Popular Science Monthly/Volume 7/June 1875/The Migration of Inland Birds
By CHARLES C. ABBOTT, M. D.
AS understood by us, the migration of a bird is simply the desertion of a given locality by that species for a certain, and always the same, portion of each year. As an example, the common house-wren (Troglodytes ædon) is migratory, in that it remains in New Jersey only from late in April until late in September, having left its Southern home for six months.
Before endeavoring to determine the causes of this movement on the part of some birds, we must first note the various features characterizing the movement itself; for it may safely be asserted that no two birds migrate alike, although the similarity is marked among the various species of the same family. The most marked feature in migration is the apparent uniformity in the time of its occurrence, i. e., of the dates of the arrivals in spring, and of the departures in autumn. Is this arrival in spring as regular as claimed by some, and supposed by most people? To the casual observer, and, indeed, to many who have for years noted the first appearances of our various birds, the arrival seems to be quite regular; and, curiously enough, we find many such observers insisting that, however late a bird may be any one season, he is never earlier than a given date. Thus we have been frequently told that a wren is never seen before the 1st of May, and usually upon that day they are here in full force. Now, let any one be determined to watch day and night for the first birds of the season; let him wander all day in or about tangled thickets, and sheltered, sunny hill-sides; let him, with sleepless eye, scrutinize every haunt of the birds, and with vigilant ear listen to every faint chirp and far-off twitter, and follow up every undetermined bird-note; let him do this, year after year, from April 1st to 30th, and he will find his note-books teeming with records of "early" birds, that will come and go, all unsuspected by the mid-day observer, who often will insist upon the absence altogether of many a summer songster, that, skulking about, withholds its joyous songs until the woods have welcomed the full company of its kind, that of old have made merry in its shady nooks. The fact is, there is more to be learned about birds, in one hour of the early morning, than in six weeks of mid-day sunshine.
The amount of variation in the dates of arrival of all of our spring birds is really considerable, and in the whole list of migratory inland birds that annually visit New Jersey, either to remain throughout the summer, or are on their way to more northern localities, there is not one that can be considered regular in the time of reaching here, by from twenty to thirty days.
The amount of variation in the dates of arrival, year after year, of the same species, say of the brown thrush, cat-bird, or yellow-breasted chat, is less, however, than that of the time of arrival of allied species; for instance, the various species of thrushes reach us very irregularly. The robin (Turdus migratorius) is a resident species; the wood-thrush appears (one or two in a neighborhood) from April 15th to May 10th; the tawny thrush (Turdus fuscescens) sometimes later by two weeks, and sometimes absent altogether; the olive-back thrush (Turdus Swainsoni) passes by irregularly, as to both time and seasons, and so, too, with the hermit thrush (Turdus Pallasi), which, however, occasionally remains throughout the summer. The brown thrush, or "thrasher" (Harporhynchus rufus) comes to us by twos and threes as early as April 20th (the first recorded by me this season, 1874, was April 17th), and not until May 3d to the 12th can they be considered as present in full force. The mocking-bird (Mimus polyglottis) is irregular, both as to years and dates, and the cat-bird (Galeoscoptes Carolinensis), never missing a year, wants the early May foliage developed, that he may skulk therein, yet often in "single blessedness," comes to his last year's haunts, and is wonderfully ingenious in his efforts to conceal himself in the leafless thickets of early April, keeping ever close to the ground, and never venturing upon the slightest attempt at a song.
The many notes we have made with reference to the warblers (Sylvicolidæ) also indicate a great degree of irregularity and uncertainty in their migratory movements. This applies to these birds as a family not only, but to all of the various species separately, of which a score or more pass through the State as a general thing. During certain seasons we have noticed a marked preponderance of some one or two warblers, which for seasons following were much less common than many others. Thus, in 1860, 1864, 1867, and 1872, the common redstarts (Setophaga ruticilla) were very abundant, not only about their natural haunts, but within the city limits, and scores of them could be seen climbing over and flitting through the branches of the shade-trees of the less-frequented streets. Since 1872 these birds have not been so numerous as usual, and far less so than many other warblers, such as the yellow-rumped (Dendroica coronata), the black-throated blue (D. cærulescens), or even the chestnut-sided (D. Pennsylvanica).
