90 The Food of Birds. 



of depredations upon the fruits of the garden and orchard, 

 and upon the grain in the fields. It is, of course, neces- 

 sary 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 injurious 

 if allowed to live. Much of the ajjparent evil for which 

 they are held responsible is also indirect; viz., the destruc- 

 tion of parasitic and predaceous insects which, if not 

 destroyed, would help to diminish the numbers of injurious 

 species. I wish, however, to call especial attention to the 

 fact that the regular mid continuous destruction of parasitic 

 and predaceous insects hy hirds 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 excessive and extraor- 

 dinary depredations of those whose numbers are subject 

 to wide fluctuations. Vegetation has become so far ad- 

 justed 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 imper- 

 ceptible.* It is far otherwise, however, with the vast hordes 

 of the Rocky Mountain locust, of the Colorado potato-beetle, 

 of the chinch-bug and of the army- worm, and many other 

 species which occasionally swarm prodigiously and then 

 almost disappear from view. The injurious species are 

 chiefly the oscillating 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 prevent the oscillation of a stable species is, therefore, 

 highly useful, while anything which tends to intensify an 

 oscillation, or to convert a stable species into an oscillating 

 one, is as highly pernicious. 



Now a species is stable because the rate of its reproduc- 



tSee Kirby and Spence's Introduction to Entomology, 4th ed., 1822, 

 Vol. I., pp. 247-258. 



