4 BULLETIN 185, U. S. DEPARTMENT OF AGRICULTURE. 



Whichever theory is accepted, the beginnings of migration ages 

 ago undoubtedly were intimately connected with periodic changes in 

 the food supply. While North America possesses enormous summer 

 supplies of bird food, the birds must return south for the winter or 

 perish. The overcrowding which would necessarily ensue should 

 they remain in the equatorial regions is prevented by the spring 

 exodus northward. No such movement occurs toward the corre- 

 sponding southern latitudes. South America has almost no migra- 

 tory land birds, for bleak Patagonia and Tierra del Fuego offer no 

 inducements to these dwellers of the limitless forests of the Amazon. 



The conclusion is inevitable that the advantages of the United 

 States and Canada as a summer home and the superb conditions of 

 climate and food for the successful rearing of a nestful of voracious 

 young far overbalance the hazards and disasters of the journey 

 thither. For these periodical trips did not just happen in their 

 present form; each migration route, however long and complex, is 

 but the present stage in development of a flight that at first was 

 short, easily accomplished, and comparatively free from danger. 

 Each lengthening of the course was adopted permanently only after 

 experience through many generations had proved its advantages. 



RELATION OF MIGRATION TO WEATHER. 



It may safely be stated that the weather in the winter home has 

 nothing to do with starting birds on the spring migration, except in 

 the case of a few, like some of the ducks and geese, which press 

 northward as fast as open water appears. There is no appreciable 

 change in temperature to warn the hundred or more species of our 

 birds which visit South America in winter that it is time to migrate. 

 It must be a force from within, a physiological change warning them 

 of the approach of the breeding season, that impels them to spread 

 their wings for the long flight. 



The habit of migration has been evolved through countless gener- 

 ations, and during this time the physical structure and habits of 

 birds have been undergoing a process of evolution in adaptation to the 

 climate of the summer home. In spring and early summer climatic 

 conditions are decidedly variable, and yet there must be some period 

 that has on the average the best weather for the birds' arrival. In 

 the course of ages there have been developed habits of migration, 

 under the influence of which the bird so performs its migratory 

 movements that on the average it arrives at the nesting site at the 

 proper time. 



The word "average" needs to be emphasized. It is the average 

 weather at a given locality that determines the average time of the 

 bird's arrival. In obedience to physiologic promptings the bird 

 migrates at the usual average time and proceeds northward at the 



