Popular Science Monthly/Volume 30/March 1887/Birds and their Daily Bread
By WILLIAM MARSHALL.
OF all animals, birds possess the quickest motions, the most energetic respiration, and the warmest blood, and they consequently undergo the most rapid change of substance, and need the most food. Although few creatures are so pleasing to the aesthetic tastes of a poetically inclined person as birds, the breeder knows that most of them are to be looked upon as hearty or excessive eaters. Any one who closely observes birds and their conduct will soon remark that all their thoughts and efforts, aside from the few days they spend in wooing and their short periods of resting, are directed to getting something to eat. With what restless earnestness do titmice plunge through the bushes and the trees! Not a leaf is uninvestigated, every chink in the bark is examined for whatever eatable it may be hiding, and a sharp look is cast into every joint of a branch. How industriously does the ousel turn and thrash the leaves on the ground of the woods all the day long, spying its game with a glance of its sharp eye, and snapping it up on the instant! After observing a few such incidents we can easily believe the stories that are related of the fish-eating powers of the cormorant, and of the fruit-eating birds that are able to consume three times their weight every day.
The result of this property of enormous appetite is an intensified activity in the competition for food among birds, and the structure of their bodies and their habits have undergone considerable modifications in consequence of the fact. It is this which has compelled some birds that should be, according to common poetic conception, creatures of the day, to hunt their prey by night. There are many transitions or connecting links between day-birds and night-birds. Day-birds may sometimes be seen pursuing their prey till late in the twilight; and, on the other hand, night-birds, impelled by hunger, will leave their hiding-places while it is still day. Chimney-swallows are often observed of summer evenings circling around high in the air, in company with the bats. The corn-kite is likewise fond of hunting in the dusk, and is late in retiring to its roost. Several owls do not shrink from the clear light, and the Strix coquimba of the Chilian coast hunts only by day. Most evidently the northern snow-owl must do its hunting in the bright glare of the sunlight, else, if it were too particular about the matter, it would become very hungry during the long polar day.
The orders of night-birds are not very numerous, and only a few plant-feeders are known among them; but many species are represented among the darkness-loving forms, constituting groups like the owls among the birds of prey and the night-swallows, or in single species, like the strigops and apteryx of New Zealand. In the great family of the waders the stone-curlew, the bittern, the snipe, and others are nocturnal, while the black skimmer and the stormy petrel, among the swimmers, partially exhibit the same peculiarity. Those among these birds which have best adapted themselves to the nightlife, however little they may otherwise be related to one another, exhibit a number of peculiarities in bodily structure. The eyes of all of them have undergone some obvious modifications. They are large, capable of considerable widening of the pupil, and otherwise especially differentiated in the elements of the retina. A degree of uniformity also prevails in the disposition of the plumage of all these forms; it is monotonous in its coloring, brownish or greenish gray, but never striking or lively, and this for closely related reasons. First, by means of these dull and therefore protective colors night-birds are enabled to live nearly hidden during the day, and are thereby saved from many disturbances and annoyances; and, secondly, because, in the nature of things, sexual selection, determined by the coloring of the feathers, can not exert any modifying influence upon them. What use has the male night-swallow for beauties which the female can not appreciate? The plumage of the real night-birds is also peculiarly soft and dense, so that their movements in the air are almost perfectly noiseless, and so ghost-like in their silence that they are regarded with superstitious awe by the natives of almost every land.
