Great weight has been placed upon the use of land-bridges and the hereditary habit of crossing seas where these land-bridges once existed but have been submerged during the great geological changes in the earth's surface. Many have insisted that wherever migrants cross the sea they do so along submerged coast-lines or over submerged land-bridges, arguing that the gradual evolution which has made the advantageous adoption of a habit of migration possible was unable to eliminate the hereditary tendency to follow the exact route by which their ancestors passed from place to place. That there have been considerable alterations in coast-lines and in the general distribution of landand water since the time when birds began to be migratory is indeed probable, but unless crossing the sea means a distinct advantage it implies the retention of a habit which would not only be useless but might be a positive danger to the species.
In the Gulf of Mexico and the Caribbean Sea there is evidence of perhaps the most recent land-bridge in the chain of islands from Florida to Venezuela, collectively known as the West Indies. Although vast numbers of North American birds winter in South America only a few of the species which annually pass from one continent to the other make use of this comparatively easy passage. One might naturally conclude that the final severance of England from the Continent was in the neighbourhood of the Straits of Dover, yet this short passage is only used by a comparatively small number of our migrants.
Mr Dixon indeed argued that there is no greater barrier to migration than even a narrow arm of the sea(26). He refers to many Continental species which are common breeders in France but are unknown as nesting species in the British Islands, and others which are found in England but not in Ireland. But this is surely but an incident of distribution; the narrow strait or even river may for a time mark the limit of expansion of a species, just as at the present time the westward and northward unseen barrier prevents the range of the nightingalefrom spreading to districts apparently well suited for its home, and until recently the turtle dove and great crested grebe were checked in their northward advance.
In the evolution of some routes land-bridges certainty appear to have played their part, but once those bridges have ceased to influence direction the shortening of the time occupied by the lengthening of the single oversea flight is only a question of generations when an advantage to a species is to be gained.
This subject will be further dealt with in connection with the actual passages performed by certain birds.
The study of migration, based on observations at our lighthouses and lightships, shows that even in the comparatively small area of the British Islands there are certain routes followed with regularity. The birds which pass along our western coasts of England and Wales do not as a rule follow the shores round Cardigan Bay or along the eastward tidal scoop of the Irish Sea towards the coasts of Lancashire; the main body passes from Pembroke to the Lleyn Peninsula, and thence to Anglesey and the Isle of Man, on its way to the southern Scottish shores.
A source of possible error in the method of deduction from these results must be taken into consideration.The observations at lightships and lighthouses are mostly made when untoward circumstances bring the birds within range of vision, and on dark and foggy nights cause them to strike the light in great numbers. What is their normal course when no great migration wave or "rush" is observed? Are the few passing stragglers noted all that go by this route in fair weather? The same uncertainty must be applied to the observation of passing birds in inland localities. The immense numbers which do pass is shown by the observation of large movements, when as occasionally happens some check to normal migration leads an army of birds to a dangerously low altitude, or when high winds hold up a portion of the host on our coasts; but even these multitudes must be small compared with the millions of birds which annually pass from zone to zone unseen. The few or many birds we meet with, either on the coast or inland, resting on passage, may represent a lost or wandering party of stragglers or weaklings from a vast army which has passed over; they may or may not be on the route or course normally followed by the majority. The cartography of bird migration is a study in itself.
Mr Abel Chapman, describing his experiences in the Mediterranean, says—"For forty hours we were passing across (or beneath) the lines of an army of migrants—say 500 miles in width; yet not a signdid we see, save only the wreckage—the feeble that fell out by the way." On April 10th a sudden bitter northerly gale sprang up, and two hours later the steamer was the goal of hundreds of birds, no longer able to face the adverse wind. These were blue-headed wagtails, swallows, martins, pipits, wheatears, nightjars, and lesser kestrels. He thinks that the strong ones may have passed on but all the others perished(12).
In the last chapter reference was made to the great height at which birds may fly on migration. Certain species, even comparatively weak-winged ones, appear normally to fly high, whilst others, often birds with pointed wings and great aerial powers, usually proceed at low elevations; but there is still much conjecture as to the actual altitude reached by any migrants.
