The term of life varies greatly in birds, and does not seem to bear the same proportion to the time of acquiring their growth, as has been remarked with regard to quadrupeds.
Most birds acquire their full dimensions in the course of a few months, and are capable of propagation the first summer after they are hatched. In proportion to the size of their bodies, birds possess more vitality, and live longer, than either man or quadrupeds: notwithstanding the difficulties which arise in ascertaining the age of birds, there are instances of great longevity in many of them. Geese and swans have been known to attain to the age of seventy and upwards; ravens are very long-lived birds—they are said sometimes to exceed a century; eagles are supposed to arrive at a great age; pigeons are known to live more than twenty years; and even linnets, and other small birds, have been kept in cages from fifteen to twenty years.
Every part of their frame is formed for lightness and buoyancy; their bodies are covered with a soft and delicate plumage, so disposed as to protect them from the intense cold of the atmosphere through which they pass; their wings are made of the lightest materials, and yet the force with which they strike the air is so great, as to impel their bodies forward with astonishing rapidity, whilst the tail serves the purpose of a rudder, to direct them to the different objects of their pursuit. The internal structure of birds is no less wisely adapted to the same purposes: all the bones are light and thin, and all the muscles, except those which are appropriated to the purpose of moving the wings, are extremely delicate and light; the lungs are placed close to the back-bone and ribs, the air, entering into them by a communication from the wind-pipe, passes through, and is conveyed into a number of membranous cells which lie upon the sides of the pericardium, and communicate with those of the sternum. In some birds these cells are continued down the wings, and extended even to the pinions, thigh-bones, and other parts of the body, which can be filled and distended with air at the pleasure of the animal.—Vide Air Cells.
It seems evident that this general diffusion of air through the bodies of birds is of infinite use to them, not only in their long and laborious flights, but likewise in preventing their respiration from being stopped or interrupted by the rapidity of their motion through a resisting medium. Were it possible for man to move with the swiftness of a swallow, the actual resistance of the air, as he is not provided with internal reservoirs similar to those of birds, would soon suffocate him.
The migration of birds would appear miraculous, did we not know their extraordinary power of flight. Speaking of this, Bewick says—“If we can suppose a bird to go at the rate of only half a mile in a minute, for the space of twenty-four hours, it will have gone over, in that time, an extent of more than seven hundred miles, which is sufficient to account for almost the longest migration; but, if aided by a favourable current of air, there is reason to suppose that the same journey may be performed in a much shorter space of time. To these observations we may add, that the sight of birds is peculiarly quick and piercing; and from the advantage they possess in being raised to considerable heights in the air, they are enabled, with a sagacity peculiar to instinctive knowledge, to discover the route they are to take, from the appearance of the atmosphere, the clouds, the direction of the winds, and other causes; so that, without having recourse to improbable modes, it is easy to conceive, from the velocity of their speed alone, that most birds may transport themselves to countries lying at great distances, and across vast tracts of ocean.”
In speaking of the flight of birds, Mr. Rennie says—“Their capability of performing flights much longer than there is any necessity for supposing, may be proved by numerous facts. Even a sparrow has been calculated to fly at the rate of not less than thirty miles an hour, and many experiments prove that the eider duck can fly ninety miles in the same time. The common kite (falco milvus) has been observed to pass, without great exertion, over a space of a quarter of a league, in a minute; and it could fly, with ease, from Cape Pruth to the Land’s End, in a single day. M. Audubon, the distinguished ornithologist, has shot the passenger pigeon of America, and on dissection, found its stomach full of fresh rice, which, to have resisted the digestive process, must have been swallowed not many hours preceding its death, but could not have been obtained within eight hundred miles of the place where it was killed. Though the nightingale, the willow-wren, and other birds of passage, fly with only half the swiftness, they may easily arrive in most parts of the south of Europe, or the north of Africa, in a few days.”—Bewick—Rennie—Montagu.