arrival in the winter-freed North, and it may be assumed that the 

 nonmigratory species that are resident in the South are also benefited 

 by the departure of the migrants. 



Nevertheless, it cannot be said that the winter or summer area of 

 every species is entirely unsuited to its requirements at other seasons, 

 for some individuals pass the winter season in areas that are frequented 

 only in summer by other individuals of their species. Such species 

 have extensive breeding ranges, presenting 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 of some birds to move southward at the approach of 

 winter is not always due to the seasonal low temperatures, since ex- 

 periments have demonstrated that many of our summer insect feeders, 

 when confined in outdoor aviaries, comfortably withstand tempera- 

 tures far below zero. The main consideration is the depletion of the 

 food supply, caused either by the disappearance or the 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 note- 

 worthy that some of our smaller birds, such as the chickadees, have 

 no fear of Arctic weather, as their food supplies are entirely arboreal 

 and so are always available. Also, when there is a good supply of 

 food in the form of pine and spruce seeds, nuthatches and crossbills 

 will remain through the winter in Canadian woods. 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. 



The Origin of Migration 



Migration has long since become a definite hereditary habit that 

 recurs in annual cycles, probably because of physiological stimulus asso- 

 ciated with the reproductive period. Its origin is locked in the ages 

 of geologic time, but by study of the history of how birds came to 

 occupy their present ranges, information becomes available from which 

 reasonable theories may be developed and explored. The two that are 

 most commonly accepted are diametrically opposed to each other. 



