RELATIONS TO ANIMATE ENVIRONMENT 113 



of ingested seec^ is retarded, and this fact, as De Vries has 

 shown, tends to produce mutation. 



While, in considering the relation of birds to their animate 

 environment, it is easy to show that they undoubtedly exert a 

 modifying influence upon the plant world, it is by no means 

 so easy to demonstrate the fact that they play a similar part 

 with regard to the animal world, including their influence on one 

 another. Yet such is the case, though from the nature of the 

 evidence this influence rests largely rather on deduction than 

 on an array of stereotyped facts. 



That many groups of invertebrates with enormous powers of 

 multiplication are kept in check by birds, there can be no 

 doubt, and nowhere is this more certain than in the case of the 

 Insecta, for a vast number of species of birds subsist entirely on 

 an insect diet ; and to these must be added a number of species 

 which during the breeding season at least become insect de- 

 stroyers. 



No less important is the part played by birds in keeping 

 down the numbers of such prolific Mammalia as the rodents. 

 How real is this check may be seen during times, when, gener- 

 ally from man's stupid interference in killing off these police of 

 nature, these mammals increase till they become a veritable 

 plague. The recurrent vole plagues in Great Britain may serve 

 as a case in point. In the last of these, which occurred in the 

 South of Scotland and North of England during 1 890-92, the 

 common field-vole {Microtus agrestis) multiplied to such an 

 appalling extent as to threaten the farmers of the infested districts 

 with ruin. And by far the most powerful agent in the final 

 suppression of these invaders was the work of birds of prey, 

 especially of Kestrels and Short-eared Owls. The latter in- 

 deed developed a remarkable fecundity under the stimulus of 

 this plenty. Normally laying from four to eight eggs, clutches 

 of thirteen now appeared. And before the end of the plague it 

 was estimated that no less than 400 pairs of Short-eared Owls 

 alone were finding sustenance in the stricken area. Further, it 

 is said that not only were clutches of eggs enormously increased 

 in size, but that at least two broods were produced by each 

 female during this time. 



But for the work of these birds the whole face of the country 

 round might well have been changed. 

 8 



