Hairy caterpillars, as a rule, are not relished by most birds, but two species, the yellow-billed cuckoo and the black-billed cuckoo, seem to prefer these to the smooth kind. Of some 109 stomachs of the yellow-billed species that were examined one was found to contain as many as 250 tent caterpillars, probably a whole colony in the young stage, and another 217 fall webworms, and this probably fell far short of the real number as these caterpillars are very small, and in many instances nothing but jaws remained undigested. These birds eat so many hairy caterpillars that the hairs, which are frequently stiff, bristly and sharp, sometimes pierce the inner lining of the stomach and remain there so that when the stomach is opened it is found to be lined with a thin coating of fur.
The cuckoos, despite their fondness for hairy caterpillars, do not by any means confine themselves to these insects but also devour beetles, grasshoppers, sawflies, and bugs of various kinds, most of them of a harmful nature. The cuckoos are extremely useful but unfortunately they are rather shy birds and prefer to remain about the edges of woodlands and groves rather than take to the more open cultivated grounds and orchards although if unmolested they sometimes gain enough confidence to frequent trees about houses and lawns.
So far I have said nothing of the many birds that are of use to us in helping to get rid of weed seeds. Preeminent among these seed eaters are the sparrows, inobtrusive birds which go about their work of doing away with vast quantities of weed seeds in a very efficient manner. Their work begins even before the seeds are ripe and continues throughout the fall and winter and even far into spring. In late summer the seed-eating habit of these birds is so noticeable as to attract the attention of even a casual observer, for by this time the seeds have ripened and the young sparrows which" have fed on an insectivorous diet are ready to turn to vegetable food. In winter, too, we may find flocks of juncoes and tree sparrows feeding in weedy fields, and it has been estimated that the tree sparrows consume per individual about one-fourth of an ounce a day and in a large agricultural state like Iowa some 875 tons annually. Only a farmer who must free his land from noxious weeds can realize what this vast consumption of weed seeds means in the saving and cost of labor. The above estimate is for a single species only, while, as a matter of fact, there are at least half a dozen species such as the white-crowned sparrow, the white-throated sparrow, the fox sparrow, the song sparrow and several others, that habitually feed during the winter on weed seeds. Some of these birds, moreover, range further south than the tree sparrow and junco so that over the entire country there is, during the colder months, a host of seed eaters at work reducing next year’s crop of useless plants.