virtually kill the young of the smaller birds and eat them that they (the jays) 

 may eventually have more insect food for their own young. When this object 

 has been attained the jays again, perhaps, allow an increase of the smaller birds, 

 the survivors of which they have unwittingly furnished with more insect food, 

 thus making conditions favorable for the increase of the smaller birds. 



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

 save the species on which they prey from overproduction and consequent star- 

 vation. 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 killing off the weaker or most unfit individuals among the species 

 on which they prey, for these are most easily captured and killed. 



We have already seen 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 enemies 

 of the smaller birds. No doubt these animals would be much more injurious to 

 the smaller birds were they without these wholesome feathered checks on their 

 increase. 



Birds are quick to assemble wherever in the woods the disappearing foliage 

 denotes the presence of great numbers of destructive caterpillars, or where patches 

 of dead and dying grass indicate that grubs are destroying the grass roots on 

 meadow or prairie. Birds flock to such places to feed on easily procured insects, 

 and to take a prominent part in repressing such insect outbreaks. This is so well 

 known as to be worthy of only passing mention here, were it not to inquire 

 whether the birds that assemble in such localities do not neglect their normal 

 and special work of holding in check certain species elsewhere. If the robin, for 

 example, which feeds normally on such ground-frequenting insects as white grubs, 

 cutworms, grasshoppers, March flies and ground beetles, goes into the woods to 

 feed on caterpillars, as is sometimes the case, does it neglect to devour any one 

 of the insects on which it usually feeds, and so give this insect a chance to increase? 

 If so, it would be merely suppressing one outbreak and permitting another. But 

 birds do not neglect any one element of their ordinary food in such cases. They 

 neglect them all, both animal and vegetable, for the time being, and turn to the 

 now abundant insect food that is more readily accessible. This I have observed 

 in studying outbreaks of cankerworms, and Professor Forbes records a similar 

 experience with birds feeding on cankerworms. 



Evidently in such cases the birds, changing their usual fare entirely for the 

 time being, remove their restraining influence from both useful and injurious 

 insects, leaving one to exert its full force as a check on the other until the urgent 

 business of the serious outbreak of grasshoppers, caterpillars or some other pest 

 has been attended to ; then the birds return to their usual haunts and food, and 

 exert the same rej^rcssivc influence as before. 



93 



