The tendency in many species to move southward at the approach of winter is not always due to the seasonal low temperatures, since experiments have demonstrated that many summer insect feeders, when confined in outdoor aviaries, comfortably withstand temperatures far below zero. The main consideration is the depletion of the food supply, caused either by disappearance or hibernation of insects, or by the mantle of snow or ice that prevents access to the seeds and other forms of food found on or close to the ground or submerged in water. Possibly also the shortened hours of daylight materially restrict the ability of the birds to obtain sufficient food at a time when the cold requires an increased supply to maintain body heat. It is noteworthy that chickadees (_Penthestes atricapillus_) and some other of our smaller birds have no fear of Arctic weather, as their food supplies are mainly arboreal and so are always available. Also, when there is a good supply of food in the form of pine seeds in Canadian woods, nuthatches (Sitta carolinensis and S. canadensis) and crossbills (_Loxia curvirostra_ and _L. leucoptera_) will remain through the winter. When these birds appear abundantly in winter at points in southern latitudes, it may be concluded that there is a shortage of their food in the North, or that they have been lured farther south by the greater abundance of this food there. THEORIES OF THE CAUSES OF MIGRATION Migration has long since become a definite hereditary habit that recurs in annual cycles, probably because of physiological stimuli associated with the reproductive period. In seeking its origin it is necessary to study the history of the birds' occupation of their present ranges, and from the information available to consider what appear to be reasonable theories. The two now most commonly accepted are diametrically opposed to each other. NORTHERN ANCESTRAL HOME THEORY According to one of these, nonmigratory birds swarmed over the entire Northern Hemisphere in earlier ages, the conditions of food and habitat being such as to permit them to remain in their haunts throughout the year. The entire northern area then afforded the two important avian requirements--suitable breeding conditions and a year-long food supply. This is the condition today in the Tropics, and it is noteworthy that many tropical birds are nonmigratory. Gradually, however, in the Northern Hemisphere the glacial ice fields advanced southward, forcing the birds before them, until finally all bird life was concentrated in southern latitudes. As the ages passed and the ice cap gradually retreated, each spring the birds endeavored to return to their ancestral homes in the North, only to be again driven south at the approach of winter. As the size of the ice-covered area diminished, the journeys made became ever longer, until eventually the habits of migration were fixed to accord with the climatic conditions of the present age.