TION, PROTECTIVE; NATURAL SELECTION.
Powers of The bones of the wing and tail support very large, strong ((quill. feathers, which, when outspread, support the bird in the air, and when moved in the proper manner carry it forward — enable it to fly, the mechanism and phenomena of which method of locomotion are explained under FLIGHT. The wing-power of most birds is very great, but the speed of their flight is often exaggerated. Few exact facts are at hand, but it is apparent that the highest speed is nearer 50. than 100 miles an hour, although the latter speed is un doubtedly reached by ducks and other swift flyers under pressure of attack or escape. En durance on the wing is more remarkable. Many sea-birds seem tireless, and swallows, among land-birds, are almost incessantly in the air. During migrations a large variety of birds, in cluding some of the smallest and feeblest, undertake rapid and extensive journeys, reach ing in some cases almost half around the world; and some regularly pass over spaces of ocean as much as 2,000 miles in width, while a flight of 500 miles from land to land is accomplished by many species. This is the more notable as a feat because in many cases they are birds which during nine-tenths of the year only flit from bush to bush. In these migratory journeys (see MIGRATION) birds often fly very high; but this is the regular custom of certain ones, es pecially vultures, which soar beyond human sight, yet will swoop to the earth in a swift dash, betraying great adaptability to sudden changes in atmospheric density. Other notable qualities are the power (largely residing in the tail) to suddenly change speed and direction, helping them to dodge and elude winged pur suers, and to catch the agile aerial insects, upon which many of the smaller species depend for subsistence. The sharpness and quick ad justability of eyesight also involved in this is noteworthy.
These abilities in flight have led to the very wide distribution of birds, which occur in every part of the world yet seen by man ; and are the most numerously represented of all terrestrial branches of animal life in the oceanic islands. Nevertheless very few are cosmopolitan, and not many range beyond the confines of a single continent, while many are more narrowly re stricted, so far at least as their habitat in the breeding season is concerned. Thus the geo
graphical distribution of birds has been found perhaps the best criterion for the mapping out of zoogeographical regions (see ZOOGEOGRAPHY ). The greater number of families of birds is tropical, and both variety of lcinds and numbers of .individuals decrease toward the poles. A strilcing fact is the great difference between the birds of the northern and the southern hemi spheres—a difference much more decided than cxists between those of Europe and North America, or of South America and Australasia.
Birds in every case repro duce their kind by means of eggs protected by a calcareous, often highly-colored shell, laid by the mother a considerable time before they are ready to hatch, which consummation is brought about by the application of warmth. This may be arranged for in two ways. A few birds bury their eggs in rotting vegetation, or in hot sand, and let the chemical heat evolved by the ferment in the former case, or the sun's rays in the latter, accomplish the desired result. The great majority, however, place their eggs in some sort of a receptacle (sometimes a mere hollow on the ground, or hole or niche in a cliff or tree, sometimes in a burrow or nest of more or less elaborate construction (see BIRDS, NESTS OF), and there brood upon, or cincubatel) them until the chick matures and emerges. In one class of birds (Prcccoces) incubation is so long continued, and the embryonic chick becomes so far advanced before leaving the shell, that it is well coated with feathers, and can at once begin to take care of itself. These birds are the sea-birds, water-birds, game-birds and their allies of comparatively low organiza tion. In another class (Altrices) of higher organization as a group, the chicks are per mitted to break from the shell before they have acquired feathers or are able to move about or obtain food. They must therefore be shielded, defended, fed and cared for by the parents for several days or weeks. Out of this condition have grown some of the most interesting, com plicated and delightful features, habits and instincts of bird-life.