Unlike the thrushes, the warblers seem to be wholly controlled by meteorological influences and sudden changes of the weather, which, unlike some birds, they seem unable to foretell, greatly influence their movements, and certainly delay their northward progress; and yet, while we have frequently known them to be caught in a "northeaster," they are not otherwise affected by it, so far as we could determine, other than by the delay, before mentioned. Even a sudden change from warm, summer-like weather to decided cold did not destroy any of them, apparently, or check their lively movements among the trees.
Let us glance at the well-known and noticed swallows. For five months of every year we have with us, in greater or less abundance, six species of swallows and one "swift," the common chimney-swallow. Of these, one, the rough-winged (Stelgidopteryx serripennis) is comparatively rare, and known only to ornithologists; the white-bellied (Hirundo bicolor) are not particularly abundant, except during certain seasons; the cliff-swallow (Petrochelidon lunifrons) is erratic, now here, about the barns and stables of a circumscribed neighborhood, for several years, and then wholly failing to appear in their former haunts, when spring comes slowly up this way, to greet May's sleeping blossoms. Not so, however, with the barn-swallow (Hirundo horreorum); with a variation in date of arrival of about ten days, we have come to us, in May, our full complement of these beautiful birds. They have decreased in numbers during the past thirty years, so observant old farmers have told us, but probably not so much as they think. It is more probably the increase in the numbers of other species that makes the numbers of the barn-swallow seem fewer. The bank-swallow (Cotyle riparia), earliest of all, is here literally by millions, and the purple martin (Progne subis), in moderate numbers, seldom fails to occupy the boxes placed for its accommodation; while, lastly, the chimney-swallow (Chætura pelagica), which really belongs to another family, nearer the humming-birds and goat-suckers, we believe, has never failed to appear in about the same numbers, year after year. We have fewer instances recorded of single swallows, seen at unusually early dates, than of birds of any other family. Some, indeed, arrive much earlier than do others, as, for instance, the bank-swallow; but the variation in date of arrival, throughout any ten years, is certainly much less than with other birds, and with some of them it is surprisingly regular, but not absolutely so, as so often asserted.
Let us now glance at the peculiarities of this family of birds, and compare them with the thrushes and warblers. One marked difference at once is seen; that is, that the swallows have a wonderful flight-power, and the thrushes and warblers are weak in their powers of flight, positively as well as comparatively: and our observations bear us out in asserting, as a law of migration, that its regularity is in proportion to and solely dependent on the flight-powers of the species. With the entire list of inland birds of New Jersey, we believe this to hold good.
We have already expressed our belief that many birds have the ability to foretell a coming storm. As this is not directly connected with the subject of our essay, as we are now considering it, we will pass to another feature of this prophetic power, as it apparently is, in birds, and that is, their ability to judge of the general character of the coming season, by a visit of a few days' duration early in spring. We have so frequently noticed that certain birds, common to a locality during the summer, occasionally fail to visit it, except one or two individuals, that in April come for a few days, that it has appeared to us that these "pioneer" birds saw satisfactory reasons for believing that there would be a scarcity of food, and so return to meet their fellows, and informing them, they all depart to "fresh fields and pastures new," just as a single crow, discovering danger, will turn a whole colony from their course as they are going to their roosting-place. This, be it understood, is our supposition, and may be wholly untrue; but how are we to interpret the meaning of any habit or particular movement of a bird, except by the human standard? An act on the part of a bird is intelligible to us only as we would interpret a corresponding act in man; and these acts in birds and men, producing allied results, indicate that close connection between all animal life which is so readily comprehended from an evolution stand-point. Now, as an instance of this "foretelling" power in birds, we noted, during the past spring, the arrival of the first chewink (Pipilo erythrophthalmus) on April 27th. Busily among the dried leaves and tangled briers it hopped, enlivening the thicket with its constant song just as a dozen of its kind had done throughout the preceding summer. In a few days it had disappeared, and not a chewink has been seen or heard for nearly six months. Now a few are noticed on their way south from the country north of us. This locality is one where these birds usually congregate, and we have often found a dozen nests in the limits of the spot. But a few miles away, these birds were as abundant as usual. In two ways we can explain the absence of these birds: either those that were accustomed to occupy it went to a new locality, and the single bird that had preceded them, finding his companions did not come, left, rather than remain alone; or he left to announce that food would be scarce, for it must be remembered, as Darwin has remarked, "most animals and plants keep to their proper homes, and do not needlessly wander about; we see this, even with migratory birds, which almost always return to the same spot." At any rate, the summer of 1874, in this neighborhood, was the driest in the past forty years, and it seemed as if the chewink knew what was coming. So, at least, we believe. During this season we noted the entire absence, during the summer, of several migratory birds, common, as a rule, and a very marked decrease in the numbers of those that did appear; but, at the same time, our note-books mention the arrival of one or more individuals of every one of our migratory birds. Many, like the chewink, foresaw what was coming and acted accordingly. It would be most interesting to determine if insect-life was less abundant than usual during the past summer, but concerning this we have only to note, as suggestive that it was so, a marked freedom of the fruit-trees and fruit itself from the attacks of their insect enemies.
