The Policemen of the Air 



BIRDS THAI' IvAT INSECTS 



Alany birds, as flycatchers, 

 warblers, swallows, and chimney- 

 swifts, live exclusively, or almost 

 so, on insects, and very many 

 more, as blackbirds, orioles, and 

 some hawks, depend on them for 

 a considerable part of their liveli- 

 hood. The little sparrow-hawk 

 lives very largely upon grasshop- 

 pers, crickets, and beetles, and 

 even one of the larger hawks — 

 the Swainson ha\\k of the west- 

 ern plains — at certain seasons de- 

 stroys enough of these injurious 

 insects, together with small ro- 

 dents, to save the western farmer 

 upwards of a hundred thousand 

 dollars a }ear. 



If all insects preyed upon vejre- 

 tation, our inquiry into the value 

 of insect-eating birds need go no 

 further, since all of them might 

 be set down as beneficial ; but by 

 no means all insects are destruc- 

 tive of vegetation, and their relations to 

 each other and to birds are very complex 

 and puzzling. The insects that feed on 

 vegetation at some stage or other of their 

 existence probably outnumber all others, 

 both in number of species and of indi- 

 viduals ; but there are two other classes 

 of insects which deserve attention here, 

 the predaceous and the parasitic. The 

 predaceous insects, either in the adult or 

 larval state, feed upon other insects and 

 hence in the main are beneficial. It 

 would seem, therefore, that in so 

 far as birds destroy predaceous insects 

 they do harm. That birds do destroy 

 a greater or less number cannot be 

 denied, but as many species of this 

 group secrete nauseous fluids, which 

 serve, in a measure at least, to protect 

 them, and as many are of retiring habits 

 and not readily found, the number de- 

 stroyed bv birds is relatively not large. 

 Moreover, some of the predaceous in- 

 sects, when insect food is not available, 

 become vegetarians, and hence assume 

 the role of enemies of the farmer; so that 

 when birds destroy predaceous insects 



From the Biological Survey 

 A BARRED OWL 



they may be doing the farmer either a 

 good turn or an ill turn, according to cir- 

 cumstances. 



The relation of birds to the so-called 

 parasitic insects is still more intricate and 

 puzzling. Parasitic insects fill a very im- 

 portant place in the economy of nature ; 

 it is even claimed by entomologists that 

 they do more eft'ective service in aiding 

 to keep true the balance in the insect 

 world than any other agency. They at- 

 tack insects in every stage of existence 

 and insure their destruction by depositing 

 eggs on, or in, the bodies of adults, their 

 larvje (the worm or caterpillar stage), 

 their pupa;, or their eggs. Now^ birds 

 recognize no fine distinctions in the insect 

 world. All is grist that comes to the 

 avian mill, and parasitic insects are 

 snapped up by birds without the slightest 

 regard to the fact that they are useful to 

 man. Hence we have a complicated 

 problem to unravel in respect to the inter- 

 relation of insect pests, of insect parasites 

 that destroy them, and of birds that de- 

 stroy both pests and their parasites. As 

 Swift phrases it : 



