vi BIRDS OF AMERICA 



ring to risk starvation rather than to retreat. The Purple Martins often arrive at their 

 nesting boxes so prematurely that the cozy home becomes a tomb if a sleet storm sweeps 

 their winged food from the air. The Bluebird's cheery warble we welcome as a harbinger 

 of spring, often only to find later a lifeless body in some shed or outbuilding where the bird 

 sought shelter rather than return to the sunny land so recently left. 



As a matter of fact, however, only a small percentage of birds exhibit these pre-seasonal 

 migration propensities. The great majority remain in the security of their winter homes 

 until spring is so far advanced that the journey can be made easily and with comparatively 

 slight danger; and they reach the nesting spot when a food supply is assured and all the 

 conditions of weather and vegetation are favorable for beginning immediately the rearing 

 of a family of young. 



If, however, a longing for home is considered the main incentive to their northward 

 flight, there arises the question as to why birds desert that home so promptly after the nesting 

 season is over. Indeed, most birds start south as soon as the fledglings are able to shift for 

 themselves. The Orchard Oriole, the Redstart, and the Yellow Warbler of central United 

 States and the Nonpareil of the south all begin their southward journey early in July, long 

 before the fall storms sound a warning of approaching winter and when their insect menu 

 is particularly varied and abundant. 



According to the opposite migration theory, the birds' real home is the Southland; all 

 bird life tends by over-production to over-crowding; and, at the end of the glacial era, the 

 birds, seeking in all directions for suitable breeding grounds with less keen competition than 

 in their tropical winter home, gradually worked northward as the retreat of the ice made 

 habitable vast reaches of virgin country. But the winter abiding place was still the home, 

 and to this they returned as soon as the breeding season was over. Thus, in the case of the 

 Orchard Oriole mentioned above, many individuals that arrive in southern Pennsylvania 

 the first week in May leave by the middle of July, spending only 25 months out of the 12 

 at the nesting site. 



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 over-crowding which would necessarily ensue should they remain in the 

 equatorial regions is prevented by the spring exodus northward. No such movement occurs 

 toward the corresponding southern latitudes. .South America has almost no migratory 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 over-balance 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. 



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 generations, and during this 

 time the physical structure and habits of birds have been undergoing a process of evolution 



