THE REGULATIVE ACTION OP BIRDS UPON INSECT 

 OSCILLATIONS. 



By S. a. FORBES. 



Attention has already been repeatedly called in these studies to 

 the fact (fundamental to this investigation) that the principal inju- 

 ries due to insects are done by a few species, existing, for a time, in 

 numbers far above the average, and soon to retire again to a much 

 lower limit. As the number of a species which reach maturity is 

 determined by the checks on its multiplication, it follows that these 

 oscillating species are held in check by variable forces, and to the 

 variations in these checks we must look for an explanation of 

 their oscillations. On the other hand, we must expect to find 

 that those insects whose numbers remain relatively constant from 

 year to year are under the control of restraining influences of a 

 much more uniform character than the preceding class. 



Concerning the effects of birds upon insect life, and through 

 this upon the interests of agriculture, there are therefore three 

 questions to answer : — 



1. Do birds originate any oscillations among the species of 

 insects upon which they feed ? That is, are their food habits ever 

 so inconstant from year to year that species which are at one time 

 principal elements of their food, are at other times neglected and 

 allowed to multiply without restraint ? 



2. Do birds prevent or restrain any oscillations of insects now 

 noxious, or capable of becoming so if permitted to increase more 

 freely ? That is, do they bring to bear upon any such species a 

 constant pressure so great that those insects would increase 

 unduly if this pressure were removed by the destruction of the 

 birds ? 



3. Do they do anything to reduce existing oscillations of inju- 

 rious insects ? Do they sometimes vary their food habits so far 

 as to neglect their more usual food and take extraordinary numbers 