Another feature of the migration of our inland birds must here be briefly referred to; and that is, the failure of late years of certain species to come, as a rule, as far north as New Jersey; and also the habit, now fully acquired by others, of remaining throughout the year, when, but a comparatively short time ago, these same birds were truly migratory.
As an instance: the summer red-bird (Pyranga æstiva), twenty years ago, was a regular visitor to Central New Jersey, arriving about May 1st and remaining until October. It nested on trees, frequently in apple-orchards, laying pretty purple-blotched, green eggs. It preferred wooded hill-sides with a growth of underbrush, and having a southern exposure. In such situations they were numerous, and to one such locality, in particular, we can well remember the charm they added to the scene by the bright gleam of their plumage as they passed from tree to tree, uttering their peculiar but not melodious notes. For the past fifteen years we have seen not half a dozen individuals, and recorded no nests since 1857. In far scantier numbers the scarlet tanager (Pyranga rubra) has taken their place, although this bird is not rare by any means, nor was it so when the preceding species was abundant.
It is much the same with the mocking-bird (Mimus polyglottis). Formerly, as regular in its appearance, if not as abundant, as the catbird, it is now among our rarest summer visitors. An occasional pair, selecting some well-tangled thicket, will come late or early, and build their nest, and then half a dozen years may elapse before we see them again. Yet, thirty years ago these birds were common.
As instances of "spring arrivals," as we will class them, that have become resident species, we will first mention the well-known bluebird (Sialia sialis), which, whatever may be the state of the weather, is as lively and full of song from November 1st to April as from April to November; yet it is still considered as a migratory species, and formerly, we doubt not, was so, even in New Jersey. More interest are the two instances yet to mention, being those of the common yellow-rumped warbler (Dendroica coronata), which, in scanty numbers, braves our winters and from the tops of the loftier pines chirps merrily while the snow-flakes fill the air, and later in the winter seeks shelter in protected nooks where the noonday sun has melted the snow and gives us a breath of spring-like air. In several such spots, since February, 1863, when we shot the first "winter" specimen, we have not failed to find several individuals of this species, during each of the winter months, and of their number that thus remain with us there seems to be a steady increase. The same remarks will apply, in part, to that beautiful but not well-known songster, Bewick's wren (Thryothonis Bewickii). They too, in scanty numbers, congregate in sheltered places having a southern outlook, and now, while we are writing (October 29th) we can hear the clear notes of this lively bird as it sits, braving a chilly westerly wind, perched on a leafless branch of a sycamore.
We have noted now the more prominent features in the migratory habits of our inland birds as they come to us in May from the South, save the one fact, the bearing of which upon the subject we cannot determine, that a large proportion of the birds perform the journey by night, the others wholly by day. At least this is the common impression, but it is difficult to demonstrate it. How little, really, we know of the precise modus operandi of migration! All through April and May, if astir at the earliest dawn, when the resident birds are just starting their morning songs, we will occasionally hear the welcome notes of some summer bird for the first time. Has it been winging its way northward through the thick, black hours of night, guided by some unknown sense, and no sooner above its old-time haunts than it checks its onward course, and from a familiar tree sings with grateful heart a loud thanksgiving glee?
If we wander about those quiet nooks and by-ways, where the first thrushes and warblers are likely to be seen these same months, we will find all the day long, and evening too, these birds "conspicuous for their absence." Not a chirp or twitter, save of the sparrows and tits of all the year, and the lingering snow-birds that seem to regret leaving our pleasant places. Far into the night we may remain, and only the startled chirp of some disturbed or dreaming bird, or the fret and scolding of little owls, greet our ears. The silence of midnight may pass unbroken, and then, as the first gray streaks of light in the hazy east herald the oncoming day, suddenly a cheerful warble from some tall cedar or tangled brier-patch breaks the dead silence, and we mark the arrival of the first spring songster of its kind. Did it reach us at sunset, and, resting a few hours, then announce its presence with its cheery song?
Both by day and by night, it may be, they come, but why at all by night, if so, must ever be a great mystery in the strange habit of migration.
