14 



While the above statement of the way in which the balance 

 of Nature is preserved may be regarded as somewhat hypo- 

 thetical, probably it approximates what actually takes place, 

 although the feeding habits of birds undoubtedly produce far 

 more complicated and far-reaching results than are outlined 

 here. 



It is a law of Nature that the destroyer is also the preserver. 

 Birds of prey benefit the species on which they prey in at least 

 two ways not noted above: (1) the more powerful bird enemies 

 of a certain bird usually prey also upon some of the weaker 

 enemies of that bird; (2) these powerful birds also check the 

 propagation of weakness, disease or unfitness by killing off the 

 weaker or most unfit individuals among the species on which 

 they prey, as these are most easily captured. 



We have seen already that jays, which are enemies of the 

 smaller birds, are preyed upon by the more powerful crows, 

 hawks and owls. These latter also destroy skunks, weasels, 

 squirrels, mice and snakes, all of which are foes of the smaller 

 birds. No doubt these animals would be much more destructive 

 to the smaller birds were they without these wholesome feathered 

 checks on their increase. 



UTILITY OF INSECT-EATING BIEDS. 



Practically all birds eat insects, and it is among insects that 

 we find the most destructive pests known to man. Most plant- 

 eating insects that live in or about cultivated lands or forests 

 are potentially injurious. Some when kept normally repressed 

 by their natural enemies may feed only on noxious plants com- 

 monly called "weeds," and so may be harmless or even bene- 

 ficial; but let the checks upon their increase become lessened 

 in any way, so that their natural food supply becomes insuffi- 

 cient for their increasing numbers, and they may at once 

 menace growing crops. Any plant-eating insect that increases 

 much beyond its normal numbers soon assumes the importance 

 of a pest, and all insects have this tendency to multiply. 



Insect pests are particularly destructive, not only because 

 of their large numbers but also because of their great con- 

 sumption of food. A certain maggot consumes in twenty-four 



