16 THE MIGRATION OF BIRDS 



journeys, at the seasons of the year when an in- 

 creasing number of young birds in the breeding 

 area drove the overgrown population to seek food 

 further from the base, and again when the sexual 

 impulses urged the birds to seek secure nesting sites. 

 The other factor is the weeding-out influence of 

 mistaken effort, the natural selection which leads 

 to the survival of the fittest. The young wanderer 

 which reached unsuitable lands must either wander 

 further or perish. Judging by the juvenile mortality 

 amongst young birds the failures would be many, 

 and only the successful competitors would return to 

 leave progeny. 



Great stress has been laid on the attachment of 

 birds to certain nesting sites, an undoubted fact, 

 and it has been argued that because, in some cases, 

 for hundreds of years certain sites have been 

 occupied by the same species, it is evident that 

 after the death of parents the young will return 

 to and occupy the home. This has even been 

 put forward as evidence that birds do not wander 

 in search of fresh nesting sites. The argument is 

 not sound. It is improbable that in most cases 

 both parents perish in the same year. Birds of 

 prey, and many of the cited instances of long tenancy 

 refer to raptorial birds, have a wonderful power of 

 finding a mate, male or female, to complete the 

 hatching and rearing of the young, when one of 