Gätke was of opinion that we do not see much of real migration, which is certainly correct, but there is no reason for his statement that it normally takes place at anything like the altitudes he mentions, 30,000 feet or more, or that birds at the time of migration undergo physiological changes which enable them to fly at immense heights and speeds and to see clearly in the dark(29). Nor need much weight be placed on the speculation of Lucanus, who contends that the height of travel must be less than 1000 metres, for above that elevation aeronautical observations show that perspective lessens. There are actual observations which, though liable to a margin of error, are proof of migratory flight at very high altitudes.
Most of these observations were at first made by accident; birds were seen through astronomical telescopes passing across the face of the moon or sun; but recently this method of observation and several ingenious plans of measuring height and speed have been made use of expressly to study migration. W. E. D. Scott, at Princetown in 1880, thought that by shape and size he could even recognise two species,Chrysomitris tristrisandQuiscalus purpureus, which passed across his field of vision at a height of at least half a mile above the earth(43). In 1888 Mr Frank M. Chapman published an account of similar observations; he calculated that the birds passed at distances varying from one to five miles, and that their altitudes must therefore have been between 600 to 1000 feet, and 3000 to 15,000 feet. He adds an important note: "A number of birds were seen flying upwards, crossing the moon, therefore, diagonally, these evidently being birds which had arisen in our immediate neighbourhood, and were seeking the proper elevation at which to continue their flight," but the direction of flight of most of the birds which were observed was parallel to the earth's surface and southerly. The average height was certainly far above the inferior limit(13).
Mr F. W. Carpenter, reviewing these astronomical calculations, says that Verey compared the apparent size of birds with lunar features, and consideringthat the small birds noticed would average 6 inches in length, calculated a height of 2000 feet above sea-level. At Detroit, Winkenwerde obtained estimates of a little over half a mile, and R. A. Bray, in England, saw birds crossing the sun in September 1894 which were invisible to the naked eye and must have been 2 or 3 miles away(11).
In December 1896 Mr H. H. Clayton, at Blue Hill Observatory, saw ducks flying at a height of 958 feet. He was at the time engaged in measuring the height and velocity of clouds, and was able to estimate the speed of the birds at nearly 48 miles per hour. In March geese passed at over 900 feet at 44.3 miles per hour. In 1905 Prof. Stebbins and Mr Carpenter worked out a scheme for ascertaining heights by simultaneous observation from different points. They based their calculations only on birds which were observed by them both, and found that these passed at various altitudes, ranging from 1200 to 2300 feet; in the following October the lowest altitude observed was 1400 feet, and the highest 5400. Allowing the 25 per cent possible error, these results are of great value.
Mr Chapman's remarks about the upward flight of some of the birds are enlightening, for when birds start on oversea journeys they frequently ascend to a great height. Dr Allen and others think that the ascent is to increase the visible distance, but it may alsobe to reach a zone or stratum of atmosphere in which flight may be more easily accomplished. Robert Service's account of the departure of migrants from the Solway shores, gives suggestion of high flight. They arrive often one by one and "seem to drop literally from the clouds," but when they actually departed it was easy to see their method. They "fly upwards and onwards, then they hesitate, fly sideways once or twice, again attempt an upward and onward flight, hesitate again, and down they come once more to earth." After repeating this manœuvre several times, "away they go over the sea." One morning he counted sixty blackbirds in one hedge, and others kept arriving, but, however closely he watched, he failed to see whence they came. "They came down from the upper air, becoming suddenly visible, sometimes three at a time." "I saw about a dozen birds thus drop into view, but I quite failed to see any indication of the point of the compass from whence they had come"(46).
Gätke frequently mentions birds raining down from the sky, appearing first as mere specks, and dropping vertically to the island, and others when departing "with breasts directed upwards and rapid powerful strokes of the wings, fly almost perpendicularly upwards."
On May 24th, 1911, I watched the departure of a spoonbill from Easton Broad on the Suffolk coast.The bird rose and soared in ever-widening circles until it was a mere speck, even when seen through powerful prismatic glasses; the great stretch of its wings alone enabled me to watch it for so long. When at a great height—I will not guess what elevation—it ceased its circling flight and made straight for the north.
In October I saw several flocks of redwings leave the Spurn. They rose to a great height before directing their flight southwards, although the Lincolnshire coast was plainly visible.
Great differences of opinion have been expressed about the speed of migrating birds, and here again Gätke's estimates, on account of the weakness of his arguments and his immense presumption, cannot be seriously considered. There are but few measured speeds, and most of these, except perhaps the ducks and geese referred to already, are of birds travelling at low elevations.
