NOCTURNAL MIGRATION , ^3 



latter month they regularly move north toward the 

 summer home. Though we may theorize as to the 

 actual origin of migration, the annual movements of 

 migratory birds are now a part of the life-cycle of 

 the individual, and as such occur at the regularly 

 appointed time, actuated by physiological condi- 

 tions. The spring migration especially has its in- 

 ception and course distinctly connected with the 

 necessity for reproduction. Food is as abundant in 

 the winter home in spring as through the winter 

 months, so there is no necessity for movement to 

 search for it. Winter is a resting period, during 

 which the bird has no cares save to avoid enemies 

 and search for food. With the approach of spring 

 the reproductive impulse awakens, and the individ- 

 ual is irresistibly driven to initiate a journey that 

 ends in its summer home. 



We?ther makes only this difference, that the life- 

 cycle has been so adjusted, and the migrations so 

 synchronized, that they agree with major phe- 

 nomena of weather and climate through the year. 

 Some species find that cold means little to them, so 

 that their northern limit in winter is in regions 

 where the climate is severe. This is true with juncos 

 and tree sparrows, and a number of other birds in the 

 United States, and with many small birds in north- 

 ern Europe and Asia. Certain waterbirds are hardy 

 and are restricted in winter range only by the line of 



