Part II.] NATURAL ENEMIES OF BIRDS. 215 



and young, and this might be equall}- true of squirrels and 

 weasels or crows and jays. 



Rats and mice, although they must be classed as enemies of 

 birds, do not constitute a menace to bird life if themselves 

 held in check by their own enemies, and they perform a great 

 service to birds by maintaining their own numbers at such a 

 point that they furnish a great surplus of individuals, and 

 become the main food supply for hawks, owls and other ene- 

 mies of birds. Therefore rats and mice, while thus serving as 

 food to maintain the numbers of birds' enemies also attract 

 the attention of these enemies sufficiently away from the birds, 

 and therefore tend to keep them from becoming too destructive 

 to bird life. 



As Professor Forbes says, the whole series of forces pressing 

 one on another is like an arrangement of springs, working one 

 against another, keeping all in place, thus maintaining the 

 general equilibrium and safeguarding the general welfare. 



USEFUL SPECIES MAY BECOME HARMFUL IF NOT HELD IN 

 CHECK BY NATURAL ENEMIES. 



There are certain bounds set by nature to the numbers of 

 each species within which it serves a useful purpose, but when- 

 ever through any fortuitous circumstances, such as may arise 

 by reason of man's interference, any species overflows this 

 high-water mark, normal habits may change and severe injury 

 to other species, even to man himself or his property, may 

 result. 



Animals considered serviceable to mankind, such as insec- 

 tivorous birds, toads, bats, shrews, etc., are useful if they are 

 kept within certain limits. The injurious species, so called, 

 are mainly those which tend to increase beyond normal bounds. 

 Then, by reason of abnormal multiplication and consequent 

 shortage of food, they become destructive. The so-called use- 

 ful species may become harmful under such circumstances by 

 changing their food habits when they outrun their usual food 

 supply. Insectivorous birds, mammals or insects may then 

 attack grain, fruits or other products of man's industry. No 

 one can tell what any animal may eat in case of necessity. 

 Carnivorous creatures then may devour grass, leaves or fruit, 



