202 THE MIGRATION OF BIRDS. 



the Southern Hemisphere. Unquestionably the one 

 grand dominating impulse of Migration in spring 

 is Reproduction. Migratory birds come north to 

 breed, to rear their young in a climate where the 

 temperature is best suited to their several require- 

 ments. That this is so seems proved by the fact 

 that the adult birds are the first to migrate 

 northwards in spring, birds whose sexual instincts 

 are mature and strong ; the young of the previous 

 year in a great many cases do not extend their 

 spring flights quite to the usual breeding-grounds 

 of their species, and in other cases actually remain 

 close to their winter quarters right through the 

 summer. Another very remarkable fact about 

 Spring Migration is the much greater rapidity with 

 which it is performed. Birds may not fly any 

 faster in spring, but they do not linger so long on 

 the road ; they seem bent on getting to their 

 summer quarters as quickly as possible when once 

 they have fairly started. It seems a wonderful and 

 very remarkable fact how the earliest spring 

 migrants time the date of their arrival to such a 

 nicety that they are back in their old haunts almost 

 to the moment that winter finally departs. But 

 this fact is really not so very amazing after all. 

 Birds as they near their northern destinations have 

 often to wait about for spring, having been too 

 eager to press onwards, and then great numbers 

 collect on the very outskirts of retreating winter, 

 ready to renew their flight at the first suitable 

 opportunity. This is not so readily remarked in 



