The theory that is, perhaps, most naturally suggested, and the one that finds widest acceptance as explaining the facts is that migration began in a search for food. That is, the food supply becoming short in the vicinity of the home (a bird's home is thus assumed to be the place where it rears its young, and may therefore be quite different from the locality where it spends the remainder of the time) they wandered away in search of food, returning again and again to the home vicinity. These journeys were extended farther and farther, the birds returning each nesting season, undoubtedly oftener at first, to or near the locality where they were born. This process went on until their wandering became a fixed habit, and finally in the countless generations of birds that have come and gone, this habit has been crystallized into what we now call, for want of a better term, the instinct of migration.
This idea has been amplified and extended by Alfred Russell Wallace ('Nature,' X., p. 459). He supposed that 'survival of the fittest' has probably exerted a powerful influence in weeding out certain individuals. He supposed further that breeding can only be safely accomplished as a rule in a given area, and that during a greater part of the rest of the year sufficient food cannot be obtained in that area. "It will follow that those birds which do not leave the breeding area at the proper season will suffer, and ultimately become extinct; which will also be the fate of those which do not leave the feeding area at the proper time." His further argument is ingenious, and, it must be added, extremely plausible. He says: "Now, if we suppose that the two areas were (for some remote ancestor of the existing species) coincident, but by geological and climatic changes gradually diverted from each other, we can easily understand how the habit of incipient and partial migration at the proper seasons would at last become hereditary, and so fixed as to be what we term an instinct."
It will probably be found, however, if anything like a satisfactory explanation can be arrived at, that this habit or instinct has arisen in more than one way, but we may appropriately turn from a consideration of theories to a review of certain observed facts of migration.
It is now abundantly established that migration is mostly carried on at night, and further mainly during clear nights. Only a comparatively few species, such as ducks, cranes, certain large hawks, swallows, swifts, and nighthawks, migrate during the daytime, and these it will be observed, are either rapacious birds or mainly those that enjoy such power of rapid flight as to be relatively safe from capture. All the vast horde of warblers, sparrows, finches, flycatchers, thrushes and woodpeckers, as well as many waders and swimmers, migrate at night. On clear, still nights during the migrations birds may often be heard calling to each other high over head, and, as will be described