THEORIES OF MIGRATION 25 



birds simply follow the sun in spring as the centre of 

 its path advances beyond the Equator, and after 

 rearing their young follow the retreating rays of the 

 heavenly orb as it moves southward, to winter 

 quarters where light is at its maximum intensity. 

 The course of migration is, in general, correlated 

 with the advance and retreat of the sun, but it 

 seems that light is a secondary cause, which reacts 

 on the bird through its effect in producing a change 

 in season, and not directly through any organic re- 

 action to the light rays themselves. On the basis of 

 light we cannot, for example, explain a migration 

 south among northern shorebirds during July, as the 

 maximum amount of light, though on the wane, is 

 still to be found in their northern breeding-ground. 

 Also in the case of the few migrant nocturnal birds, 

 for example, various goatsuckers, it would seem ad- 

 vantageous to react against light rather than to fol- 

 low it, since prolonged darkness is better suited to 

 their activities, which are actually curtailed by 

 lengthening days. In fact, this is probably one of 

 the factors that limits the summer range of some of 

 the strictly nocturnal species. A whippoorwill, for 

 example, would be decidedly out of place near the 

 Arctic Circle, where the hours of night are elimi- 

 nated or greatly curtailed by the movements of the 

 midnight sun. Further, we should expect birds of 

 such habits to find their optimum conditions as re- 