Let us next study our birds during the autumn.
A careful examination of the many notes, jotted down at irregular intervals, during the months of September, October, and November, with respect to the departure south of such of our birds as are summer residents, and of some that, having passed the summer in regions far to the north, are now, likewise, seeking their accustomed winter-quarters, indicates a similar apparent regularity in the southward movements of our birds as in spring, and at the same time an actual degree of variation in the dates of departure exceeding the irregularity of the dates of arrival.
If we consider the several circumstances that would necessarily influence their migratory movements, this actual irregularity, in autumn, is just what should be expected; but in the spring, as every bird returns to its own home and former nest, if possible, they will not linger on the way, as they know too well the length of the journey, and the coming duties of incubation speed them on, and we wonder why they are not more regular in their movements. In autumn, all this is changed. Now nothing need hurry them, and, so long as they find an abundance of food, they leisurely move along, just keeping ahead, as it seems, of the chilling frosts of the coming winter, which they can easily endure, but which robs them of the food they must have. This is especially true of insect-eating birds. Considered in this light, we are not surprised to find, then, as a rule, that the warblers, swallows, and such other birds as depend wholly upon insects for their sustenance, leave more promptly and in larger numbers, at one time, than do granivorous birds, and those that can subsist on seeds, while they consume insects so long as they can find them.
The weather, both during September and October, is exceedingly variable, and this fact causes the southward movements of the migratory, insect-eating birds equally so, inasmuch as these birds are not larvae-hunting species, but depend upon insects that can be caught upon the wing, or are to be found resting upon the leaves and twigs of the trees; therefore, just so long as the heavy white frosts are delayed, these insectivorous birds will linger with us. Up to a certain date, about October 1st on the average, these birds largely increase in numbers, consequent upon the daily accession of those from the north, and after the maximum is reached (October 1st or earlier, in accordance with the weather), their number steadily decreases, until but a few stragglers remain.
We feel quite confident that in exceptionally mild winters many more migratory birds winter in Southern New Jersey than ornithologists suspect; and we can see, in the lingering remnant of the great flight of warblers that annually pass through the State, that gradual adaptation to surrounding conditions, on the part of birds, that as centuries roll by, evolve, by that mystery of mysteries, the "survival of the fittest," new species from the old.
Again, long after the true insect-eaters, such as the fly-catchers (Tyrannidæ), the vireos (Vireonidæ) and the swallows (Hirundinidæ), with the chimney-swallow, humming-bird (Trochilus colubris), whippoorwill (Antrostomus vociferus), night-hawk (Chordeiles popetue), and the two cuckoos (Coccygus Americanus and erythrophthalmus), have passed southward, beyond the limits of the State, and scarcely a leaf is left upon the forest-trees, when not one straggling fly-catcher, in a day's walk, can be found hovering about the many spots so lately tenanted by myriads of their kind, we have yet the pleasure of seeing, in our rambles, many a blithe sparrow, either in the fields or about leafless hedges, or haunting the still green but nearly deserted swampy meadows, and even, late as it is, an occasional grosbeak, as it half conceals its gorgeous ruby and black plumage in some dark cedar, while it utters in broken cadences a fragment of its glorious song.
Of our many sparrows, of which several are resident species, we have noted down for several years, when the severity of the winter was yet to come, even as late as December 14th, the presence of the pretty bay-winged bunting (Poocætes gramineus), and in less scanty numbers the quiet little field-sparrow (Spizella pusilla). In the wet, reedy meadows, it is not until winter has encased in ice the tangled grasses, that the swamp-sparrow quits its home. For two years past, we have noticed that in the dry upland fields, all through November's hazy Indian summer, the sprightly black-throated bunting (Euspiza Americana) still remains, in little companies; and in the quiet woodlands, ever and anon, a retiring grosbeak (Hedymeles melanocephalus) lingers, until biting north winds drive him from his summer haunts. Last year, the indigo-bird (Cyanospiza cyanea) until the 20th of November remained with us, singing as merrily from the bare branches of the maples as when, during the summer, they cheered their brooding mates with almost ceaseless song. The bobolink, in spite of the persecution they suffer from sportsmen, hold to their reedy haunts, in scattering pairs, often until the first fall of snow, and this same bird—"reed-bird" in autumn (Dolichonyx oryzivorus)—being seen so early in the spring, occasionally, may possibly remain, but if so, very rarely. A few red-winged blackbirds (Agelaeus phœniceus), we know, withstand our winters, and seem to find food somewhere and how, even when the thermometer is at zero.
