88 THE BIOLOGY OF BIRDS 



depend to some extent on other birds. The golden eagle 

 accounts for a good many grouse, and probably ser\'es as a 

 useful eliminator, keeping up the standard of racial fitness. 

 Not a few birds, such as crows, steal the eggs of other birds. 

 Mr. Beebe relates that a South African raven breaks ostrich 

 eggs and an Australian Black-breasted Buzzard breaks 

 emeu eggs by letting a stone fall on them. The Secretary 

 Bird's bill of fare includes snakes and our rooks allow fresh- 

 water mussels to fall from a height so that the shells are 

 broken. The ranks of frogs are thinned by storks, and of 

 fish-eaters there is a long and varied list from the flight- 

 less penguins to the albatross, from the hovering osprey 

 catching the trout in its talons to the diving cormorants, 

 from the patient heron to the restless Frigate-bird in the 

 open sea. 



Cuttlefishes are captured by penguins ; the oyster- 

 catcher is skilful in opening mussels ; rooks take clams and 

 sea-urchins to a height and let them drop on the stones ; 

 all sorts of shore animals are eaten by birds. The thrush 

 breaks the shells of the wood-snails on its stone anvil, and the 

 Pied Wagtail is fond of the little fresh-water snail, Lifmicea 

 truncatula, which harbours the young stages of the liver- 

 fluke. 



One of the most important facts in regard to birds is 

 that they check the tendency that many insects have to 

 over-multiplication — a tendency which is a continual 

 menace to the balance of nature. Mr. Beebe writes : " It 

 has been said that without birds, within a space of ten 

 years, the earth would not be habitable for man, owing to 

 the unrestricted increase of noxious insects. There is 

 doubtless not a single group of insects which does not 

 suffer from the appetite of one or more species of bird." 

 The cuckoo is partial to hairy caterpillars ; the swallow 

 and the swift hawk insects, usually small, in mid-air ; the 

 goatsuckers with their big gape work by night and the 

 flycatchers with their sharp eyes work by day ; the hen 

 scratches up the eggs and larvae hidden in the ground and 

 some of the woodpeckers gouge grubs out of holes in the 



