BIRD MIGRATION". 3 



Son Mi America. But the habit of migration had been formed, and 

 when the ice receded toward its present position the birds followed 

 it northward and hi time established their present long and diversified 

 migration routes. 



Those who thus argue that love of birthplace is the actuating im- 

 pulse to spring migration call attention to the seeming impatience 

 of the earliest migrants. Ducks and geese push northward with the 

 beginnings of open water so early, so far, and so fast that many are 

 caught by late storms and wander disconsolately over frozen ponds 

 and rivers, preferring 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 ex- 

 hibit these preseasonal migration propensities. The great majority 

 remain in the security of their winter homes until spring is so far ad- 

 vanced that the journey can be made easily and with comparatively 

 slight danger; and they reach the nesting sp6t when a food supply is 

 assured and all the conditions of weather and vegetation are favora- 

 ble 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. Data 

 recently collected at the Florida lighthouses by the Biological Survey 

 show that southward migration begins at least by the 10th and proba- 

 bly as early as the 1st of July. Indeed, most birds start south as 

 soon as the fledglings are able to shift for themselves. The orchard 

 oriole, the redstart, and the summer 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 ap- 

 proaching 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 overproduction to overcrowd- 

 ing; and, at the end of the glacial era, the birds, seeking in all direc- 

 tions 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 2^ months out of the 12 at the nesting site. 



