Popular Science Monthly/Volume 60/February 1902/The Journeyings of Birds

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THE sudden appearance of certain familiar birds in spring and their disappearance at the close of summer has excited the attention and interest of all classes of observers from the earliest times. "The stork in the heaven" says the prophet Jeremiah, "knoweth her appointed time; and the turtle and the crane, and the swallow observe the time of their coming." Much curious speculation has been indulged in to account for this periodic appearance and disappearance, one ingenious writer of the early part of the eighteenth century arguing that when the birds leave in the fall they retire to the moon. He presumed that they required about two months in passing thither, and that, after arriving above the lower regions of the air they will have no occasion for food. Concerning the great distance, he adds, 'between the moon and the earth, if any shall still remain unsatisfied, I leave only this to his consideration, whether there may not be some concrete bodies at much less distance than the moon, which may be the recesses of these creatures, and serve for little else but their entertainment,' just as the rocky islands of the sea which he says are 'of no other manifest use than for sea fowl to rest and breed upon!'

Hardly less absurd but wonderfully more persistent has been the notion that birds hibernate during the winter in hollow trees, caves and holes, and, at least in the case of swallows, in the mud at the bottoms of lakes and ponds. Linnæus and Cuvier, as well as a great number of lesser lights, believed that swallows spent the winter in a torpid state in mud, and even as late as 1878, a writer in a prominent natural history journal in this country described the finding, in midwinter, of two swallows in the mud at the bottom of a spring in a logging camp in Maine. When taken out they are said to have revived and to have flown about in a warm room.

These absurd ideas have gradually given way to more rational views, and at the present time the whereabouts of a great majority of our birds is known accurately for the entire year. Their coming and going on these long journeys has been under intelligent, though often desultory, observation for more than a century and, although we have learned much, it seems likely that we are hardly advanced beyond the borderland of this intricate and fascinating subject. The object of the present paper is to bring together some of the more important and striking of recent results for the purpose of showing that progress, although slow, is actually being made, and also in the hope that it may lead to further observation and record, for which there is undoubtedly abundant need.

In the first place, it may be well to define briefly certain phases of bird movement that are often overlooked or confounded with the generally accepted understanding of what migration covers. In the popular mind, and, it may be added, this is the correct view, a migratory species is one that regularly resorts to a given locality for the purpose of rearing its young, after which both old and young retire to some other, often widely different locality, where they pass the time before the next breeding season. In all temperate countries the migratory birds may be separated along these lines into two classes: first, those which come in spring, spend the summer and retire towards autumn; and second, those which pass through in spring to a breeding ground nearer the pole, and in the fall while on their journey south. The distinction between these two classes is obviously one of degree rather than kind.

The birds that come to us only in winter, such as Juncos, snowflakes, redpolls and Lapland longspurs, are not usually thought of as migrants, yet it requires but a moment's reflection to show that they are strictly so, and this leads to the general proposition that most birds throughout the world are constantly changing their location, but, as the individual is merged in the species, it is often difficult to obtain exact data on the subject. Because we see individuals of a certain species constantly about us, we call that a resident species, but, as a matter of fact, it is more than likely that not the same individuals are continuously under observation.

There is also another class known as occasional visitors, as the pine grosbeak and snowy owl, which may be absent for years, then of a sudden appear in great numbers. Their coming is supposed to be the result of a deficient food supply in their natural habitat far to the north, but the evidence for this is theoretical rather than actual. Hardly to be distinguished from these occasional visitants are the sudden incursions of species in a locality in which they have never been before known, as when a vast horde of nutcrackers spread over all Europe in 1844, or the erratic sand grouse, a bird of Central Asia, which has penetrated to England. But the climax of this restless and roving tendency in birds is reached in the stragglers that now and then are found hundreds, even thousands, of miles away from their homes, as when the Old World skylark is found in Greenland and the Bermudas, the American black-billed cuckoo in Italy, and our catbird and brown thrasher in Europe. While it may not be quite logical to class all these bird movements under the head of migration, as narrowly defined, they are more or less clearly manifestations of the same influences and go to make up the sum total of this wonderful ebb and flow of bird life.

