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 theirown 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? 
Flies would, of course, be there in plenty, but there 
might well be a dearth of juicy larva. 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 
BB 