The food itself of birds is of the most diversified character. While some of the tribe are omnivorous, others appear to be adapted to special kinds of food. Usually forms may be distinguished which feed either exclusively on animal or on vegetable meats, or on both. With most birds the last is, in a greater or less degree, the case. Even the so-called "noble" birds of prey occasionally eat berries and herbs when compelled by necessity. Only a few classes of the animal kingdom, perhaps only those which live entirely in the depths of the sea, fail to contribute of their bodies and lives to the repasts of birds; but vertebrates and insects are their favorite game. One bird will eat only living game which it has caught itself, another only carrion, and a third both. Falcons, eagles, hawks, and owls feed upon living, usually warm-blooded creatures; and, while the presence of these may in some cases be regarded as a calamity, it may in other cases be looked upon as a benefit. Alturn, by examination of the hair-balls of indigestible remains of food ejected through the mouth, found that the smaller owls are of the highest advantage in agriculture on account of the prodigious numbers of field-mice they destroy. Yet the farmers kill these benefactors, and nail their skins upon their sheddoors as trophies and admonitions. This is bad enough, even though it be done in ignorance and not in malice. Toads and creeping things are pursued by numerous wild birds—the snake-eagle, the secretary, the curious South American cuckoos called saurothers, and many other feathered foes. The dumb host of fishes finds eager hunters among the birds, even more than among men. Cormorants and pelicans, gulls and sea-swallows, petrels and albatrosses, have made themselves masters of the sea in a higher sense than the most formidable pirate, for the latter is limited to the water, while the birds, not only by swimming and diving snatch the inhabitants of the waters from their own element, but they also capture those fish which, leaving the water for a moment to escape the Scylla of danger from other fishes, throw themselves into the air and into the power of a hungry and greedy Charybdis. And where is any fish in fresh water safe from the pursuit of birds?
No shell or other shelter affords adequate protection to the poor mollusks; but they, too, have to find their way through the maws of birds into the circulating stream of matter. It is true that they usually serve only as a temporary substitute for something else, and few of the inhabitants of the air live exclusively upon them; of these are those curious finches, so like the haw-finch, that live in the Galapagos Islands, descendants from South American castaways, which found no corn-grains on the rocks, but a nutritious though unaccustomed substitute in the shell-fish of the beach, whose shells their tough bills could crack with ease.
The hosts of birds consume immeasurable quantities of insects every day. In no place and no condition are creatures of this class secure from their pursuit. The fat larva, leading a tranquil existence in the depths of decaying wood, is the prey of the sagacious woodpecker. The portly spider, in her dark corner, does not escape the penetrating eye of the redstart. Little beetles and gnats, carried up into the air by ascending currents, are a welcome game to the high-flying swallows.
No more than animals can plants escape the demands of the birds' appetite. Of whatever plants can furnish, excepting only the wood, the birds take their tithes. Humming-birds, besides the nectar, devour the insects that go in after it. The Australian licmetis digs into orchid-roots and lily-bulbs with a bill admirably adapted to that use. The Polyborus chimango, of Chiloe, digs up freshly planted potatoes, to the ruin of the farmers, and eats them with relish. Some South American birds feed on aromatic leaves, and the New Zealand nightparrot prefers a delicate liverwort to all other food. But so slight is the nourishing power of this plant that the bird has to consume immense quantities of it, and his crop is swelled out after each meal enormously. Seeds, berries, and fruits furnish a most welcome subsistence to innumerable flocks of birds in all parts of the world. But it makes a great difference to the plant species whether a bird prefers a fruit for its flesh or for its kernel. The yellow thrush and cherry-finch are both fond of cherries—one for the sake of their juicy envelope, the other for the piquant meat of the pits. In the latter case harm comes to the plant; in the former case good. The colored, fragrant, sweet-tasting berries are there to be eaten, and that principally by birds, while the seeds pass out undigested and with vitality unharmed, but rid of their coverings, which would have to be consumed by decay if not eaten, and thereby, in some cases, even better adapted for planting and growing than before.
The birds in this way contribute much to the distribution of plants. For the appetizing reward which they enjoy, they afford the species, if not the single tree or plant, the most important service; and when we say that this bird damages the cherries and that one the grapes, we speak from a selfish point of view that which is fundamentally false, for the birds are ultimately useful to the plants. The Smyrneans call the rose-starling the devil's bird when it visits their fruit-trees in July, and forget that they welcomed the same bird as a saint when it cleared away the locusts in May.
