BIRD-MIGRATION 



cuckoos, which, having no domestic cares, are free to 

 leave as soon as the stock of caterpillars shows signs 

 of failing. 



Though the increasing cold of the Polar regions has 

 driven birds farther and farther south, temperature 

 probably is but the secondary cause of the migratory 

 movement; the immediate one is the effect of cold 

 upon the food-supply. Swallows cannot exist with- 

 out flies, nor hard-billed birds without seed. Flies 

 are much more sensitive to cold than the birds that 

 prey on them; hence when the chill of English 

 autumns checks the hatch of flies, swallows must 

 move to regions where they find a fresh supply. When 

 snow buries the lowly vegetation of Scandinavia, 

 buntings must betake themselves to Scottish shores, 

 where, on rushy moors and in stackyards, they may 

 make sure of subsistence. Cuckoos depend on cater- 

 pillars, and nightjars on moths and cockchafers; so 

 these birds come to us late, and depart betimes; but 

 even so they experience a wide range of temperature. 

 The cuckoo haunting the gorse bushes on the sunny 

 links of Nairn enjoys a much more genial climate than 

 his kinsman half-way up the side of Ben Nevis, though 

 both are in nearly the same latitude. Swallows, too, 

 are patient of a very low temperature, so long as they 

 have a full larder. The autumn swarms of bluebottles 

 which gather on the sunny walls of houses in frosty 

 weather, sometimes tempt swallows to linger so long, 

 that in the end a failing food-supply leaves them 

 without strength to undertake their southward flight. 



