THE EVOLUTION OF MIGRATIONS 107 



Unduly lengthened days would have as detrimental 

 an effect on crepuscular species as protracted nights 

 on the diurnal. As summer birds they have no 

 place in the far north. 



Assuming again that the migratory rhythm can 

 become inherent, the enormous migration of some 

 of the far northern breeders — golden plover, Arctic 

 tern, etc. — become comprehensible. They are 

 merely an exaggerated form of lesser migrations and 

 any able flyer overshooting the mark of the original 

 wintering ground on its road south would tend to- 

 ward such exaggeratipn. If repeated sufficiently 

 often such an expanded migration might become the 

 custom. Its success would depend on the nature of 

 the territory added. If no antagonistic factor were 

 encountered, the addition would have no detrimen- 

 tal effects on the numerical status of the species and 

 would not be eliminated by selection. Among the 

 most powerful flyers we might thus expect to find 

 what we actually get, an annual trip from virtually 

 one end of the globe to the other and back again. 

 As was pointed out earlier, many species are in- 

 cessantly on the wing and whether the daily mileage 

 is done more or less in one direction or in circles has 

 no bearing on the amount of energy expended. 



Finally, an exceptional type of migration must be 

 noted. The flight of the Pacific golden plover from 