Swallowed seeds undergo a superficial change in their passage through the digestive apparatus, which is, however, not detrimental, but rather, like the maceration which the gardener gives to his seeds, only favorable to their vegetation. Acorns which the nut-hatch has macerated in its crop, when dropped, are much surer to grow than those which the forester plants. Some plants are absolutely dependent on birds for their diffusion, and a kind of mutualism exists between the two—one of those remarkable phenomena in which very different kinds of beings are reciprocally advantageous and adapted to one another. A carious example of such adaptation is seen in the case of the mistletoe and the mistle-thrush. The mistletoe, a parasite of trees, is green and evergreen, though few other parasitic plants are so colored; and the color does not perform the same office to the mistletoe as the green of other plants, for this plant derives its nourishment directly from the sap of the tree on which it is growing without the help of chlorophyl. But it has its uses, one of the chief of which is to attract the thrush to itself. Its fruit-bearing time is when the host-tree has shed its leaves, and it, being all upon the trunk that is green, is conspicuous. Its fruit, a berry, inclosing the seeds in a tough, adhesive envelope, is eagerly sought by the thrush, which can recognize it from a distance by means of the sign it throws out. The seeds hang by their sticky gum to the bird's bill after the fruit has been eaten, and the bird, to remove the nuisance, is obliged to rub its bill against the bark of the tree. This enables the seeds to plant themselves in the crevices of the bark, which afford just the soil they need to sprout and grow in.
Strawberries are a choice delicacy for the wood-hen, and their seeds are minute enough to escape being ground up among the millstones of its crop. The fowls, therefore, unknowingly sow the seeds in the forest along with their dung, and help the plant to find new beds in fresh soil. Geese are especially fond of the leaves of Potentilla anserina, and with them eat multitudes of the minute seeds of the plant. Thereby, while the natural home of the potentilla is by brooks, it is transplanted by the geese to quite different localities; and in mountainregions, where nearly all the towns are on little streams, the potentillais to be found as far away in the neighboring fields and along the borders of the woods as the geese are driven in the fall to feed upon the stubble.
When the Spaniards settled in Chili they brought with them their native apple and other fruit-trees. The fruits of these orchards, which were planted, of course, only in the neighborhood of dwellings, were fallen upon by the native parrots, and they carried the apples and undigested seeds into all parts of the country, so that the traveler may now find whole forests of wild apple-trees in places which have received no other touch of human cultivation. In a similar way crows have spread the opuntia over the uninhabited islands of the Canaries, and the nut-crackers in the Alps the seeds of the vetch in places where the winds and man could never have carried them.
Not a few birds are impelled, by the instinct to get their food in as much security as possible, to form relations with other animals. Starlings are found frequently associated with flocks of sheep, making themselves at home on their backs and playing the part of selfish benefactors of the suffering animals as they explore their wool for appetizing ticks and lice. So there are birds, according to eminent travelers, in Africa that perform similar service for elephants, camels, horses, the rhinoceros, and the hippopotamus. These prudent and vigilant birds are ever to be found in company with their huge providences, and give the sleeping monsters timely warning, with their shrill, familiar cries, of approaching danger. Lichtenstein relates that ostriches and zebras are on good terms with one another, to their mutual profit. The droppings of the zebra afford a breeding-ground for innumerable beetle-grubs, and these are a choice delicacy to the ostriches. When any danger approaches the company—if, for instance, a horseman appears in the distance—it is at once perceived by the tall, sharp-eyed birds, which take to flight in a direction away from the threatening object; and the dull-sighted zebras, without knowing what is really going on, sagaciously intrust themselves to the guidance of their careful associates. Similar harmonies in behavior may be observed among animals in very remote quarters of the world—between the rheas and the stags and guanacas of Brazil, and between the royal pheasant and the wild goat of the Caucasus. The ancients had a story of a bird—the trochilus—which made its living by picking the teeth of the crocodile, and in return warned it against its enemies, which has been laughed at as fabulous; but, in the light of modern observations, there is nothing incredible in it.
Hunger sometimes causes birds to take to the most unaccustomed foods. When sparrows gather around fresh horse-dung and turn it over with evident relish, it is on account of the undeconiposed oats it contains; but there are real coprophagists or dung-eaters among birds. Some vultures, according to Brehm, live chiefly on human excrement, and one of the handsomest gulls of the far North feeds largely on the droppings of seals and walruses.
