seek a warmer climate and more to eat; more particularly probably for sustenance.
The original and simple theory that the majority of birds migrate for food or warmth is not overthrown by modern observations. That appears to be the primary impulse, though others may be traced or reasonably imagined. To suppose, as has been put forward, that birds are endowed with a migratory instinct for the express purpose of keeping down their numbers, in order, that is, that they may perish in crossing the sea, is really too absurd for serious consideration. If that were the end in view, it would be most easily obtained by keeping them at home, where snow would speedily starve them. On the contrary, it will appear to any one who walks about woods and fields that migration is essential to the preservation of these creatures. By migration, in fact, the species is kept in existence, and room is found for life. Apart from the necessity of food, movement and change is one of the most powerful agencies in renewing health. This we see in our own experience; the condition of the air is especially important, and it is well within reasonable supposition that some birds and animals may wish to avoid certain states of atmosphere. There is, too, the question of moulting and change of plumage, and the possibility that this physiological event may influence the removal to a different climate. Birds migrate principally for food and warmth; secondly, on account of the pressure of numbers (for in good seasons they increase very fast); thirdly, for the sake of health; fourthly, for sexual reasons; fifthly, from the operation
