470 University of California Publications in Zoology [Vol. ] i. 



turist and horticulturist, any factor, even though it be one of 

 many factors, is important in insect control. Birds destroy 

 insects; they are therefore a natural factor in the control of 

 insect life. Since they do not fluctuate in numbers so greatly 

 as do parasitic insects, they are a more constant factor than 

 parasites. Insectivorous birds feed on the insect most abundant, 

 hence they are the more important in limiting the numbers of 

 insects. The proportion taken apparently varies directly as the 

 numbers of the insects. The greatest toll on insect life comes 

 during the nesting season, a time when the insect population is 

 at its maximum. This in itself establishes the fact that birds 

 have a real and an important part to play in the interaction of 

 organisms. 



Almost all insects are potentially injurious. Injurious insects 

 in small numbers cause practically no damage. Neutral and 

 beneficial insects in large numbers may become injurious. Hence 

 the destruction of neutral and beneficial insects, if they are 

 potentially destructive, may become at times of utility. 



Insects have in turn adapted themselves to the constant drain 

 on their numbers. This becomes very evident when we study 

 the rate of reproduction of insects. However, it is the balance 

 we need. Birds and insects both have a part to play in the 

 balance. They are supplementary and indispensable. 



This is an important viewpoint. Emphasis on the relation 

 of birds to insects viewed from the standpoint of the interaction 

 of organisms is rightly placed. Birds are important because they 

 evidently are an indispensable factor in maintaining an equi- 

 librium of organic life. If all birds play this important part, 

 the destruction of any particular species of bird means a dis- 

 turbance in the balance. The nearer man adjusts organic life 

 to his desires the more important will become each natural factor 

 concerned. Artificial means of adjustment often lack the effi- 

 ciency of natural means. 



It is readily acknowledged that birds are not the only checks 

 on the increase of insects. The very large toll taken by them, 

 however, places them in the front rank as insect destroyers. 

 Parasites can become abundant only when their host becomes 

 abundant and do their work effectively only after the insect has 