The origin, or perhaps better the origins, of this habit or instinct of bird migration is exceedingly obscure. Many theories have been advanced to account for it, but perhaps none has yet been offered that explains satisfactorily all its multitudinous phases. For instance, it has been suggested that migration is the result of the development or acquirement of the power of flight. That flight has had much to do in making long extended migrations easily possible no one can deny, but that it has been the cause is not logically evident, for certain mammals, as the bison and antelope, are to a limited extent migratory, and certain flightless birds, as the penguins and the great auks, are strictly so, or rather were in the case of the latter species which is now extinct.

According to Mr. F. M. Chapman ('Bird Studies with a Camera,' p. 194) 'the desire for seclusion during the breeding season' is a 'good and sufficient cause for the origin of bird migration.' He applies this theory especially to birds nesting in colonies in secluded spots, as the Ipswich sparrow which is known to nest only on Sable Island, off the Nova Scotia coast, the gannets (Sula bassana), which nest in the western hemisphere only on three islets in the Gulf of St. Lawrence, terns on Muskeget and Penikese, and the brown pelicans of the Indian River region of eastern Florida.

This theory may afford an explanation for the migrations of birds that congregate in such colonies during the breeding season, but it should not be overlooked that 'survival of the fittest' may have been an equally important factor in weeding out those individuals of such colonies that did not seek these secluded or isolated localities for breeding sites. These birds may at first have nested in scattered situations and have been driven by predatory animals or other causes to seek inaccessible locations, and seclusion and isolation may thus have been a resultant rather than a cause. It is also difficult to apply this theory to land birds. Take, for example, the warblers of the genus Dendroica. Some species barely reach the United States during the nesting season; a few stop in the southern tier of states; others only reach to southern New England, while the bulk of the species press on from northern New England to Hudson's Bay. If seclusion were the only point aimed at, it would seem that the warblers which pass farthest north to breed could have found it in the mountains of the southern and middle states as some now do. Again, certain species, as the cliff and barn swallows, phoebe and summer warbler, seek the vicinity of human habitations during the nesting season, and, moreover, have greatly increased in numbers since the country became thickly settled.

The theory that is, perhaps, most naturally suggested, and the one that finds widest acceptance as explaining the facts is that migration began in a search for food. That is, the food supply becoming short in the vicinity of the home (a bird's home is thus assumed to be the place where it rears its young, and may therefore be quite different from the locality where it spends the remainder of the time) they wandered away in search of food, returning again and again to the home vicinity. These journeys were extended farther and farther, the birds returning each nesting season, undoubtedly oftener at first, to or near the locality where they were born. This process went on until their wandering became a fixed habit, and finally in the countless generations of birds that have come and gone, this habit has been crystallized into what we now call, for want of a better term, the instinct of migration.

This idea has been amplified and extended by Alfred Russell Wallace ('Nature,' X., p. 459). He supposed that 'survival of the fittest' has probably exerted a powerful influence in weeding out certain individuals. He supposed further that breeding can only be safely accomplished as a rule in a given area, and that during a greater part of the rest of the year sufficient food cannot be obtained in that area. "It will follow that those birds which do not leave the breeding area at the proper season will suffer, and ultimately become extinct; which will also be the fate of those which do not leave the feeding area at the proper time." His further argument is ingenious, and, it must be added, extremely plausible. He says: "Now, if we suppose that the two areas were (for some remote ancestor of the existing species) coincident, but by geological and climatic changes gradually diverted from each other, we can easily understand how the habit of incipient and partial migration at the proper seasons would at last become hereditary, and so fixed as to be what we term an instinct."

It will probably be found, however, if anything like a satisfactory explanation can be arrived at, that this habit or instinct has arisen in more than one way, but we may appropriately turn from a consideration of theories to a review of certain observed facts of migration.

