The Regulative Action of Birds upon Insect Oscillations. 21 



per cent, of the food of all the birds congregated in this orchard 

 should have consisted of a single species of insect, is a fact so 

 extraordinary that its meaning can not be mistaken. Whatever 

 power the birds of this vicinity possessed as checks upon 

 destructive irruptions of insect life, was being largely exerted 

 here to restore the broken balance of organic nature. And while 

 looking for their influence over one insect outbreak we stumbled 

 upon at least two others, less marked, perhaps incipient, bvit evi- 

 dent enough to express themselves clearly in the changed food 

 ratios of the birds. 



2. The comparisons made show plainly that the reflex eifect of 

 this concentration on two or three unusually numerous insects was 

 so widely distributed over the ordinary elements of their food 

 that no especial chance was given for the rise of new fluctuations 

 among the species commonly eaten. That is to say, the abnormal 

 pressure put upon the canker-worm and vine chafer was compen- 

 sated by a general diminution of the ratios of all the other ele- 

 ments, and not by a neglect of one or two alone. If the latter 

 had been the case, the criticism might easily have been made that 

 the birds, in helping to reduce one oscillation, were setting others 

 on foot. 



3. The fact that, with the exception of the indigo bird, the 

 species whose records in the orchard were compared with those 

 made elsewhere, had eaten in the former situation as many cater- 

 pillars other than canker-worms as usual, simply adding their 

 canker-worm ratios to those of other caterpillars, goes to show 

 that these insects are favorites with a majority of birds. 