There are aberrations of taste among birds as well as among men and other animals, the exact origin of which can be definitely traced out. Tame birds—geese, fowls, and doves, the last of which in particular are strict vegetarians—can, by the "withdrawal of their accustomed grain, be won over to the enjoyment of animal diet till they prefer it, and may finally have their new taste so highly cultivated that they will spurn vegetable food. These changes do not mean much when they are compulsory; but they become very significant when they are wholly voluntary. Our common sparrow has within a few years gained legal, scientific, and even political fame by reason of its change of food-habit; and, amid all the controversy that has raged about its character, it is clear that it has become extremely ravenous and destructive to broods of young song-birds. Yet the sparrow is originally an eater of animal food, living on larvæ, worms, and snails, from which the step is not so very great to the devouring of a little wee, helpless chick; but the perversion of taste which the New Zealand kea (Nester productus) has undergone is of a different character. The nature of its home as well as its organization made it clearly a vegetable feeder, and there was no apparent occasion in its normal state for a bloodthirsty taste or a predatory inclination to arise within it. "With the introduction of sheep-raising in New Zealand the bird's nature seems to have been changed. It learned to feast upon the blood of the slaughtered animals, and, having once tasted blood, it conceived such a relish for it that it was no longer able to satisfy its appetite with the occasional slaughter of a sheep, but began itself to take the initiative by falling upon animals which were suffering from slight wounds and opening the sores to make the blood flow. From this it advanced till it learned to help on the accident and make wounds for itself.
Birds living naturally on flesh are sometimes turned, without any stress of necessity, to become vegetarians. A considerable number of insect-feeders in Southern Europe eat fruits, especially figs, with evident relish, and the habit abides with them not only there, but also in other regions when they pass through them. I once became very indignant in Corfu at seeing an old, lank fellow, with a fowling-piece likewise old and lank, shooting the little singing-birds, among which I recognized my particular favorite, with unerring aim. On my remonstrating with him, he said they were fig-peckers and destroyed the fruit. I laughed at him, and, to convince him that he was wrong, took out my pocket-knife and cut open one of the dead birds. It was his turn to laugh, for the maw of the bird was distended to the full with the soft, seedy fig-pulp! The green woodpecker, whose whole structure should make it a first-class insect-eater, is very fond of service-berries, and is, according to Pallas, destructive to the grapes around Astrakhan. Another woodpecker is fond of hazel nuts, and has learned how to crack them; but it may be that it was first introduced to this sort of food by exploring the nuts for worms. In fact, there is great room for the development of individual tastes in birds. But there are limitations in the matter, and the maxim "What is one man's meat is another's poison" applies well in this case. Housedoves can eat beans without harm, but geese after partaking of them sicken and sometimes die. The cause of the difference is mechanical. The doves grind up the beans in their crop, while the geese have to digest them, and are sickened or killed by the swelling they undergo in the process. A real physiological puzzle is presented, however, in the case of some chemically acting poisons which a few animals, including some birds, can partake of with impunity, while they are deadly to all other creatures.
The birds' work is not always done when they have captured their food. Sometimes they have to prepare it to a certain extent. Many of the smaller birds of prey pluck their game before eating it. Others wash their meat, as I once saw a stork do; and while this queer bird would swallow frogs without any ceremony, it gave the mice that were thrown to it a thorough soaking, probably because their hairiness interfered with the comfortable swallowing of them. A sagacious canary-bird I once knew had learned the art of soaking in its drinking-vessel the crumbs that were too hard for its bill to break up. Shell-fish give birds much trouble, but the heron proves itself a match for them by swallowing them whole, shells and all, and keeping them in its maw till the animals are killed by the pressing in of the digestive fluids, and the shells open; then it spits them up and feasts upon the soft parts. Crows carry them up into the air and drop them upon stones, whereby the shells are broken up. The 1am mergeyer, according to Krüger's observations, does the same with marrow-bones and turtles; and it is possible that the eagle that killed Thales by dropping a tortoise upon him, mistook the shiny skull of the philosopher for a big, extra hard stone!