It is now abundantly established that migration is mostly carried on at night, and further mainly during clear nights. Only a comparatively few species, such as ducks, cranes, certain large hawks, swallows, swifts, and nighthawks, migrate during the daytime, and these it will be observed, are either rapacious birds or mainly those that enjoy such power of rapid flight as to be relatively safe from capture. All the vast horde of warblers, sparrows, finches, flycatchers, thrushes and woodpeckers, as well as many waders and swimmers, migrate at night. On clear, still nights during the migrations birds may often be heard calling to each other high over head, and, as will be described later, may be actually seen by powerful telescopes. Woods and hedgerows that were untenanted one day may become fairly alive with birds at daylight the next morning, showing that they have arrived during the night. They remain to feed and rest during the day, and, if the weather be favorable, may practically all disappear the next night. That they only venture on these journeys during clear nights is shown by the fact that on such nights very few birds are killed by lighthouses, monuments or other obstructions, whereas on cloudy or rainy nights, especially such as opened clear and later become overcast, thousands of birds become confused and dash themselves against these obstructions. Thus over 1,500 birds have been found dead at the base of the Bartholdi statue in New York harbor in a single morning, and 230 birds of one species—black-poll warblers—were killed in a single night (Sept. 30, 1883) by the Fire Island light. The Washington monument, although not illuminated at night, causes the death of hundreds of birds annually.

The height above the earth at which migrating birds travel has been made the subject of some interesting observations, the first of which appear to have been by Mr. W. E. D. Scott, on the night of October 19, 1880, at Princeton, New Jersey. In company with a number of visitors he was being shown through the astronomical observatory at that place, and after looking at a number of objects through the 91/-inch equatorial, they were shown the moon, then a few days past its full phase. His attention was at once arrested by numbers of small birds that could be more or less plainly seen passing across the field of observation. Most of the kinds seen were the smaller land birds, among which were plainly recognized warblers, finches, woodpeckers and black-birds. He was able to identify with much certainty the characteristic undulating flight of the goldfinch, and the broad boat-shaped tail of the purple grackle. The flight of the birds noted was apparently nearly at right angles to the field of observation, and they were passing at the rate of 4% per minute. As nearly as could be estimated their height above the earth was between one and two miles.

In the following year similar observations were made by Scott and Dr. J. A. Allen, but the results were not as striking, only 13 birds passing in any quarter of an hour. They were also apparently flying lower than on the first occasion.

Some years later observations on nocturnal flight were taken up by Mr. Chapman, who spent three hours on the night of September 3, 1887, at Tenafly, New Jersey. During this time 362 birds passed across the moon's face. Of these 233 were computed to be at a height of from 1,500 to 15,100 feet, and curiously the lowest birds seemed to be flying upward, as though they 'had arisen in the immediate neighborhood and were seeking the proper elevation at which to continue their flight, but after that time the line of flight was parallel to the earth's surface.' He was able to identify positively only comparatively few species, such as the Carolina rail, grackle and a large snipe.

But perhaps the most satisfactory observations of all were those made also by Chapman, who, in company with a number of ornithologists, spent the night of September 26, 1891, at the Bartholdi Statue, New York. The weather proved to be exceptionably favorable, being clear during the early and later portions of the night, with an intermittent rain storm lasting for three hours between. As early as eight o'clock the birds began to be seen and heard, but almost simultaneously with the beginning of the rain there occurred a very marked increase in the number of birds seen about the light. They came singly, in troops, and in thousands, were visible for a moment and passed on into the darkness beyond. "The birds chirped and called incessantly. Frequently, when few could be seen, hundreds were heard passing in the darkness; the air was filled with the lisping notes of warblers, and the mellow whistle of thrushes and at no time during the night was there perfect silence."

The latest recorded observations were made by Mr. O. G. Libby ('Auk,' XVI., 140), who studied the nocturnal migrations at Madison, Wisconsin, in September, 1897. His first place of observation was a small elevation in the vicinity of three small lakes, where he undertook to make a record of the number of bird calls heard. During the night a total of 3,800 calls were recorded. The number of calls varied greatly, sometimes running as high as two or three per second and again falling to that number per minute. The largest number counted was 936.