This difference between the insect-eating and the granivorous birds, the more prolonged autumn stay of the latter, we think, explains itself. In the spring, there is an object ever in view, while on their journey north—in autumn, their sole care is to be home in time; not so much to escape the coming cold, as to avoid being pinched by hunger.
We have seen that the first frost that but little affects vegetation does materially decrease insect-life; the swallows even anticipate this first frost, and, gathering in immense flocks, wing their way southward long before it comes. We can clearly see that the weather greatly influences, indeed governs, the migratory movements, in autumn, of the insect-eaters. It bids them depart, and, in general, they heed the bidding; but long after this, while there are yet berries, seeds, and fruits, to be obtained, the migratory vegetarians linger, in varying numbers, by the way.
Let us now glance at the abundant and well-known purple grakle or crow-blackbird (Quiscalus purpureus). The numbers of this (with us) partially migratory species that remain throughout the winter, as compared with those which are here during the spring and summer months, are about as three to one hundred, as near as we can judge; and, in proportion as the winter is mild, the percentage of those that remain is increased. In Massachusetts, this bird is strictly migratory; the great bulk of those that depart from the north, and from New Jersey, wintering in the Carolinas and Georgia. In this species, therefore, we have an example of a migratory bird that is gradually becoming more and more accustomed, not to the rigors of winter which birds are better able to withstand than they are supposed to be, but to the methods of our winter residents, such as woodpeckers, jays, and titmice, in procuring such food as can then be procured. Food, as a matter of course, and an abundance of it, must necessarily be obtained, and, on examination of the stomachs of grakles killed in January, we have found them filled with a half-digested mass of what appeared to be both animal and vegetable matter. If the grakles that remain during the winter are of a hardier constitution than those that migrate, then, as they mate very early in the year, and before the great bulk of the southern sojourners reach us, their offspring will naturally inherit equally vigorous constitutions, and, like their parents, will be more disposed to remain; at least a large proportion of them will be, and in this way, wholly through natural selection, a race of grakles, otherwise undistinguishable from the whole number of this species, will be evolved, that in time will wholly replace(?) the now migratory and semi-migratory individuals. If we have now correctly explained a change now in progress, in the habits of this and other species, then can we not, from it, gain a clew to one, at least, of the original causes of the habit of migrating?
But this we will discuss in the concluding part of our essay.
The act of migrating being the passage from one distant point to another, it is evident that the cause or causes of this movement is one or more that operates at either terminus of the journey. A warbler that winters in Florida, and breeds near the arctic circle, is operated upon by a cause that exists at each terminus, or by two differing causes, each peculiar to its own location, and it is wholly incredible that it is the same cause that induces both the visit to northern regions and the return' to a southern clime; therefore there must be at least two causes for the habit—one inducing it in the spring, another compelling the migrating bird to return. If it be possible now to demonstrate what these causes are, and. how the same cause can influence all migratory birds, considering that their habits are otherwise so totally different, it will not then necessarily follow that it was the originating cause of the habit. When, indeed, did this migration commence? How far back into the world's geological history must we go, to trace the first bird that was forced to seek another and far-distant land, wherein to rear its young and find for its offspring and itself sufficient food? What conditions of heat and cold, land and water, summer and winter, then obtained, that birds must need fly from coming rigors of scorching sun, or ice and floods, or perish where they were? Was it from living in such a world that migration originated, and became, strangely enough, characteristic of only a fraction of the whole number? How, too, could birds have learned the oncoming of disastrous times, and know just where to seek a safe harbor and secure rest? Clearly it could have been only by a very gradual accumulation of experiences extending over many generations, before the few progenitors of our many birds gained the happy knowledge, that here in the North we have months of sunny summer weather and a wealth of pleasant places. We will not go back, then, of the Glacial period, but rest content with it as having been the starting-point in time of birds' migratory movements. The progenitor of our score of warblers, the one tyrant flycatcher, from which all our species have sprung, the vireos, the goatsuckers, and cuckoos, then very few in species, if indeed there were more than one of each, must have been influenced by the presence of the icy barriers that shut them off for the time being from a vast portion of the northern world, and at the close or closing of that wonderful period it may be that migration commenced, yet why and how, we can but guess. Knowing that it commenced then or recommenced, if previously a feature of bird-life, we have now to inquire what are its apparent causes at present; but, before inquiring into these, may we not, after all, ask if migration be not an inherited habit, the originating causes of which are not now in operation? The conditions not obtaining that necessitate migration, does it not become a case of survival of habit, just as in man many customs now exist, the origin and proper meaning of which are wholly lost? That this is true of the migration of all birds we do not believe, but that it partially holds good with some species we are fully convinced. As an inherited habit, but one now not absolutely necessary to the bird's welfare, we can see why it should be, as it frequently is, so greatly influenced by surrounding circumstances and conditions.
