead of one clawless finger as modern birds have, and feathers. They resembled the small dinosaurs so closely that only the presence of feathers has prevented them from being placed among the reptiles.
Subclass Neornithes (Modern Birds)
The neornithes include all present-day birds in addition to a number of extinct species which resemble modern birds in most respects. The chalk beds of western Kansas, which were laid down at a much later date than the Bavarian slates of Jurassic times, have yielded the fossil remains of extinct birds with teeth, for example, Ichthyornis, in form resembling a tern, and Hesperornis (Fig. 49), a flightless loon-like water bird, of which over a hundred specimens have been found. Excluding the toothed ancestral birds of Kansas, modern birds may be divided, according to flying ability, into two unequal subclasses, Ratitae and Carinatae.
The ratites, none of which are indigenous to North America, are flightless running birds that have powerful legs and small wings. They include ostriches, cassowaries, emus, rheas, and the curious wingless Apteryx, or “kiwi” of New Zealand (Fig. 50), that, in the absence of ability to escape by flight, has survived the perils of a hostile world by burrowing in the ground.
In New Zealand also, a land of special interest to the biologist, have been found abundant fossil remains of Dinornis, the largest of all known birds, commonly called the “moa” (Fig. 51), which reached a height of at least 18 feet. It is likely that this species of gigantic ostrich-like bird has become extinct within the memory of man for when the whites first came into contact with the native Maoris of New Zealand they had legends about these birds that had been handed down to them from their fathers.
The carinates are flying birds whose wide breastbone has developed an expansive keel, or carina, for the attachment of muscles of flight. Ribs made up entirely of bone hold the breastbone firmly in place.
According to Gadow of the British Museum the carinates comprise thirteen orders as named below. A few more or less familiar examples are given to represent each order.
1. Colymbiformes: loons, grebes.
2. Sphenisciformes: penguins.
3. Procellariiformes: petrels, albatrosses.
4. Ciconiiformes: pelicans, herons, cormorants, flamingos, storks.
5. Anseriformes: geese, ducks, swans.
6. Falconiformes: vultures, hawks, eagles, condors.
7. Tinamiformes: tinamous.
8. Galliformes: turkeys, fowls, quail.
9. Gruiformes: rails, and other marsh birds.
10. Charadriiformes: plover, sandpipers, and other shore birds, gulls, terns, auks, pigeons, doves.
11. Cuculiformes: cuckoos, parrots.
12. Coraciiformes: kingfishers, owls, whip-poor-wills, swifts, humming birds, woodpeckers.
13. Passeriformes: flycatchers, sparrows, swallows, vireos, wrens, nuthatches, kinglets, thrushes.