No. 4.] BIRDS USEFUL TO AGRICULTURE. 59 



except, perhaps, the crow. The cut worms have no worse 

 enemy. 



It may be inferred from all of the foregoing that most 

 birds are believed to be useful to agriculture. 



Food Relations of Birds and Other Animals. 



And now a word about the food relations of birds. It is a 

 general rule that any animal or plant which, through circum- 

 stances particularly favorable to its multiplication, becomes 

 abnormally numerous, will at once, in the struggle for ex- 

 istence, become unduly destructive, and therefore a pest. 

 When crows, blackbirds, rabbits or any bird, mammal or 

 insect become too numerous, or when too many of any one 

 species are crowded too closely on a limited area, then only 

 do they become pests. What animal so harmless as a 

 rabbit ? Yet in Australia great tracts of land have been 

 made barren by the multiplication of these little animals. 

 Man himself tried in vain for years to check them. 



It is often said that birds destroy both useful and benefi- 

 cial insects or other animals, and that this fact detracts from 

 their usefulness. But it must be remembered: (1) that 

 the species of injurious insects out-number the so-called bene- 

 ficial insects enormously, therefore the vast majority of insects 

 destroyed by birds are the injurious ones, and that these 

 fewer beneficial insects alone could never hold the many 

 injurious insects in check; (2) certain beneficial insects 

 themselves would become injurious if not for the check put 

 upon them by birds which devour the surplus. 



One never can tell what any creature will eat if pressed by 

 hunger, or what change may occur in the food habits of any 

 species when that species becomes abnormally numerous. 

 Any arl)itrar3'^ classification of animals by their food habits, 

 as absolutely insectivorous, carnivorous, etc., will fail in 

 view of this fact. The animals themselves are not aware of 

 our distinctions, and refuse to be bound by them. In my 

 earlier years I was surprised to find the wolf, a carnivorous 

 animal, subsisting on berries, and swallows, insectivorous 

 birds, also living on the same food. Later we found a 

 climbing cut worm eating the pupte of the tent caterpillar. 



