9 



the smaller insectivorous birds that they are able daily to con- 

 sume and assimilate quantities of insect food enormously be- 

 yond their apparent capacity. 



When one begins to study the food of birds he finds that ex- 

 ceedingly complex food relations exist between the bird and 

 the animals and plants on which it feeds. The food preferences 

 of a bird may produce complicated, far-reaching and unexpected 

 results. It is not often possible for the investigator who studies 

 a bird's food to measure fully the effect of its feeding habits. 

 In ordinary circumstances a tent caterpillar and a climbing cut- 

 worm both would be considered destructive, as both are known 

 to consume the foliage of trees, but when we find the climbing 

 cutworm destroying the living pupae of the tent caterpillar 

 moth, it seems questionable at first sight whether the bird that 

 eats both is rendering any valuable service in nature. Likewise, 

 when we find birds feeding on tiny parasitic insects which kill 

 injurious insects by living and feeding within their bodies, or 

 when we see birds destroying the larger predaceous insects 

 which kill and eat so-called injurious insects, we are inclined to 

 wonder whether birds when engaged in destroying such bene- 

 ficial creatures are not themselves injurious. 



Let us examine the effect of this practice among birds. First, 

 it should not be forgotten that the chief function of birds is to 

 perform their part in holding in check the whole great body of 

 insect life, and to help in preventing its undue increase. There- 

 fore, since birds are helping to hold in check all insect life, their 

 reduction of what we call beneficial insects is a negligible harm, 

 if they are at the same time reducing in greater measure the 

 numbers of the far more numerous injurious insects. Man's 

 own measures to control pests (as by spraying) destroy many 

 useful parasitic and predaceous insects; but spraying is not 

 thereby condemned. Under normal conditions birds and other 

 predatory enemies of insects are of chief importance. Parasitic 

 insects, though often performing remarkably efficient service, 

 are ordinarily of secondary value for the following reason: 

 birds and other predatory enemies of insects destroy their 

 prey at once, while most parasites allow the insect pests to 

 continue injurious activity until the latter have nearly or 

 fully passed their feeding period. The parasite, therefore, in 



