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THE REGULATIVE ACTION OF 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 