From the nature of the data it was manifestly impossible to estimate the number of birds represented by these calls, but the effect was impressive in the extreme. He says: "Nothing but an actual experience of a similar nature can adequately convey the impression produced by such observations. The air seemed at times fairly alive with invisible birds as the calls rang out now faintly and far away, now sharply and near at hand. All varieties of bird calls came sounding out of the darkness that evening. The harsh squawk of a water bird would be followed by the musical chink of the bobolink. The fine, shrill notes of the smaller sparrows and warblers were heard only close at hand, but the louder ones came from all along the line, east and west. More than once an entire flock, distinct by the variety of their calls, came into range and passed out of hearing, keeping up their regular formation with the precision of a rapidly moving, but orderly body of horsemen. The great space of air above swarmed with life. Singly or in groups, large and small, or more seldom in a great throng, the hurrying myriads pressed southward."

The second station chosen by Mr. Libby was the Washburn Observatory, where for three nights he watched the birds passing across the face of the moon. During the three nights a total of 583 birds were counted, the largest number in any fifteen-minute period being 45. Considerable diversity in the direction of flight was noted. Thus up to ten o'clock the prevailing direction was south, but after this time the diversity increased, until it reached its maximum between twelve and two o'clock, when eight principal points of the compass were represented by numbers varying from 3 to 28. However, two-thirds of the number were still maintaining a southerly direction.

Libby attempted to estimate roughly the total number of birds that passed his point of observation during the three nights, but as he well says, 'when one recalls the relatively small size of the moon's surface as compared to its path from east to west, within the range of vision,' the difficulty becomes evident. As nearly as could be made out about 9,000 birds were passing per hour or a grand total of 168,000.

The rate of speed at which birds travel during the migrations, and also at other times, has been made the subject of observation, although the results, as might be expected from the confusing elements which must enter into such an inquiry, are far from complete or satisfactory. If the speed often attained by powerful and swift-flying species, such as ducks, geese, swallows, etc., could be maintained, it is obvious that the time occupied in migrations would be inconsiderable. But, as will be shown later, the maximum speed appears to be rarely or never realized at this time.

Frank Forrester records 90 miles an hour for ducks, as noted by telegraph from point to point, and an albatross has been known to cover 3,150 miles in 12 days. The actual distance flown by the latter bird was probably at least twice as great, for they rarely fly far in a straight line.

Some years ago Griffitt made some observations (recorded in 'The Field,' Feb. 19, 1887) in a closed gallery on the speed attained by 'blue-rock' pigeons and English pheasants and partridges. The two first mentioned flew at the rate of only 32.8 miles per hour, while the partridge made but 28.4 miles, and these rates were all considerably in excess of what they made in the open. The carrier pigeon is a rather fast flying bird, yet the average speed is not very great. Thus the average made in 18 matches ('The Field,' Jan. 22, 1887) was only 36 English miles an hour, although in two of these trials a speed of about 55 miles was maintained for 4 successive hours. In this country the average racing speed is apparently about 35 miles an hour, although a few exceptionally rapid birds have made short distance flight at the rate of from 45 to 52 miles an hour. The longest recorded flight of a carrier pigeon was from Pensacola, Florida, to Fall River, Mass., an air-line distance of 1,183 miles, made in 1512 days or only about 76 miles a day.

Herr Gätke, whose observations on Heligoland, a small island in the North Sea, extended over a period of fifty years, would give to birds a speed that is incredible. For example, the gray crows were believed by him to pass over the 360 miles between Heligoland and Lincolnshire at a rate of 120 miles an hour, and curlews, godwits and plovers are said by him to cross from Heligoland to the oyster beds lying to the eastward, a distance of a little more than 4 miles, in one minute, or at the astonishing rate of 240 miles an hour. The error in these observations, as suggested by Newton ('Dictionary of Birds,' p. 566), probably lies in the impossibility of identifying the individuals that leave one of the given points with those first arriving at the other end of the line. Professor Newton also calls attention to the fact that few birds, even swallows and quail, fly as fast as an express train from whose windows they may be observed. It is a common experience, when a train is passing along at no great speed, for various birds to be flushed by it, but after flying vigorously for a few hundred yards they quickly drop behind.

