BIRDS, a class (AvES) of animals so conspicuously different from all other classes that they can be separated off by the two words "feathered bipeds." A little more content is given to the definition if two other terms be added—"warm-blooded" and "oviparous." But an attempt must be made to reach a more comprehensive diagnosis, and emphasis may be laid, first of all, on the intensity of the bird's life. The temperature of the body is higher than that of mammals, and the warm-bloodedness, or ability to sustain a uniform temperature, is almost perfect except in young nestlings. The high temperature is connected with the large proportion of muscle, the muscles of flight sometimes weighing about half the whole animal; and the conservation of heat is secured by the non-conducting plumage. The great ad vantage of a constant high temperature is that the essential chemical processes of the body (metabolism) go on more uni formly and rapidly. Intensity of life is also expressed in the rapid beating of the bird's heart, the quick breathing movements, the richness of the blood in the oxygen-carrying red corpuscles, and perhaps also in the perfection of the digestive processes, as in ferred from the relatively small amount of faecal matter. As contrasted with the reptiles, from which they evolved, the ratio of katabolism to anabolism averages much higher.
With a spare muscular body, quick breathing, a strong heart, perfect digestion, rich blood, and a covering of feathers, it is natural to correlate great activity—in running, leaping, climbing, parachuting and eventually flying The bipedal habit is perhaps ancestral, being indicated in extinct reptiles such as the Pseudo suchia and the Oroithischia, and this emancipation of the fore-limb from being a support opened up the possibility of a new function. The patagium, extending from the shoulder-joint along the pre axial margin of the arm, hints at a webbed fore-limb, useful as a parachute in leaping, before a feathered wing had fully evolved. Antecedent to and also accessory to flight was the improvement in the circulatory system by the formation of the dorsal aorta out of a single systemic arch. Thus the body is supplied by purely arterial blood and the heart is four-chambered. This implied an invigoration of life. It may be recalled that there is a f our chambered heart in Crocodilia, which have however a dorsal aorta made of two aortic arches and thus a supply of mixed blood to the greater part of the body. It was doubtless gradually that thermotaxis evolved ; it implied the specialization of a heat-regu lating centre in the brain, apparently in the corpus striatum, which reacts to slight changes in the temperature of the blood, and ad justs in relation to these the body's production and loss of heat. As contrasted with the wings of insects, Pterodactyls and bats, those of birds are feather-wings and the acquisition of feathers was one of the main steps in avian evolution. But feathers may be correlated with the highly developed vascular system and the abundance of blood-vessels in the bird's dermis. While feathers are in a general way homologous with reptilian scales, they arc quite unique integumentary structures, and no intermediate structures are known (see FEATHER).
Adaptations Effecting Improvement in Flight.—When flight had begun, adaptations effecting its improvement would follow. Thus the skeleton would become more lightly built, the sternum would acquire a keel f or the insertion of the muscles of flight, the neck would become long and supple, and the beak would take on the work of a hand. The flying movements would assist in the expiration of air from the lungs, so different from the active inspiration in mammals; and the need for keeping the total size of the bird relatively small may be correlated with the large internal surface of the lungs. It is useful to think of accessory adaptations being added to the essential features, which may be associated primarily with the high intensity of metabolism. One of the marked differences between birds and reptiles is the great increase in the relative size of the brain. This may be associated with the very perfect locomotion, which is in part co ordinated by the cerebellum, and with the fine manipulations in volved in nest-building, which have their centres in the cerebral hemispheres. Most birds are small and delicate, with little in the way of armour or weapons, and they are therefore more depend ent than reptiles on brains and wits. The high development of the senses of sight and hearing, essential to life-saving alertness, is also to be correlated with increased size of brain.
The flying habit made it possible f or birds to deposit their eggs in safe places; economized reproductivity became practicable ; and with this there must have been correlated variations in the direction of increased -parental care. The comparative safety secured for the eggs and young allowed a long period of develop ment before hatching, a possible prolongation of sheltered in fancy in the nest, and the further possibility of lengthening out the nurture period in which the capacities of the young creatures are educated and their new departures tested. All this would favour the improvement of the brain and its associated mentality.
It is plainly advantageous that flying creatures should have a sharply punctuated reproductive period, and that the gonads should not be large throughout the year. This punctuation of the reproductive period probably means an intensification of the emotions in courtship and in family life, and may be correlated with the high tide of feeling expressed in song. This points the way to increased utilization and possible socialization of the voice, so familiar in rooks. In this respect birds stand out from reptiles as men from apes, with parallel brain-increase in the two cases.
The object of this introduction is to suggest that many of the salient features of birds are congruent or correlated, and also to indicate the deep constitutional characteristics which may be reasonably regarded as the pre-conditions of flight. These deep characters are intensity of metabolism, spare muscular habit of body, rapid bipedal progression, the supply of the body with arterial blood exclusively, and that of fine quality, the perfect warm-bloodedness except in early youth, the assisting of respira tion by locomotion, and a well-developed brain. Given these features, it is easier to understand how the evolution of feathers would make flight possible. Then would follow a score of acces sory adaptations to this mode of locomotion.