Storing up of food is quite a common practice in the bird-world. A European species of nut-hatch collects hazel-nuts when they are abundant, and hides them in the hollows of trees; and the enow-hen of Greenland does something of the same kind. The pine nut-cracker collects vetches, and the common nut-hatch, which appears frivolous enough in its every-day life, acorns. One of the most remarkable instances of avian providence is that related by Saussure, of the Mexican Colaptes. The sides of the extinct crater of Pizarro, in Mexico, are covered during the winter months with the dried-up hollow stalks of the last year's agave-flowers, over which a yucca-tree here and there casts a scanty shadow. Hither, at the time of the ripening of the acorns, come large flocks of the birds with acorns, which they have brought from a great distance, for no oak-tree grows within many miles of the spot. Beginning at the bottom, they bore with their deft bills a succession of holes, at short distances apart, in the hollow agave-stems, and through them deposit their acorns in the interior cavity. The acorns look, when the magazine is filled, laid one after another, like the beads of a rosary. When the time of scarcity arrives, these insect-eating birds hasten to their store-houses, extract the acorns from them, fly with them to the yucca-trees, and, boring in the bark of them holes large enough to hold the acorn, as an egg-cup holds an egg, break open the fruit and eat it in comfort. Saussure avers that the acorns in question are sound, and free from worms.
Other birds, not looking so far into the future, provide only for temporary wants. A species of owl, according to Naumann, feeling the approach of a storm, lay up a stock of mice to last them through the nights when they will be unable to hunt. When the red-backed shrike has caught more than its appetite demands, it spits the surplus—young birds, frogs, and larvæ—on thorns; whence it has been called the thorn-turner, or, because the people believe that the number of its victims is always nine, the nine-killer. Curiously enough, a relative of this shrike, the Collurio Smithii, of Africa, employs, to accomplish a similar object, the more difficult method of slipping one end of a plant-fiber around the victim's neck, and hanging it by the other end to a bush, thereby giving its store-room a kind of resemblance to a gallows-yard. Not only are these lesser special peculiarities, these side-features of the bird's habits, as we might call them, determined by the kind of food and the method that has to be employed to obtain it, but more important and fundamental features of its life, its migrations, its distribution, and many of the motives of its propagation-history, can be traced back to them. The origin of the migratory instinct has usually been sought in climatic causes, and in the desire to avoid the cold and hardships of northern winters. This is an error. The little titmice and modest wrens, which are able to find the most carefully hidden larva in its winter-quarters, and can discover the most minute insect-egg, and which will not disdain a berry or a seed, stay with us through ice and snow; but the larger and stronger cuckoo remains in the north only while insect-life is at its height, and starts early on its southward journey. The great shrike is a permanent resident in Europe, going away only rarely, in case of dire extremity, but its three indigenous relatives are real birds of passage, appearing in May and leaving again in August. The former bird is large and strong enough to capture other grown birds and mice, and can thus always find a bounteous table spread for itself, while the others, confined by their more limited powers to smaller creatures, have to go away when, with the end of summer, these fail. The carrion-crow, to which everything that it is only possible to swallow furnishes a delicious feast, is not troubled by the severity of winter; but the rook, whose bill of fare is more limited, is in no condition to endure the scarcity of the winter months, and, therefore, on the approach of fall, it flies in large flocks over the Alps and Pyrenees, to the luxuriant fields of Southern Europe and Africa.
