214 BOARD OF AGRICULTURE. [P. D. 4. 



comes out in the open and lies in the sun. There it is readily 

 detected and killed by hawks, and thus the progress of the 

 contagion is stayed. 



A similar selection is seen in the removal and destruction by 

 natural enemies of the dull, slow-witted, deformed or other- 

 wise unfit individuals which, in the long run, are the first to 

 be caught and killed, while the active, quick-witted, strong 

 and well-formed individuals are more likely to escape. A 

 sharp-shinned hawk dashed into a flock of juncos feeding on 

 the ground. All escaped by flight but one, which seemed to 

 have lost its wits or failed to make use of them, 'and, crouch- 

 ing close to the ground while its companions were already in 

 flight, fell a victim to its swift rapacious enemy. Individual 

 birds which vary widely from the protective coloring of the 

 species to which they belong are more readily seen, followed 

 and destroyed by their enemies. Thus albinos, which because 

 of their whiteness are conspicuous, are weeded out, and the 

 protective color of the species is kept true. 



Evidently, then, the tendency to increase is serviceable in 

 maintaining a species, provided only that there exist effective 

 checks to this increase to regulate the species, preserve its fitness 

 and prevent it from increasing too much in numbers. Natural 

 enemies provide such checks. 



SOME NATURAL ENEMIES REGULATE OTHERS. 



In a brief treatise like the present this subject cannot be 

 handled exhaustively, but there is another way in which natural 

 enemies benefit the species on which they prey which cannot 

 be overlooked here, and that is the regulation by some of 

 them of the numbers of certain other natural enemies. For 

 example, one must count among the foes of birds hawks, owls, 

 crows, jays, shrikes, foxes, weasels, minks, squirrels, snakes, 

 rats and mice, bjt certain large hawks destroy smaller hawks; 

 large owls destroy smaller owls; hawks and owls kill crows, 

 jays, shrikes, weasels, minks, squirrels, snakes and rats; weasels 

 destroy squirrels, snakes eat other snakes, and all catch mice. 

 If rats, ground mice and climbing mice, lacking these and 

 other checks, were allowed to increase too much in numbers 

 they might exterminate most birds by destroying their eggs 



