It cannot be said that the winter or summer area of every species is entirely unsuited to the requirements of all of its members at other seasons, because some individuals pass the winter season in areas that are frequented only in summer by other individuals of their species. Such species may have extensive breeding ranges with wide climatic variations so that some individuals may actually be permanently resident in a region where others of their kind are present only in winter. Also, some individual song sparrows and blue jays, for example, have been known to change their migratory status (e.g., a particular bird may migrate one year and not the next or vice versa). Thus, different individuals or populations within these species appear to have different tolerances for climatic conditions.
The tendency of some birds to move southward at the approach of winter is not always due to seasonal low temperatures. Experiments have demonstrated many of our summer insect feeders, when confined in outdoor aviaries, comfortably withstand temperatures far below zero as long as abundant food is provided. The main consideration then, is depletion of the food supply, caused by either the disappearance or hibernation of insects or the mantle of snow or ice that prevents access to seeds and other food found on or close to the ground or submerged in water. Also, shortened hours of daylight may restrict the ability of birds to obtain sufficient food at a time when low temperatures require increased energy to maintain body heat. It is noteworthy that some of our smaller birds, such as the chickadees, can withstand a cold winter because their food supplies are always available above ground on trees. When there is a good supply of pine and spruce seeds, red-breasted nuthatches and crossbills will remain through the winter in Canadian woods, but when these birds appear abundantly in winter at southern latitudes, it may be concluded there is a shortage of these foods in the North.
Modern views based on studies of bird behavior and physiology indicate migration is a regular, annually induced movement, modified by local weather conditions, but largely independent of them. Migration is a phenomenon far too regular to be created anew each season merely under stress of circumstances, such as need for food; and it begins before the necessity for a change in latitude becomes at all pressing. Swallows, nighthawks, shorebirds, and others may start their southward movement while the summer food supply in the North is at peak abundance. American robins and bluebirds may leave abundant food in the South and press northward when food supplies there are almost entirely lacking and severe cold and storms are likely to cause their wholesale destruction. Regularity of arrival and departure is one of the most impressive features of migration, and since birds travel in a rather strict accordance with the calendar, we might ask: "What phenomena, other than the regular changes in length of day, occur with sufficient precision to act as a stimulus for migration?"