The interactions of animals and plants upon each other and their relations to their inorganic environment present some of the most complicated problems that can occupy the attention of the philosopher and scientist. It is a well-recognized fact that any cause acting upon one group of animals or plants creates a change of some kind in other groups with which the first is in relation. The relations between species or groups are rarely more intimate than when one preys upon the other. If, through any cause, predaceous species become abnormally abundant, the species more extensively preyed upon will be reduced in numbers, and conversely, if predatory species become less numerous, the species they prey upon will increase in numbers.

Birds and insects constitute two groups so related, and as the latter include some of the worst pests with which the farmer has to deal, it may be profitable to study some of the relations between the two.

For centuries it has been a matter of general observation that the common singing birds subsist largely upon insects, and in modern times it has come to be generally understood that insects form one of the most important items of food for the great majority of birds.  This has been confirmed by careful scientific investigation. Many species are now known to live almost wholly on insects, and in some large groups of species the average quantity of insects consumed forms a high percentage of the whole food. In the family of woodpeckers, for instance, insects form over 65 per cent of the yearly food. In the California flycatchers examined, insects formed 96 per cent of the food; in the warblers of the same State, 94 percent; and in the wrens, 95 per cent. On the other hand, the stomachs of a large number of sparrows and finches from the Pacific coast contained an average of only 12 per cent of insects, while those of a like number of eastern sparrows and finches contained about 25 per cent of animal matter. Insects, however, constitute the larger part of the food of shore birds, the cuckoos, the goatsuckers, the swifts, the swallows, the vireos, the creepers, and the titmice; while in the stomachs of the crows and jays, the blackbirds and orioles, and the thrushes, the amount of animal and vegetable food is nearly equal. Many of the game birds and the smaller birds of prey eat a considerable amount of insects, especially during the breeding season.

While no series of stomachs of any species has yet been examined that did not contain some vegetable matter, it is probable that in many cases this element is purely accidental. When insects are picked from the ground, or from plants, small seeds or bits of leaves are easily taken and swallowed at the same time. In the case of the western yellow-throat, animal food amounted to 99.77 per cent of the whole contents of the stomachs, which shows the insignificance of the vegetable part, and tends strongly to prove that plant food was not sought at all, but was taken by chance with an insect morsel.

In view of the above facts, one is impressed with the conviction that the avian tribes must exercise an important influence upon the relative abundance of insects. That birds are an efficient check upon insect multiplication seems impossible of denial, and it is doubtful if anywhere in the animal kingdom any other restraining influence so important can be found. A predatory insect, for instance, actually destroys at best only a comparatively small number of insects each day; parasites, indeed, may deposit eggs in several hundred hosts, each of which eventually will be destroyed, so that under favorable conditions they may perform a very efficient service in checking the increase of insect pests. In many cases, however, the parasite does not prevent the immediate harm done by its host; it only prevents a future generation.

To illustrate the destructive capacity of birds it may be mentioned that from 3,000 to 5,000 insects have been found in a bird’s stomach at one time. It is true that birds are not so numerous as are the predatory and parasitic insects, but it is doubtful if this disadvantage is sufficient to overcome the advantage of greater size, with corresponding capacity for destruction.

Another point in favor of birds is their ability to travel long distances, so that in case of a local outbreak of any species of insect they are able to rally quickly to the spot and render good service in checking the further increase of the pest. On this point Professor Forbes says:

Especially does the wonderful locomotive power of birds, enabling them to escape scarcity in one region which might otherwise decimate them, by simply passing to another more favorable one, without the loss of a life, fit them, above all other animals and agencies, to arrest disorder at the start—to head off aspiring and destructive rebellion before it has had time fairly to make head.¹

That they do so is proved by numerous instances. Professor Forbes has shown that in an orchard badly infested with canker worms birds were abnormally abundant and had added to their usual food a very considerable quantity of these insects, as was shown by comparing the contents of their stomachs with those of others of the same species taken at other places.

Mr. O. E. Bremner, in a letter to the Biological Survey, dated at San Francisco, March 16, 1908, says:

The canker-worm episode is quite a common one with us here. In one district * * * there has been a threatened invasion of the prune trees several times, but each time the [Brewer] blackbirds came to the rescue and completely cleaned them out. I have often seen bands of blackbirds working in an infested orchard. They work from tree to tree, taking them clean as they go. If a worm tries to escape by webbing down they will dive and catch him in mid-air.

When the Rocky Mountain locust invaded the fertile plains of the Mississippi Valley, Professor Aughey found that it was preyed upon by every species of land bird, and even by some water fowl. Birds that normally fed upon other food, attracted by the unusual abundance of these insects, ate them freely and continuously while they lasted. The above facts prove that birds are attracted by an abundance of food ; and that propinquity as well as palatableness has some influence on the selection.

