BIRDS OF NEW YORK 39 



of their stomach contents in the laboratory, reveals the fact that they 

 destroy incredible niunbers of insects, for they have the most voracious 

 appetities of all warm-blooded animals. When we consider that one 

 Whippoorwill has been known to devour thirty-six good-sized moths within 

 one hour and that a warbler has been seen to swallow five hundred 

 seventy-six plant lice in four minutes, it is evident that an abundance 

 of bird life in the field and forest can not fail to keep down the number 

 of insect pests. Furthermore, the freedom with which birds move from 

 place to place suggests the especial value of the birds' work, for, by reason 

 of their migratory habits and their unrestricted activity, both over the 

 ground and through the trees and in the air, they are able to discover 

 danger centers of insect life and prevent serious outbreaks in many cases 

 when insect parasites might be too slow in their attacks or weather con- 

 ditions unfavorable to the pest might fail to appear. The especial value 

 of each family of birds or of each order will be found briefly summarized 

 in the pages of this book immediately following the family and ordinal 

 descriptions. 



Carnivorous birds. Every one knows that hawks and owls feed on 

 birds and mice. It is also a fact that nearly every species of hawk and owl 

 feeds, especially in summer and fall, upon large quantities of insects, 

 although this is especially true of the smaller species like the Sparrow 

 hawk and Screech owl. Other members of this family vary their diet 

 with fish, frogs and reptiles, as the occasion offers, so that the order Rap- 

 tores can not be considered exclusively carnivorous, although the main 

 food of all the larger species is composed of some kind of flesh. Besides 

 the hawks, owls and vultures that are typically carnivorous species, many 

 other birds at times kill smaller mammals or even other birds, as is the 

 case with gulls, jaegers, herons, and bitterns, which occasionally capture 

 mice or young birds; some Red-headed woodpeckers are known to feed 

 on the young of other birds; crows, jays and grackles are especially fond 

 of nestlings and also capture small mice in the field; and shrikes are adapted 

 for capturing birds, which they impale on thorns and partly devour. It 



