12 USEFCL BIRDS. 



the smaller birds, the survivors of which they have unwit- 

 tingly furnished M^ith more insect food, thus making con- 

 ditions favorable for the increase of the smaller birds. 

 These oscillations or alternate ex})ansions and contractions 

 in the numbers of birds or insects are usually so slight as 

 to escape common observation. It is only in those cases 

 where they are carried to extremes that they result disas- 

 trously. Under nature the checks on the increase of birds 

 are essential, else they would increase in numbers until 

 their food suppl}' had become exhausted, when they would 

 starve, and other consequences even more grave and much 

 more complex would then follow. 



While these examples of the wa}- in which the Imlance of 

 nature is preserved ma}' be regarded as somewhat hyi)othet- 

 ical, they probably approximate what actually takes place, 

 although the feeding habits of birds undou))tedly })roduce 

 far more complicated results than are here outlined. 



It is a law of nature that the destroyer is also the protector. 

 Birds of i)rey save the species on which they prey from 

 overproduction and consequent starvation. They also serve 

 such species in at least two other ways: (1) the more 

 powerful bird enemies of a certain bird usually prey upon 

 some of its weaker enemies; (2) these powerful birds also 

 check the propagation of weakness, disease, or unfitness, by 

 killine: oil' the weaker or most unfit individuals amons: the 

 species on which they prey, for these are most easily captured 

 and killed. 



We have seen already that Jaj-s, which are enemies of 

 the smaller birds, are preyed upon by the more })Owerful 

 Crows, Hawks, and Owls. These latter also destroy skunks, 

 weasels, squirrels, mice, and snakes, all of which are also 

 enemies of the smaller birds. Xo doubt these animals would 

 be much more injurious to the smaller liirds were they Avith- 

 out these wholesome feathered checks on their increase. 



In a state of nature, albino birds or those that are rendered 

 cons})icuous to their enemies by any unusual mark or color 

 are soon captured by some bird of prey, and seldom live to 

 perpetuate their unfitness. 