The point has been raised, however, that in the matter of insect consumption birds are indiscriminate, and eat insects without regard to species or to their economic significance. It has been asserted that in devouring useful insects birds counteract all the good they do by eating harmful ones. It is quite true that they destroy many useful insects. The Carabide, or predaceous ground beetles, are eaten by the ground-feeding birds, especially in the spring, when the birds first return from their southern migration. The useful parasitic Hymenoptera are eaten by flycatchers, and form a very respectable percentage of the food of some species. While at first sight this may appear to be an argument against the usefulness of birds, a broader philosophy will show that it is exactly what they should be desired to do.

Against the uprising of inordinate numbers of insects, commonly harmless but capable of becoming temporarily injurious, the most valuable and reliable protection is undoubtedly afforded by those predaceous birds and insects which eat a mixed food, so that in the absence or diminution of any one element of their food their own numbers are not seriously affected.²

Whoever expects to find in birds beneficent organisms working with a sole view to the benefit of the human race will be doomed to disappointment. Birds eat food to sustain life, and in their selection are guided entirely by considerations of their own. If all species of insectivorous birds be considered as a whole, it is found that they eat insects of the various species in about the proportions in which these species exist in nature. But it must not be inferred that each species of bird eats all kinds of insects to the same extent. Flycatchers and swallows, which take the greater part of their food upon the wing, eat largely of Hymenoptera, Diptera, and flying Coleoptera— insects which spend most of the daylight hours in flying about, and so fall an easy prey to the more agile species of birds. Ground-feeding birds, like robins, meadowlarks, and blackbirds, find and feed upon the predaceous ground beetles and other terrestrial Coleoptera and grasshoppers; cuckoos, orioles, warblers, and vireos find most of their food among the leaves of trees, and so destroy caterpillars and leaf-eating beetles; titmice, nuthatches, and creepers scramble over the trunks and larger limbs of trees, where they get insects’ eggs, pupæ, hibernating insects, small moths, and some beetles; while woodpeckers dig into both sound and rotten wood, from which they secure wood-boring larvæ and ants.

It is probable that no species of insect is so completely protected by its habits of life that it is not found and preyed upon, at one or another stage of its existence, by some species of bird. Even in those cases where so-called “protective devices” have been developed, investigation of the contents of the stomachs of many birds has shown that they are effective only to a limited extent; that in spite of protective coloration, protective or mimetic forms, nauseous odors, acrid secretions, and defensive armatures, insects so protected are found and eaten by birds, and in many cases form a considerable percentage of the average annual food.

Thus among Hemiptera the Pentatomide have a most nauseous smell and taste, as many discover when they accidentally take them into the mouth with a berry; in fact they have received the vernacular name of stink-bugs. It is evident, however, that birds do not find them nauseous or in any way disagreeable, for they eat them freely;Chlamys plicata and Exema conspersa, when they draw in their limbs and antenne, so nearly resemble lumps of dirt or droppings of a large caterpillar that only by the closest inspection can they be distinguished as living creatures, yet both of them frequently appear in the stomachs of birds. Many if not all species of snout beetles will, if disturbed, “play ’possum,” that is, fold up their limbs, press their snouts close to their bodies, and drop to the ground, where, in addition to being concealed by rubbish, they very closely resemble lumps of dirt or bits of twig. This resemblance, however, by no means always saves them from the keen eyes of the ground-feeding birds, and over 40 individuals of a single species have been found in the stomach of a blackbird. In fact the snout beetles as a group are among the insects most commonly found in birds’ stomachs.

From these considerations it would appear that the true function of insectivorous birds is not so much to destroy this or that insect pest as it is to lessen the numbers of the insect tribe as a whole—to reduce to a lower level the great flood tide of insect life. That this is the true relation of birds and insects should be inferred from the fact that the two have lived together for countless ages, and the balance of nature has been preserved except as disturbed by.the operations of man.  Birds have not wholly destroyed predaceous and parasitic insects on the one hand, nor on the other have they, so far as we know, exterminated any vegetable-eating pest, but they have successfully held the balance between the two, and kept both at such a level of relative abundance as has subserved the best interests of both the animal and the vegetable world; and it is only where man has interfered with this balance that oscillations have taken place which have resulted in damage to him and to the products of his labor.

Had birds preyed exclusively upon harmful—that is, upon vegetable-eating—insects, they, together with the predaceous and parasitic insects, might have, completely exterminated their natural prey.  In that case both birds and predatory and parasitic insects would be without their natural food, and in consequence must themselves perish, unless they could find some other source of subsistence. In the meantime, vegetation would have enormously increased, producing complications difficult to foresee. Fortunately birds eat insects indiscriminately, so that the two great opposing forces, the vegetable eaters and the birds and insects that feed upon them, are kept in a state of practical equilibrium. This is the ideal natural condition.

