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largely on cutworms, as well as on the white grubs of the May 

 beetle. When these insects are few in number, a part of the 

 usual food supply of both robin and crow is cut off. This being 

 the case, the hungry crows would be likely to destroy more 

 young robins and other young birds than usual in order to make 

 up the supply of animal food for themselves and their ravenous 

 nestlings. This, again, would decrease perceptibly the number 

 of robins and other small birds, and would be likely in turn to 

 allow an increase of May beetles and cutworms. Should these 

 insects become more plentiful, the crows would naturally turn 

 again to them, neglecting the young of robins and other birds 

 for a time, and allowing them to increase once more, until their 

 multiplication put a check on the insects, when the crows would 

 of necessity again raid the robins. 



The blue jay may be taken as another instance of this means 

 of preserving the balance of Nature. Hawks and owls kill blue 

 jays, and crows destroy their eggs and young; thus the blue 

 jays are kept in check. Jays are omnivorous feeders. They 

 eat the eggs and young of other birds, particularly those of 

 warblers, sparrows and vireos, birds which are active cater- 

 pillar hunters. But jays themselves are extremely efficient 

 caterpillar killers. They atone in great measure for destroying 

 other caterpillar-eating birds when they (the jays) turn to 

 killing caterpillars which have increased in numbers in conse- 

 quence of the destruction by jays of eggs and young of smaller 

 birds. Like the crow, they virtually kill the nestlings 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 may perhaps again allow an increase 

 of the smaller birds, the survivors of which they have indirectly 

 furnished with more insect food, thus making conditions favor- 

 able for their increase. These oscillations, or alternate expan- 

 sions and contractions, in the numbers of birds or insects often 

 are so slight as to escape common observation. It is only in 

 those cases where the alternations are carried to extremes that 

 they result disastrously. Under Nature the checks on the in- 

 crease of birds are essential, else birds would multiply until their 

 food supply became exhausted, when they would starve, and 

 other consequences much more complex and more serious to 

 mankind would quickly follow. 



