48 THE RIDDLE OF MIGRATION 



ing in their own element, may forego feeding alto- 

 gether on the breeding run. 



As far as the southward flight of many northern 

 species of birds is concerned it may, perhaps, be 

 classed as a feeding movement since continuance 

 in the north means inevitable starvation, but it is 

 only half the story. What of the return passage? 

 Food is neither obviously, nor certainly, in spite 

 of the assertions of many writers, a compelling 

 factor. If birds find ample sustenance on their 

 winter range they should find even more there in 

 the summer, the season of universal abundance. 

 The assumption that the bird can foresee a possible 

 forthcoming shortage of food as a result of impend- 

 ing spring increase, and therefore moves elsewhere, 

 is wholly gratuitous and unwarranted. Not even 

 man can certainly state that such would be the case. 

 There are good reasons for believing that it would 

 not. The same may, of course, be said of breeding, 

 i.e., that the birds could probably breed as well in 

 the south as in the north and therefore breeding is 

 not the urge, but at this juncture the point is es- 

 sentially immaterial. We shall return to it later. 

 Breeding is involved in the northern passage whether 

 constituting the inducing factor or not and this 

 is the element in true migratory movements that 

 makes them distinctive. 



