THE MIGRATION OF BIRDS 



birds, the possibility of nesting in the south. Per- 

 haps in some cases the bird's constitution is such 

 that it cannot mate and nest without the subtle 

 stimulus implied in a return to the conditions of the 

 original birthplace. Perhaps too there are lingering 

 memories of the abundant and pleasant food 

 whether berries or mosquitoes to be had in the 

 north. As regards both nesting and feeding, there 

 may be a sort of constitutional home-sickness. 



It is difficult to say anything very useful at 

 present in regard to the origin of the migratory 

 habit. Perhaps the establishment of two homes or 

 territories has been wrapped up with the history 

 of climates which have changed greatly in the course 

 of ages. Perhaps it had more to do with the food- 

 supply. Many birds are prolific, and overcrowding 

 is apt to occur. Instead of crowding in one area 

 all the year round, and involving themselves in 

 want, birds learned, like the Swiss peasants, to 

 exploit two areas, each for about half of the year. 

 They tended to push farther and farther northward 

 in spring, exploring and exploiting new grounds, 

 staying as long as they could, and retreating before 

 the breath of winter to their home in the south. 

 It was probably most effective to go as far north as 

 possible before settling down to family life. A 

 noteworthy fact is that the birds with the largest 

 families tend to have the widest migratory range. 



However the migratory habit originated, it is not 

 difficult to understand that individual birds who 

 were dull, sluggish, wilful, or foolish would tend to 

 be sifted out year after year, age after age, and that 



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