But granting that the occasional speed is very considerable, the actual speed of most migrating birds appears to be surprisingly low. Observations tending to prove this were made some years ago under the direction of Prof. W. W. Cook, in the Mississippi Valley. The services of over one hundred observers were enlisted, at stations ranging from the Gulf to Manitoba. The date at which a certain species was first noted at the most southern point was compared with the first appearance of that species at the most northern point; the distance in miles between these two stations is then divided by the number of days between the observations. Thus the Baltimore oriole was first seen at Rodney, Mississippi, April 7, and was not observed at Oak Point, Manitoba, until May 25. The distance in a straight line between these two places is 1,298 miles and as it took 48 days the average speed was 27 miles a day. The records of fifty-eight species for the spring of 1883 gave an average speed of 23 miles a day for an average distance of 420 miles, while in the following year a slightly smaller number of species gave exactly the same average speed over an average distance of 861 miles. In the case of individual species the results were of much interest. Thus the robin, cowbird and yellowhammer traveled at an average speed of about 12 miles a day, while the average for the summer redbird, ruby-throated humming bird and night hawk was 28 miles a day. It is, however, necessary to take so many things into account in arriving at these conclusions that it is easy to see the possibilities of error. For example, meteorological conditions play an important part during migrations, a rain storm or an unusually cold spell may retard progress for days. Even if the conditions are favorable, it is hardly probable that the same individuals migrate for more than a night or two without intermission, so that while the species may be making progress the individuals are alternating a night or two of travel with often several days of rest and recuperation. Again, it was found that most species traveled considerably faster during the latter part of the journey than during the first part. Thus six species showed an increase of 77 per cent, in speed for the northern half of their journey, and the same general result was obtained by calculating the average speed of twenty-five species separately for each of the different months in which migration is performed; the average for March being 19 miles, for April 23 miles and for May 26 miles a day. The species which are late migrants also move faster than those which start earlier and take more time about it.

The persistence with which birds cling to established lines of travel during the migrations is one of the most remarkable facts within the range of bird life, and this in not a few cases can only be interpreted in the light of past geological conditions. Thus certain species which breed in Europe and spend the winter in Africa now cross the Mediterranean at one of the widest points, a seemingly needless waste of energy. But soundings between these points have shown that the sea for much of the distance is relatively shallow, and that a moderate subsidence has changed what may have been narrowest to what is now one of the broadest points. This subsidence was undoubtedly slow and first resulted in the formation of a series of islands and lagoons, and the birds easily passed from one island to another, and even after the last bit of land had disappeared they still followed the old route established by their remote ancestors.

Many shore and water birds that spend the breeding season in and about the arctic circle to the north of Europe and Asia, follow lines of travel during their migrations that were undoubtedly established under past continental or oceanic conditions. Thus certain species take a circuitous route over what is now a wide expanse of open ocean, while others pass far inland through the Russian and Central European lowlands-Those of the first class are simply still following an ancient shoreline, and those of the second class the location of an inland shallow sea.

The Old World migratory quail (Coturnix coturnix) is one of the comparatively few migrants among the so-called game birds. During the migrations they wander far from places of their birth, reaching South Africa, Persia and India. The individuals inhabiting Great Britain, or at least a part of them, long ago established a migration route in a southeasterly direction. When examples from Great Britain were introduced into New England, they adapted themselves readily to their new surroundings and reared young, but when the season for migration arrived the inherited tendency to go in a southeasterly direction asserted itself, and, according to Mr. Wm. Palmer, of the U. S. National Museum, they all passed out into the broad expanse of the Atlantic and were lost.

For several decades it has been noted that a few species of birds from Western Asia have been gradually extending their summer range into northern Scandinavia. When these species migrate, instead of going south through central Scandinavia or southwest along the coast line, as do the original Scandinavian residents, they turn back east to the point in Siberia whence they came, before turning southward to spend the winter on the borders of India.