Taking the movement from its proper starting-point, which we assume to be the movement from south to north, in the three spring months, we must now look for sufficient causes to induce the undertaking of such long journeys. These causes are suggested by the two principal objects effected, on their arrival at their northern destination, viz., rearing of their young, and procuring suitable and sufficient food for both themselves and offspring. If migration is for these two purposes only, then it should prove to be the case that food was not sufficiently abundant in the south for both its resident and migratory birds. This certainly could not have been the case, and we believe, therefore, that migratory movements, at the outset, were to a very limited extent only; a few birds at a time seeking to avoid their enemies, and have undisturbed possession of a locality, by pushing out from their accustomed haunts, for, comparatively speaking, a few miles. The young of such pioneer birds would naturally leave the neighborhood of their nest, and return to their parents' usual haunt with them; but, on the return of another breeding-season, they would themselves seek a resting-place near where they themselves were reared, and the older birds would go to the same nest or nesting-place that a year ago they occupied. This is precisely what occurs now, year after year. Now, as birds increased, century after century, the limits of this northward movement would be extended, until it became in time the journey of thousands of miles that it now is.
Assuming, then, that migration arose for the dual purpose of safe nidification and a certainty of sufficient food, we are met by the ugly question, "Why do not all the southern birds come-north?" If, when the whole avi-fauna was concentrated at the south, there was any struggle whatever for favorable feeding or breeding grounds, then, naturally, the weaker would go to the wall, or, in other words, would be driven beyond the limits of their accustomed habitat. These weaker birds, taken together, having once formed the habit of visiting certain localities at stated times for given purposes, or periodically were forced to do so, would vary in their methods of reaching these localities, in their choice of regions wherein to remain, and the length of their annual visit, just in proportion as their habits generally varied from those of both other species of the same family and from species of other families. For instance, to avoid a common enemy, a number of species might have gradually learned to migrate at night; while others, although forced to migrate, had not this same enemy to contend with. In this way, the habit of nocturnal migration would long ago have been formed, and it would, by inheritance, be continued by their descendants, even after the enemy had been long extinct.
Having reached the northern summer homes, and, free from molestation, reared their broods, clearly, if all things needed for their comfort were to be obtained, it cannot be supposed that these same birds would unnecessarily retrace their long flight to the distant South. This suggests that if we are correct in assuming that birds first appeared in a tropical climate, and from such climate migration started, it is probable that by gradually prolonging their northern visits and accustoming themselves to northern insect and vegetable life, these regions became populated by their resident species. It is evident that the present migratory species are simply compelled to return, and three compelling causes are demonstrable. Primarily, the sudden increase of cold at the close of the brief northern summer, which starts southward those farthest at the north. This accession of intense cold necessarily decreases the amount of food, and the birds are now forced to find it elsewhere. Farther and farther south they come, just in advance of the cold, and slower and slower they proceed, as they enter our more temperate latitude, and here, resting as it were, they linger until a keen frost kills their insect-food, and, scattering the leaves, robs them of their main shelter from their enemies, happily fewer now than formerly; and now still southward they proceed, until they reach a home in lands blessed with perpetual summer.
We have now traced these migratory species from south to north, and back to their southern habitat, and endeavored to point out the several operating causes of the movement as we did so. We have already suggested the possibility of migration being an inherited habit not now necessary. Now, be this true or not, it is evident that the habit is not so fixed a one that ordinary changes in surrounding conditions do not greatly influence it. This, we think, is shown by the irregularity of the movement that really occurs, and the tendency on the part of many species to modify the habit by occasionally halting much to the south of their usual breeding-grounds, and by remaining later and later in autumn; and, again, by the fact that many birds are now only partially migratory, and others by occasionally migrating simply in search of food, thus exhibiting, as it were, traces of a habit they have long lost, as to its full meaning and accomplishment.
In the migration of a bird, then, we see simply a temporary sojourn in a distant locality for the purpose of rearing its offspring in safety; the cause being implied by the term "safety," i. e., freedom from enemies, and an abundance of food.