Amongst the Accipitres the kestril (Falco Tinnunculus, L.) devours abundance of insects. A friend of mine, upon opening one found its stomach full of the remains of grasshoppers and beetles, particularly the former, which he suspects constitute great part of the food of this species. One of the shrikes, also, or butcher-birds (Lanius Collurio)—and it is probable that other species of this numerous genus may have the same habits—is known to feed upon insects, which it first impales alive on the thorns of the sloe and other spinous plants, and then devours. If meat be given it, when kept in a cage, it will fix it upon the wires before it eats it. Lanius Excubitor also impales insects, but Heckewelder denies that it feeds upon them. If he be correct, the object of this singular procedure with that species, may be to allure the birds, which it preys upon, to a particular spot[519].
Amongst the Picæ or Pies the Crotophaga, called the Ani, which is a native of Africa and America, lives upon the locust and Ixodes Ricinus, which it picks in great numbers from the backs of cattle; but none are greater devourers of insects in this order than rooks. It is for the grubs of Melolontha, Tipula, &c., that they follow the plough; and they always frequent the meadows in which these larvæ abound, destroying them in vast numbers. Kalm tells us, that when the little crow was extirpated from Virginia at an enormous expense, the inhabitants would willingly have brought them back again at double the price[520]. The icteric oriole is kept by the Americans in their houses for the sake of clearing them of insects; and the purple grackle is so useful in this respect, that when, on account of their consuming grain, the American farmers in New England offered a reward of threepence a head for them, and they were in consequence nearly extirpated, insects increased to such a degree as to cause a total loss of the herbage, and the inhabitants were obliged to obtain hay for their cattle not only from Pennsylvania but even from Great Britain[521]. Of this order also is the bee-cuckoo (Cuculus Indicator) so celebrated for its instinct, by which it serves as a guide to the wild bees' nests in Africa. Sparrman describes this bird, which is somewhat larger than a common sparrow, as giving this information in a singular manner. In the evening and morning, which are its meal-times, it excites the attention of the Hottentots, colonists, and honey-ratel, by the cry of cherr, cherr, cherr, and conducts them to the tree or spot in which the bees' nest is concealed, continually repeating this cry. When arrived at the spot, it hovers over it, and then alighting on some neighbouring tree or bush, sits in silence, expecting to come in for its share of the spoil, which is that part of the comb containing the brood[522].—The wryneck and the woodpeckers, the nut-hatch and tree-creeper, live entirely upon insects and their eggs[523], which they pick out of decayed trees and out of the bark of living ones. The former also frequents grass-plats and ant-hills, into which it darts its long flexible tongue and so draws out its prey. The woodpecker likewise draws insects out of their holes by means of the same organ, which for this purpose is bony at the end and barbed, and furnished with a curious apparatus of muscles to enable them to throw it forwards with great force. Some species spit the insects on their tongue, and thus bring them into their mouth. In America, the tree-creeper is furnished with a box at the end of a long pole to entice it to build in gardens, which it is found to be particularly useful in clearing from noxious insects.
Amongst the Grallæ or Waders, many of the long-billed birds eat the larvæ of insects as well as worms: and they form also no inconsiderable part of the food of our domestic poultry, especially turkeys, which may be daily seen busily engaged in hunting for them, and, as well as ducks, will greedily devour the larger insects, as cockchafers, and in North America Cicadæ. Mr. Sheppard was much amused one day in July last year with observing a cow which had taken refuge in a pond, probably from the gad-fly, and was standing nearly up to its belly in water. A fleet of ducks surrounded it, which kept continually jumping at the flies that alighted upon it. The cow, as if sensible of the service they were rendering her, stood perfectly still though assailed and pecked on all sides by them. The partridge takes her young brood to an ant-hill, where they feast upon the larvæ and pupæ, which Swammerdam informs us were sold at market in his time to feed various kinds of birds[524]. Dr. Clarke also mentions having seen them, as well as the ants themselves, exposed to sale in the market at Moscow as a food for nightingales[525]. Latreille tells us that singing birds are fed in France with the larvæ of the horse-ant (Formica rufa).
