224 THE MIGRATION OF BIRDS. 



behind it! The Cuckoo {Cuculus canorus), with 

 his fly-line reaching from the North Cape to South 

 Africa, migrates early, passing Heligoland in July. 

 In this species, it might here be remarked, the old 

 birds migrate before the young, an anomaly due 

 to the bird's parasitic instincts, which free it from 

 all parental duties, and allow it to set off very early 

 for the south ! The Great Spotted Cuckoo (Cucu- 

 lus glandarms) is an equally early migrant, a parasite 

 leaving its summer quarters before its offspring. 

 Indeed it seems doubtful whether these Cuckoos 

 would migrate at all, if it were not for the influence 

 of temperature on the nidification of their eggs, 

 for they just come north to lay them, early in 

 spring, when small insectivorous birds are nesting, 

 and very soon afterwards draw south again. But 

 the Cuckoos that breed in the normal manner are 

 equally normal in their migration. The Yellow- 

 billed Cuckoo and the Black-billed Cuckoo (Coccy- 

 zus americanus and erytfirophthalmus) of America 

 are comparatively late autumn migrants, but they 

 build nests and hatch their own eggs in the 

 usual way. It is also interesting to remark that 

 the summer area of Parasitic Migratory Cuckoos 

 is dependent upon that of the various species which 

 play the part of foster parents, and the Migradon 

 Flight is consequently affected by the same cause. 

 Were no small insectivorous birds to visit the 

 Arctic regions in summer, the Common Cuckoo 

 would never have extended its range to the Arctic 

 Circle, just as the narrower summer area of the 



