Before presenting some of the present theories concerning the
origin of bird migration, it seems well to consider briefly the
ends served by this annual round trip between breeding grounds and
winter quarters. It is apparent that the migratory habit enables a
species to enjoy the summers of northern latitudes while avoiding
the severity of the winters. In other words, migration makes it
possible for some species to inhabit two different areas at seasons
when each presents favorable conditions. In the performance of its
reproductive duties every pair of birds requires a certain domain,
the extent of which varies greatly in different species. Generally,
however, this territory must be large enough to provide adquate food,
not only for the parent birds but also for the lusty appetites that
come into being with the hatching of the eggs. Thus, if all birds
were to remain constantly either in tropical or in temperate regions,
there might be intolerable overcrowding during the breeding season.
By the spring withdrawal to regions uninhabitable earlier in the
year, the migrants are assured of adequate space and ample food
upon their arrival in the winter-freed North, and it may be assumed
that nonmigrant species resident in the South are benefited by the
withdrawal of the migrants.
Nevertheless it cannot be said that the winter or summer areas are
entirely unsuited to the requirements of every migrating species at
other seasons, for some individuals pass the winter in areas that
are frequented only in summer by other individuals of their species.
The extensive breeding ranges of such species present wide climatic
variations, so that some individuals may actually be resident in a
region where others of their kind are present only in winter.
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.