xiv MIGRATION 369 



a counterbalancing gain. The perilous flight over, 

 they are in a genial climate with plenty of food. To 

 say that Africa could support all the Swallows that 

 come to us in spring is to speak positively on a sub- 

 ject of which we know very little. There may seem 

 to be abundance of flies for all comers, but the large 

 flocks of birds that fly northward, each with a voracious 

 appetite requiring many hundreds of gnats or other 

 small insects to satisfy its daily wants, might well 

 get to the end of the supply. Over bird-population 

 is certainly a possibility. Eagles will not allow their 

 own young ones to stay within what they have marked 

 out as their own domain. The same jealousy is found 

 not only in other birds of prey, but in Robins, and 

 probably other species. The Nightingales of the 

 Jordan Valley seem to be in excess of what the 

 country can support, for some remain there to nest 

 while others fly northward. It has been well urged, 

 too, that in the tropics in the height of summer the 

 country becomes parched, whereas in the north there 

 are hosts of succulent caterpillars and other grubs. Mr. 

 Seebohm found abundance of insect life in the valleys 

 of Asia Minor in May and June. But would the case 

 have been the same a month or two months later ? 

 Flics would, of course, be there in plenty, but there 

 might well be a dearth of juicy larvae. And the mass 

 of grub-eating migrants, it must be remembered, come 

 from the parched regions near the equator. We have 

 some direct evidence that food is the magnet that 

 attracts. The Rice-bunting or Bobolink, an American 

 bird which winters in Central and Southern America, 

 is enlarging its northern range as the growing of rice 



B B 



