BIRD-MIGRATION 7 



kind in regard to the migration of starlings. These 

 are the first birds to make their appearance in Heli- 

 goland at the beginning of the southward move- 

 ment, as early as the last week in June; but the 

 earliest flights consist entirely of young starlings 

 of the same season. The next travellers to put in 

 an appearance are old 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, tem- 

 perature 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 without 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 caterpillars, and 

 nightjars on moths and cockchafers ; so these birds 