While this brief glance at the connection between the yearly migrations of birds and their food-supply must suffice for that point, the consideration of the distribution and accidental wanderings of these remarkable creatures, so far as it is related to their commissariat, demands more attention. It is evident that an animal species can live and thrive only when its accustomed provision, or some substitute similar in character to it, is present; where this is wanting, it can not gain a permanent footing; or it must adapt itself to its new relations, undergo gradual modifications in its habits and organization, and thus in the course of time become another and new variety. But wherever it can find its accustomed subsistence in the usual quantity and season, it will readily make itself at home without having to undergo any modification, unless it is provoked by other causes. If we ask what creatures—what birds, in the present case—have under particular circumstances the greatest chance for a wide distribution, the answer will be that they are those which, like the crow, the thrush, and the true shrike, are least particular in the choice of what they eat; and next, those whose appropriate food is most widely diffused. But the more closely a bird is adapted to particular kinds of food, and the more limited the circle of its distribution, the narrower will be the field of its residence. This is the case, not only with species, but in a wider sense with genera and families, and it is not to be overlooked that very widely distributed species are also frequently permanent residents. The most uniformly distributed animals under all conditions of climate and season are vertebrates, especially fishes and the smaller land-mammalia, and also some insects; and they also exhibit less important and diversified variations, and demand less on the part of their pursuers, than insects. Next to these are certain invertebrates of fresh and salt water; and among these the shore-species, particularly within their seasonal limits, display a great uniformity in all parts of the earth. Birds of prey, except the carrion-eaters, live on land-vertebrates. Three of their four families include quite or nearly cosmopolitan genera, and, what is rare among land-birds, only three species are wanting in very small districts. Fish and water animals constitute, with few exceptions, the food of the large groups known as the waders and swimmers. Of the fourteen families of the former groups, four are quite and one nearly cosmopolitan; and of the eight families of the latter group, among which are some powerful fliers, only three have not a universal diffusion; and the two groups exhibit more genera occurring over nearly the whole earth (eighteen out of ninety-six and eleven out of seventy-nine) than all the other families of birds put together. But we can not overlook the fact that some of these genera stand on a very feeble footing.
It is interesting to observe that, in the other orders of birds, those kinds that live on fish are very widely spread. Thus, the kingfishers are cosmopolitan, and the genus Ceryle, which Brehm says includes "the strongest, most active, and most ravenous members of the family," is especially wide-spread, and is without representatives only in Australia, Madagascar, Europe, and Xorthern Asia. The nine not greatly differing species of water-ousel, whose habits are much like those of the true kingfishers, are very widely diffused wherever there are cool, clear, and rapid mountain-brooks.
The distribution is less wide of such birds as live principally on land-invertebrates, especially on articulates. Their occurrence is largely dependent, like that of plant-feeders, indirectly but intimately on the character of the vegetation; and those forms among them which, like titmice, wrens, and woodpeckers, live on insects at rest, as well as on eggs and pupæ, or on larvæ, either concealed or living in wood, and changing place but little, have the most extensive range; and, since larvæ pass the winter in one of the forms mentioned, they are permanent residents in temperate climates. In the measure that a bird pursues perfect insects, particularly those which fly, it becomes possible for it to be an established inhabitant only in warm climates. To dwell in colder regions, if it usually refuses vegetable food, it must be a good flier, and is then compelled to go away on the approach of the cold season and the accompanying disappearance of its food.
The presence of plant-eating birds is still more predominantly dependent on their food; and it is a matter of interest to observe how the opening out of the vegetation of a country reacts upon its ornithic population. Africa, wherever it is not wooded or desert, the Europeo-Asiatic steppes, and the prairies of America, are covered with grass and other mealy-seeded herbs, and are also the dwelling-places of hosts of weaver-birds, larks, and other grain-eaters, the flocks of whose numerous species are numbered by the thousand; and wherever they can find their food in winter, they abide. Groups of berry-bearing plants cover extensive regions of the north, and follow, as they stretch down toward the south, the cooler regions of the higher mountains; and in their suite we find everywhere, in the tundras of Siberia, on the slopes of the Himalayas, and in the Peruvian Andes, fruit-eating birds of similar genera. In the fruit-rich woods of the tropics, especially of South America, more than half of the native birds, though representing very different families, live chiefly, and for the most part exclusively, on the fruits; a thing that would not be possible in a temperate climate, where the development of the fruits is limited to a small part of the year. The immense northern pine-woods of the Old and New Worlds have their denizens from the bird-world living exclusively upon their seeds in the cross-bills, which are quite as much tree-birds as the tropical parrots.
