By withdrawing in the spring to regions uninhabitable earlier in the year, migrant species are generally assured of adequate space and ample food upon their arrival in the winter-freed North, and those nonmigratory kinds, which stay behind to nest, are also assured of ample space for these activities.
Every pair of birds requires a certain amount of territory for the performance of its reproductive duties, the extent of which varies greatly between different species. This territory must be large enough to provide adequate food, not only for the parent birds but also for the lusty appetites of their young. In the Arctic summer, 24 hours of daylight allow the young to feed or be fed almost continuously and rapid growth is apparent. The short breeding season in northern latitudes exposes the vulnerable young to predation for a brief period and prevents a build up of predator populations.
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.