Page:Popular Science Monthly Volume 52.djvu/815

of temperature, permits birds to divide their time between widely separated regions, and, whether the choice be conscious or unconscious, the breeding places of migratory birds are selected on account of their safety, and not because they furnish all that a permanent home must supply. If we believe, with Professor Marsh, that the power of flight was acquired by birds after they became arboreal, we must look for the primitive home of the migratory birds in the great tropical and subtropical forests where arboreal reptiles and arboreal mammals still abound; nor can we believe the great armies of northern birds which find abundant food in southern lands in winter are driven out by scarcity on the approach of spring. Enemies are numerous in the tropics, but no animals have sharper senses or better means of escape than birds, and, trusting in their power of flight and their quick sight and hearing, they venture into danger with confidence. The great charm of birds to us is the fearlessness with which they approach man, who is the most dreaded enemy of all other vertebrates; but while adult birds are eminently fitted for taking care of themselves, the opposite is true, in even greater degree, of nestlings, for no animals are at the same time more helpless and more exposed to danger than many young birds, while eggs are not only absolutely helpless but also very tempting to enemies, although there is no group of animals in which the safety of the eggs and young is more important. Among birds a high birth rate is incompatible with flight, for their eggs are large and heavy, and the preservation of each species imperatively demands that every egg shall be cared for with unceasing solicitude; for while, in other animals, increased danger to eggs and young may be met by an increase in the birth rate, this can not be much increased in birds without corresponding loss in the power of flight. Every one knows how quickly birds are exterminated by the destruction of their eggs and young, and the low birth rate of all birds of powerful flight is a sufficient reason for migration, for at the same time that flight limits the birth rate it permits the birds to seek nesting places beyond the reach of their enemies; and as there is rigorous selection of the nestling's which are born in safe nests, it is easy to understand how the instinct has been gradually fixed by selection, and how, as it has become more and more firmly fixed, and as the safety of the eggs and young has become assured by the remoteness and isolation of the nests, the birth rate has been still more reduced and the power of flight correspondingly extended. Many sea birds that nest on desolate rocks in midocean lay only one egg each year, and have the power of flight in its highest perfection. The power of the storm petrel to wander is as boundless as the ocean, and while it lays only a single egg, it is said to be the most prolific