The abundance of grains, fruits, insects, and even of mammalia, such as mice, is, on account of the varying local weather conditions on which they are dependent, much more variable in temperate than in tropical regions. One year may be rich in these products, and the next very poor; unusually favorable conditions for their growth may prevail for the time in one locality of an extensive territory, while in neighboring districts, from causes which it is often hard to explain, the contrary may be the case. Such circumstances must, of course, act with great effect upon the bird-world. At some times the birds of a region may, on account of the failure of their principal food and consequent famine, be driven from their hereditary quarters, and compelled, becoming wanderers, to resort to districts that are strange to them. When the pine-nuts fail in the north and on the mountains, the nut-crackers that feed upon them, obeying necessity, and not doing according to their custom, come south; and, when it is a bad year for birch-seed, multitudes of the northern linnets visit Europe. During long, snow-bound winters these northern and northeastern birds may be seen abundantly in Germany, and away down in Southwestern Europe, and the superstitious countrymen see in the unusual visitors the heralds of coming disasters.
On the other hand, an unusual abundance of food attracts birds into places to which they are not accustomed, and leads them into strange companies. Naumann relates that the thistles once got so firm a hold in the pasture-lands near his home that in many places the previously luxuriant grass wholly disappeared; then there came great flocks of green-finches, attracted by the abundance of their favorite food, and more in the next year at the time of the ripening of the seed, till, in a few years, all the thistles were exterminated through the agency of these birds. In another place he tells of a pine-woods of only thirty acres, to which hundreds of cuckoos resorted, attracted by the enormous number of caterpillars. It has also been frequently observed that horned owls are to be found by the thousand in localities that are plagued by an unusual visitation of mice.
Numerous birds attach themselves to droves of wandering animals. The passage-falcon travels regularly with the birds of passage; schools of herring are accompanied by throngs of fish-eaters, especially by the gannet, the presence of which is regarded by fishermen as the surest indication of the neighborhood of herring. So, swarms of locusts, pressing toward the West, are attended by birds to which they are food, usually peculiar to the East; lemmings, by the snow-owl; and in former wars of great severity, the armies by wolves, hawks, and ravens; and I have been told by eye-witnesses that the relics of the Grand Army were pursued in their disastrous retreat from Russia by thousands of ravens. Many birds owe their specific diffusion to their eeking out men, the products of their civilization, or their domestic animals, to live upon them; and many formerly strange species no doubt find their way into new countries after the introduction of grain and fruit-raising.
The relation of subsistence to the propagation of birds is of the highest importance; upon it largely depend the time of breeding and the number of the eggs and of the brood. We are accustomed, without sufficient grounds, to assign the reason of our putting fowls to sit in the spring, as well as the migrations of birds, directly to climatic influences. "We speak of the awakening of Nature, and it seems self-evident to us that the bird's nest with its tender chicks is a necessary part of it. Yet the time of year has hardly anything to do immediately with the propagation of birds, which is rather determined by the presence of a sufficiency of proper food.
In warm countries, such as Egypt, Ceylon, or Brazil, birds hardly observe a fixed period in their nesting, but each pair sets about the business whenever it finds itself in the most favorable conditions of subsistence, so that we can find nests and eggs of the same species in every month. The Falco eleonoræ of Southern Greece breeds at the unusual season (for a bird of prey) of August, and has its young in September. Quails are plentiful in the country at that season, having come down fat from the north, and in their multitude and helplessness fall a ready prey to the young falcons, and a much richer support than the birds can find in the spring, when, according to Erhardt's observations, no quails are ever taken in Greece. The Bombicilia Carolinensis of North America breeds as far north as 40° in June; it feeds its young on berries and cherries. The cross-bill never asks about the season or the temperature. It fixes its household indifferently in winter amid ice and snow, and in the height of summer, provided only sufficient nourishment for its children is present. The barn-owl likewise breeds irregularly. Its eggs and young have been found in October and November, but always in those years and places in which field-mice are unusually prevalent.
These few examples may suffice to show how, with birds, nearly everything turns upon subsistence; how the fact holds good not only for individuals and species, but that it is a matter of fundamental importance in their structure, their spread, and their habits, and how we may, with perfect right, apply to birds the maxim, "Tell me what you eat, and I will tell you who you are."—Translated for the Popular Science Monthly from "Unsere Zeit."