But the Linnean order of Passeres affords the greatest number of insectivorous birds; indeed almost all the species of this order, except perhaps the pigeon-tribe, and the cross-bill and other Loxiæ, more or less eat insects. Amongst the thrush tribe, the blackbird, though he will have his share of our gooseberries and currants, assists greatly in clearing our gardens of caterpillars; and the locust-eating thrush is still more useful in the countries subject to that dreadful pest: these birds never appear but with the locusts, and then accompany them in astonishing numbers, preying upon them in their larva state. The common sparrow, though proscribed as a most mischievous bird, destroys a vast number of insects. Bradley has calculated that a single pair having young to maintain, will destroy 3360 caterpillars in a week[526]. They also prey upon butterflies and other winged insects. The fly-catchers (Muscicapa) and the warblers (Motacilla), which include our sweetest songsters, are almost entirely supported by insects; so that were it not for these despised creatures we should be deprived of some of our greatest pleasures, and half the interest and delight of our vernal walks would be done away. Our groves would no longer be vocal; our little domestic favourites the red-breast and the wren would desert us; and the heavens would be depopulated.—We should lose too some of the most esteemed dainties of our tables, one of which, the wheat-ear, is said to be attracted to our downs by a particular insect[527]. Lastly, insects are the sole food of swallows, which are always on the wing hawking for them, and their flight is regulated by that of their prey. When the atmosphere is dry and clear and their small game flies high, they seek the skies; when moist and the insects are low or upon the ground, they descend and just skim the surface of the earth and waters; and thus by their flight are regarded as prognosticating fair or wet weather. I was last summer much interested and amused by observing the tender care and assiduity with which an old swallow supplied her young with this kind of food. My attention was called to a young brood, that having left their nest before they were strong enough to take wing, were stationed on the lead which covers a bow window in my house. The mother was perpetually going and returning, putting an insect into the mouth first of one and then of the others in succession, all fluttering and opening their mouths to receive her gift. She was scarcely ever more than a minute away, and continued her excursions as long as we had time to observe her. When the little ones were satisfied, they put their head under their wing and went to sleep. The number of insects caught by this tribe is inconceivable. But it is not in summer only that birds derive their food from the insect tribes: even in winter the pupæ of Lepidoptera, as Mr. White tells us, are the grand support of those that have a soft bill[528].
I shall close my list of the indirect benefits derived from insects, by adverting to the very singular apparent subserviency of some of them to the functions of certain vegetables.
You well know that some plants are gifted with the faculty of catching flies. These vegetable Muscicapæ, which have been enumerated by Dr. Barton of Philadelphia, who has published an ingenious paper on the subject[529], may be divided into three classes: First, those that entrap insects by the irritability of their stamina, which close upon them when touched. Under this head come Apocynum androsæmifolium, Asclepias syriaca and curassavica, Nerium Oleander, and a grass described by Michaux under the name of Leersia lenticularis. The second class includes those which entrap them by some viscosity of the plant, as many species of Rhododendron, Kalmia, Robinia, Silene, Lythrum, Populus balsamifera, &c.[530] And under the third class will arrange those which ensnare by their leaves, whether from some irritability in them, as in Dionæa, Drosera, &c., or merely from their forming hollow vessels containing water, into which the flies are enticed either by their carrion-like odour, or the sweet fluid which many of them secrete near the faux, as in Sarracenia, Nepenthes, Aquarium, Cephalotus, &c., the tubular leaves of which are usually found stored with putrefying insects. In this last class may be placed the common Dipsacus of this country, the connate leaves of which form a kind of basin round the stem, that retains rain-water in which many insects are drowned. To these a fourth class might be added, consisting of those plants whose flowers smelling like carrion (Stapelia hirsuta, &c.) entice flies to lay their eggs upon them, which thus perish.
The number of insects thus destroyed is prodigious. It is scarcely possible to find a flower of the Muscicapæ Asclepiadeæ that has not entrapped its victim, and some of them in the United States closely cover hundreds of acres together.
What may be the precise use of this faculty is not so apparent. Dr. Barton doubts whether the flowers that catch insects, being only temporary organs, can derive any nutriment from them; and he does not think it probable that the leaves of Dionæa, &c., which are usually found in rich boggy soil, can have any need of additional stimulus. As nothing however is made in vain, there can be little doubt that these ensnared insects are subservient to some important purpose in the economy of the plants which are endowed with the faculty of taking them, though we may be ignorant what that purpose is; and an experiment of Mr. Knight's, nurseryman in King's Road, London, seems to prove that in the case of Dionæa, at least, the very end in view, contrary to Dr. Barton's supposition, is the supplying the leaves with animal manure; for he found that a plant upon whose leaves he laid fine filaments of raw beef, was much more luxuriant in its growth than others not so treated[531]. Possibly the air evolved from the putrefying insects with which Sarracenia purpurea is sometimes so filled as to scent the atmosphere round it, may be in a similar manner favourable to its vegetation.