Forty or more species of migratory birds occur as summer residents in the Yukon Basin, Alaska. Of these some fourteen species are Pacific coast birds. With a single exception they are all thought to reach the upper Yukon by crossing the Alaskan coast range of mountains. This exception, according to Mr. W. H. Osgood, of the U. S. Department of Agriculture, is the varied thrush (Hesperocichla nœvia), which apparently reaches its summer home by going up the coast to the mouth of the Yukon, and thence following this river for almost 2,000 miles. Equally abundant with it in this summer home is the common snowbird (Junco hyemalis) of the eastern United States, which reaches the Yukon Basin by way of the Mississippi Valley.

Perhaps the longest straight-away flight made during the migrations is accomplished by certain shore and water birds, as the tattler (Heteractitis incanus), sanderling (Calidris arenaria), turnstone (Arenaria interpres), and the pintail and shoveler ducks, which nest in islands in the Bering Sea and spend the winter in the Fanning and Hawaiian groups, a distance of some 2,200 miles. As the shore birds above enumerated are probably unable to rest on the surface of the water, the entire distance must be accomplished in a single flight. It is difficult indeed to see how this line of migration could have been established. Following the analogy of the Old World species before mentioned whose path marks an ancient shore-line, we might presume that there was at one time a land connection, or at least a chain of islands between the Aleutian and Hawaiian groups, but on the contrary the depths of the Pacific are profound between these points, and there is not the slightest geological evidence on which to base a former land connection. When it is recalled how slight a deviation at the point of departure would suffice to throw them to the one side or the other of the Hawaiian islands the accomplishment is truly marvelous. In the absence of familiar landmarks and surrounded by a waste of sky and water, they make their way with the precision of a rifle bullet, and it would seem at hardly less speed.

The plovers, sandpipers and kindred species take migratory journeys often of extraordinary length. Thus the American golden plover (Charadrius dominicus) breeds in Arctic America and migrates through the entire length of North and South America to its winter home in Patagonia. The little sanderling just mentioned is almost cosmopolitan in distribution, breeding in Arctic and sub-Arctic regions and migrating in the New World to Chile and Patagonia, a distance of eight thousand miles, and in the Old World along all the shores of Europe, Asia and Africa. The Bartramian sandpiper (Bartramia longicauda) nests from eastern North America to Nova Scotia and Alaska, and goes south in winter to southern South America. The solitary sandpiper (Totanus solitarius) breeds mainly to the north of the United States and winters as far south as Brazil and Peru. The buff-breasted sandpiper (Tryngites subruficollis) rears its young in the Yukon district of Alaska and from the interior of British Columbia to the Arctic coast, and journeys in winter well into South America. The turnstone (Arenaria interpres), a little shore bird about the size of the song thrush of Europe, is also cosmopolitan, breeding in high northern latitudes and at other times of the year being found along the coast of Europe, Asia, Africa, North America, South America to the Straits of Magellan, Australia and the Atlantic and Pacific islands. It is one of the species mentioned as making the wonderful flight from islands in the Bering Sea to the Hawaiian Islands.

The ducks form another interesting group, although their journeys during the migrations are not nearly as extended as the birds just mentioned. The larger number breed mainly to the north of the United States and many within the Arctic Circle. Certain species, as the eider duck, only come south in winter to the coast of northern Maine, others, as the old squaw, may reach the Potomac and the Ohio, while most of them, as the bald-pate, blue-winged teal, pin-tail, goldeneye, bufflehead, etc., visit Mexico, Guatemala, northern South America or the West Indies.

Certain of the familiar birds of lawn, hedgerow and field, for whose coming we watch so anxiously, may claim a moment's attention. The bobolink, so dear to the hearts of the residents of New England, makes his appearance in his summer home in May. By the last of July or the first part of August the young are reared, the old males have lost their bright dress, and with a musical chink as their only note, they start southward. In the region of the Chesapeake they begin to congregate in vast flocks, where they are known as reed-birds, but in a few weeks they pass on to the rice fields of the South to become the dreaded rice-bird. But by October the last one has disappeared, and some by way of Cuba, others by way of Central America, where a few may linger, the main body presses onward beyond the Amazon into central and southeastern Brazil. On the return journey they reach the southern border of the United States in March and April.

