30 THE MIGRATION OF BIRDS 



breeding season, the natural food supply would be 

 taxed to its limits ; the falling temperature drove 

 some and finally all to seek food further south, and 

 their short migration to lands already filled with old 

 and young birds, caused pressure and overcrowding 

 further south. Further outward and usually south- 

 ward movement was necessary and the zone of stress 

 was gradually extended, though probably in those 

 early days no particular species took long passages. 

 The winter passed and the vacuum was again pro- 

 vided, and the rebound to fill it would create a 

 slackening force all along the line ; birds would 

 spread from congested districts so soon as food 

 supplying areas opened to receive them. 



Mr Taverner, arguing on these lines (51), shows 

 that competition would be originated in areas con- 

 taining the earliest breeders, and be severest in 

 the most productive districts. Weaker and later 

 breeders would be driven out or prevented from 

 colonizing by the stronger and earlier species, and 

 the evicted ones would encroach on others, forcing 

 them in turn to trespass on a wider circle of species. 

 He then argues how the gradual recession of the glacial 

 ice would increase the possible northward breeding 

 area, and cause longer migration, and that this migra- 

 tion would delay breeding and conversely delayed 

 breeding would assist the evolution of migration. 



But the lengthening of the journey might surely 



