ing season. It is a fact that some species spend very little time on 

 their breeding grounds; the orchard oriole, for example, spends only 

 2^/2 months in its summer home, arriving in southern Pennsylvania 

 about the first week in May and leaving by the middle of July. 



Both of these theories assume that migration is an ingrained habit, but 

 both have been criticized on biological and geological grounds, so 

 neither should be accepted without qualification as definitely account- 

 ing for the origin of bird migration. It is apparent, however, that 

 whether the ancestral home of any species was at the northern oi 

 southern limits of its present range, or even in some intermediate re- 

 gion, the search for favorable conditions under which to breed in 

 summer and to feed in winter has been the principal factor underlying 

 the origin of migration. 



Theory of photoperiodism 



A modern view based on studies of living behavior, suggests also 

 that there is good reason for believing that migration is an annually 

 induced movement. If such be true then the theory of photoperiodism 

 as propounded by some recent investigators should receive some 

 consideration. 



This theory holds as its major premise that quantity of light and 

 length of day are the stimulating causes of migration. Its proponents 

 urge that migration is a phenomenon far too regular to be created anew 

 each season merely under stress of circumstances, such as need for food ; 

 and that it begins before the necessity for a change in latitude becomes 

 at all pressing. Swallows, nighthawks, shore birds, and others may 

 start their southward movement while the summer food supply in the 

 North is at peak abundance; while robins, bluebirds, and others may 

 leave an abundant food in the South in spring and press toward north- 

 ern points when food supplies there are almost entirely lacking and 

 when severe cold and storms are likely to cause their wholesale de- 

 struction. The regularity of arrival and departure is one of the most 

 impressive features of migration, and since birds travel in almost strict 

 accordance with the calendar, the proponents of the theory ask : "What 

 phenomenon to which we may attribute the stimulating impulse occurs 

 with such precise regularity as the constantly increasing amount of 

 light in spring?" 



Experimental work has abundantly demonstrated the effect of in- 

 creased light upon the growth, flowering, and fruiting of plants. 



9 