The catbird is found in summer throughout the eastern United States and British Provinces, and in winter in the southern States, Cuba and Middle America to Panama. Our common robin is very erratic in habits of migration. Occasionally a few may winter in dense swamps as far north as southern Canada and Maine, but the majority spend the winter in the Southern States. The chimney swift is found in summer in eastern North America and thence north to Labrador and the fur countries. The winter is spent to the south of the United States. Cliff and barn swallows, which are found over nearly all North America in summer, may penetrate to Brazil, Paraguay and the West Indies in winter. The scarlet tanager passes the winter in the West Indies, Central America and northern South America, and the familiar indigo bird may go as far as Veragua.

The great group of warblers, of which some 70 species are found in the United States, has been mentioned before. They are all strongly migratory and mainly pass beyond our southern borders in winter, although a few individuals of a single species—the yellow-rumped warbler—have been known to winter on Cape Cod. Some of them visit the West Indies but the larger number, after rearing their young in the dense coniferous forests of the Hudson's Bay region or even in Alaska, spend the winter in Mexico, Central America or northern South America.

The sparrows as a group are also strictly migratory. Quite a number, such as the tree sparrow (Spizellla monticola), snowflake (Plectrophenax hyperhoreus) and longer spur (Calcarius lapponicus) breed far to the north of the United States in Arctic districts, and come down in winter into the northern states or irregularly farther south. Many species which breed mainly north of the United States only go into the middle and southern states during the winter, while a few may reach the West Indies, Mexico, Central America or northern South America.

But after having described these migration routes and the wonderful journeys over continents and vast oceans, the mystery of mysteries—How is it possible for the birds to find their way so unerringly?—still remains without a wholly satisfactory answer. As in the case of theories propounded to account for the origin of migration, so numerous suggestions have been made to explain this wonderful faculty. Thus Dr. Von Middendorff, a distinguished naturalist who studied exhaustively the migrations in the Russian Empire, suggests that because all the spring movements in that country are toward the magnetic pole, the migrating bird knows the location of this point and is enabled to direct its course accordingly. It is perhaps needless to say that this theory is not only unsupported by any serious facts but, as has been shown by Baird, is opposed to the facts of migration in North America.

If during the migrations the older and stronger birds always led the way, it might be said with plausibility that this faculty is due in large measure to experience, but here again the facts are either conflicting or directly opposed to such a view, for it seems to have been demonstrated with reasonable certainty that in Europe the young birds not only precede the old, during the fall movement, but often travel by a wholly different route. In this country, however, observations on this point are limited and authorities differ, but the tendency is to believe that the old birds do actually lead. Observation is much needed to settle this question.

In the case of birds migrating over land areas, sight is supposed by some to have an all-important function, especially when it is recalled that a bird two miles above the earth is surrounded by a horizon line of 90 miles on either side. As already shown, they have been observed at a height of three miles, which would easily keep them within sight of prominent landmarks, and would even permit them to cross considerable bodies of water without entirely losing themselves. That they depend to some extent on such landmarks to guide them on their course seems to be shown by the fact that they migrate mainly on clear nights and are obliged to seek the earth on the approach of cloudiness and storms. But in the case of birds migrating over hundreds or even thousands of miles of open water, vision must play an unimportant part. Möbius ('Das Ausland,' Aug., 1882) suggests that in such cases they may be guided by observing the roll of the waves, but while this may be true in a few instances, it cannot possibly be so in the majority of cases. We, therefore, seem inevitably led to the conclusion that birds are possessed of a 'sense of direction.' This homing' faculty or power of orientation which is, for example, so strongly developed in the carrier pigeon, is by no means unique among birds. It is possessed in a greater or less degree by many animals, by most savage races of men and, not infrequently by individuals among civilized races, more especially those accustomed to life away from centers of civilization, in forest and on plain—just how it is to be explained is difficult to say. Some would give it the dignity of a sixth sense and would fix its seat in the semi-circular canals of the ear.