Man, however, when he settles in a new country, proceeds at once to overturn the natural equilibrium by cutting off the forests, plowing up. the prairies, draining the marshes, or irrigating the deserts, thus producing marked disturbances in the animal and plant life.  Some insects, deprived of their natural food, turn to the introduced plants, and in many cases find them more abundant and more palatable than their former food, and so thrive and increase rapidly.  The birds, not being able to multiply with such facility, are unable at first to deal with the greatly increased supply of food, except to the extent that they increase by migration from surrounding territory. Moreover, the seed and fruit eating birds have, like the insects, suffered a loss of their natural diet, and so turn to the farmer’s crops for their supplies. He, in turn, seeing his crops preyed upon on all sides, declares indiscriminate war upon all animal life; and as birds, being more conspicuous than insects, are more easily killed, he slays without consideration both those that feed upon his crops and those that prey upon the insect spoilers.

After years of misdirected effort, man is at last learning the lesson that Nature’s adjustments are not to be lightly set aside; that when undisturbed by his influence each species maintains a certain normal maximum of abundance at which it does the most good and the least harm; and that its fluctuations either above or below this normal are temporary and local—from which it follows that his best efforts should be directed to restore and maintain this harmony, and, in all places where he is obliged to disturb it, he should seek for means of counterbalancing the mischief. In the case of insect depredations, while more immediate remedies may be necessary at first, there is little room for doubt that the protection and encouragement of insectivorous birds offer, in most cases, the surest means of relief.

The objection that birds destroy useful and harmful insects indiscriminately also applies to most modern. insecticides. Spraying the trees for scales destroys the beetles which may be feeding upon the scales. When caterpillars are killed, either by spraying or by any other wholesale method, the larvæ of parasitic Hymenoptera which they contain are destroyed also.

The eminent French entomologist, Paul Marchal, writing upon this point, says:

Some authors, struck with the eminently useful role played by parasites in some invasions of insects, have gone so far as to advise the cessation of destructive measures in the fear of killing at the same time the parasites which they harbor or the predaceous insects which prey upon them * * * In the great majority of cases, on the contrary, it may be said that however useful the parasites may be, the fear of destroying them ought never to prevent the taking of all measures having for their object the direct destruction of harmful insects * * *. An intervention by destructive methods, far from being dangerous, would permit us, on the contrary, always to obtain a double result; first, it will immediately stop the damage and save the products of that year in a more or less complete manner, and second, it is not likely that in the great majority of cases the caterpillars will be more abundantly parasitized in that particular spot than in any other portion of the country. So that in destroying a certain number of nonparasitized caterpillars one will diminish for the whole region a number of possible adults, which would insure the generation of the following year, and that without changing the existing proportions between the parasites and the representatives of the injurious species.³

There is probably no way of destroying insects on a large scale not open to the objection that it is liable to kill friends as well as foes.  And it should be remembered also that nature destroys indiscriminately, and, as we have endeavored to show, thus produces in the long run the greatest good to the greatest number. Marchal has also taken the same view of the relation of insectivorous birds to insects that he has of the relation of parasitic and predatory insects to harmful ones. He says:

The assertion that insectivorous birds can cause more harm than good by attacking either the useful species or the larvæ parasitized by them does not appear to us well founded, and seems to us to be refuted by analogous argument. In spite of the theory formerly proposed by Perris and ably defended of recent days by Berlese and Severin, the protection of insectivorous birds appears to us not at all susceptible of thwarting the beneficial action of useful insects.⁴

That birds do little or no harm by eating insects indiscriminately may perhaps be better shown by an illustration. Let us suppose that half of all of the individuals of every species of insect in the world were suddenly destroyed ; half of the cotton boll weevils, half of the Colorado potato beetles, half of the chinch bugs, half of the codling moths, half of the innumerable host of other pests to the farmer and fruit raiser, and also half of the vast multitude of predatory and parasitic species swept away at one fell swoop. Is there any farmer or horticulturist who would not welcome such destruction? Would it not be a blessing to vegetation as far as cultivated crops are concerned? Many insects that are now troublesome would by this reduction be rendered comparatively innocuous, while in other cases the farmer would be able to cope successfully with the remainder. Now, this reduction would leave entirely undisturbed the internal relations of the insects themselves. The predatory beetles remaining would have proportionally just as many scales or larvæ to feed upon as before. The parasitic Hymenoptera would have just as many hosts to infest and the scales and larvæ would have just as many enemies to prey upon them. That a great increase of vegetation would take place is probable, but this would very soon be counterbalanced by the unusual supply of food offered to rodents and other herbivorous mammals, and in fact in a short time the insects themselves would, through the increased facilities for multiplication, resume their normal numbers unless there arose some other factor to hold them in check, such, for instance, as a great increase in the number of birds.

In closing the writer can not do better than again quote Professor Forbes:

To avoid or mitigate the evils likely to arise and to adapt the life of his region more exactly to his purposes, man must study the natural order as a whole and must understand the disturbances to which it has been subject.  Especially he must know the forces which tend to the reduction of these disturbances and those which tend to perpetuate or aggravate them in order that he may reenforce the first and weaken or divert the second.
   The main lesson of conduct taught us by these facts and reasoning is that of conservative action and exhaustive inquiry. Reasoning unwarranted by facts and facts not correctly and sufficiently reasoned out are equally worthless and dangerous for practical use.