. [Articles about birds from National geographic magazine]. Birds. The Policemen of the Air BIRDS THAI' IvAT INSECTS Alany birds, as flycatchers, warblers, swallows, and chimney- swifts, live exclusively, or almost so, on insects, and very many more, as blackbirds, orioles, and some hawks, depend on them for a considerable part of their liveli- hood. The little sparrow-hawk lives very largely upon grasshop- pers, crickets, and beetles, and even one of the larger hawks— the Swainson ha\\k of the west- ern plains—at certain seasons de- stroys enough of these injurious insects, together with smal
. [Articles about birds from National geographic magazine]. Birds. The Policemen of the Air BIRDS THAI' IvAT INSECTS Alany birds, as flycatchers, warblers, swallows, and chimney- swifts, live exclusively, or almost so, on insects, and very many more, as blackbirds, orioles, and some hawks, depend on them for a considerable part of their liveli- hood. The little sparrow-hawk lives very largely upon grasshop- pers, crickets, and beetles, and even one of the larger hawks— the Swainson ha\\k of the west- ern plains—at certain seasons de- stroys enough of these injurious insects, together with small ro- dents, to save the western farmer upwards of a hundred thousand dollars a }ear. If all insects preyed upon vejre- tation, our inquiry into the value of insect-eating birds need go no further, since all of them might be set down as beneficial; but by no means all insects are destruc- tive of vegetation, and their relations to each other and to birds are very complex and puzzling. The insects that feed on vegetation at some stage or other of their existence probably outnumber all others, both in number of species and of indi- viduals ; but there are two other classes of insects which deserve attention here, the predaceous and the parasitic. The predaceous insects, either in the adult or larval state, feed upon other insects and hence in the main are beneficial. It would seem, therefore, that in so far as birds destroy predaceous insects they do harm. That birds do destroy a greater or less number cannot be denied, but as many species of this group secrete nauseous fluids, which serve, in a measure at least, to protect them, and as many are of retiring habits and not readily found, the number de- stroyed bv birds is relatively not large. Moreover, some of the predaceous in- sects, when insect food is not available, become vegetarians, and hence assume the role of enemies of the farmer; so that when birds destroy predaceous insects. From the Biological Survey A BARRED OWL
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