LAND BIRDS 



307 



vireos, kinglets, and many more guard the leaves. The insects 

 of the air are preyed upon in the daytime by the diurnal birds, 

 such as the swallows, swifts, kingbirds, and fly-catchers. Cre- 

 puscular insects are caught by such birds as the whip-poor- 

 will, night-hawk, and small owls. 



Hawks and owls destroy many rats and mice and other young 

 rodents, while the vultures are very useful as scavangers, since 

 they subsist largely on carrion. The South African secretary 

 bird (Fig. 252) belongs in the list of friends. 



Fig. 252. Secretary bird (Gypoger'anus reptiliv'orus). A South African 

 snake-killer protected by law. (After Houssay.) 



Certain kinds of birds are of especially great value on account 

 of their specific food, insect pests which are making such havoc 

 with particular crops or with certain trees. Thus, the swallows, 

 which eat almost nothing of value to man, prey upon the cotton- 

 boll weevil, which they catch upon the wing. Forty-seven adult 

 weevils have been found in the stomach of a single swallow. It 

 should be remembered that the swallows, which are such inde- 

 fatigable insect destroyers here in the breeding season, migrate 



