90 
The Food of Birds. 
The direct injuries due to birds commonly take the 
form of depredations upon the fruits of the garden and 
orchard, and upon the grain in the fields. It is, of course, 
necessary to know the species chargeable with these, and 
the ratio which such injuries bear to the benefit likewise 
attributable to them. The good done by birds is almost 
wholly indirect, consisting chiefly in the destruction of 
insects which would become directly or indirectly inju- 
rious if allowed to live. Much of the apparent evil for 
which they are held responsible is also indirect ; viz., the 
destruction of parasitic and predaceous insects which, if 
not destroyed, would help to diminish the numbers of in- 
jurious species. I wish, however, to call especial atten- 
tion to the fact that the regular and continuous destruc- 
tion of parasitic and predaceous insects by birds is not 
necessarily an evil. Paradoxical as this statement may 
seem, it is* fully borne out by the following facts : 
The most serious losses of the farmer and gardener 
due to insects are not consequent upon the ordinary and 
uniform depredations of those species whose numbers 
remain nearly constant, year after year, but upon exces- 
sive and extraordinary depredations of those whose 
numbers are subject to wide fluctuations. Vegetation 
has become so far adjusted to our crickets and ordinary 
grasshoppers, etc., that the foliage they eat can be spared 
without injury to the plant, and the damage done by 
them is commonly imperceptible.* It is far otherwise/ 
however, with the vast hordes of the Rocky Mountain lo- 
cust, of the Colorado potato-beetle, of the chinch-bug and 
of the army-worm, and many other species which occa- 
sionally swarm prodigiously and then almost disappear 
from view. The injurious species are chiefly the oscillat- 
ing ones, and the dangerous species are those which 
show a tendency to oscillate. Anything which tends to 
limit the fluctuations of an oscillating species, or to pre- 
vent the oscillation of a stable species is, therefore, high- 
ly useful, while anything which tends to intensify an os- 
cillation, or to convert a stable species into an oscillating 
one, is as highly pernicious. 
* See Kirby and Spence's Introduction to Entomology, 4 tli ed., 1822, 
Vol. I, pp. 247-258. 