Many birds, especially day-migrating swallows, hooded crows and other birds, frequently travel at slight elevations; it is not unusual to see birds at sea flying a few feet only above the waves. Mr W. Eagle Clarke, whose systematic observations demand the profoundest respect, again and again urges that the direction of the wind has little effect upon migration, but that the force of the wind may make migration impossible. At the Eddystone, where hespent a month in the autumn of 1901, he noticed birds passing at heights varying from 20 to 200 feet, all flying southwards. He concluded that "the wind is certainly the main factor in migration meteorology—I am convinced that thedirectionof the wind is, in itself, of no moment to the emigrants, for they flitted across the Channel southwards with winds from all quarters"(16). When the velocity of the wind, however, was above 28 miles per hour (a fresh breeze, force 5 on the Beaufort scale), no migration was observed.
Allowing this, and also taking into consideration the undoubted fact that birds are frequently held up by strong winds on the shore before starting oversea journeys, is there any proof that they do not actually avail themselves of fairly steady strong winds when they reach the upper air?
Mr F. J. Stubbs makes some useful suggestions(50). He points out that Gätke and others imagine that a bird flying with the wind would suffer inconvenience through the wind ruffling up its feathers. It is surely evident that a bird supported in a moving medium could not progress at any slower rate than that medium; the bird is not flying on one stratum of air with another moving with a different speed immediately above it; it is actually in a current, not on it. If the bird flies at 20 miles an hour, and the wind or moving air is progressing at 10 miles anhour, the bird will cover a distance of 30 miles in one hour, though the force exerted by the bird is the force necessary for 20 miles in a calm. Conversely, if a 10-mile air current meets it, it will unconsciously be carried only 10 miles. If the speed of the bird is the same as the opposing force of the wind, it will remain stationary; I have seen ducks in a blizzard rise head to wind and fly rapidly, making no progress but maintaining their position over the water, to which they dropped again when the storm passed. Some black-headed gulls on the same water did not attempt this manœuvre, and in a few seconds had vanished down wind. The swimmer, in a swiftly-flowing river, may hold himself in position so long as he can swim at the same rate as the stream he is contending with, but he cannot make headway if the speed of the water exceeds his. He may, however swiftly the stream moves, swim in any direction, but his actual progress will be down stream; if he aims to swim directly across, his real course will be diagonal.
The fact that birds fly in any direction in a wind, and when at low elevation pay little heed to the direction of the wind, when the breeze is light, simply means that they can fly faster than the medium they are in; if the medium travels faster than they do, they will be carried in it to their advantage or disadvantage.
Even in these days of upper-air investigation we really know very little about the speed, direction and steadiness of upper-air currents, but we do know that at a moderate elevation—some two or three thousand feet—the strength is usually greater than nearer the earth.
Mr Abel Chapman, in "Wild Norway," makes this pertinent remark—"Except by aid derived from the operation of physical laws, the nature and extent of which are unknown to me, and by taking advantage of 'Trade-wind' circulation in the upper air, I believe that migration is impossible for short-winged forms of sedentary habits—but that aid, and those advantages, may facilitate, and perhaps vastly accelerate, a process which is otherwise impossible."
In "Bird Life of the Borders" he goes further. "Birds are warmer-blooded than ourselves or other mammalia, and are capable of sustaining life in rarified atmospheres where these could not. By a simple mechanical ascent, they can reach, within a league or two, regions and conditions quite beyond human knowledge: where, selecting favouring air-strata, they may be able to rest without exertion; or find meteorological or atmospheric forces that mitigate or abolish the labours of ordinary flight, or possibly assist their progress.... It is in the upper regions of open space where, I suggest, the final clue will be found"(12).
A warbler flying leisurely, say at 10 miles per hour, in a current of air which was travelling at 20 or more miles an hour, could accomplish the journey across the North Sea—say 300 miles, in ten hours. Allowing much higher rates of speed for strong-winged species, and greater force of wind, some of the marvellous distances covered by migrating birds cease to be mysterious. Prof. J. Stebbins and Mr E. A. Fath made careful calculations from observations with the telescope, and found that birds passed at rates varying from 80 to 130 miles per hour, and these were the minimums, for if the birds were not flying absolutely at right angles with the line of observation, they must have travelled a greater distance in the time occupied between their passage of the observation points(47).
The question, How do birds find their way? is answered by many ingenious and often purely speculative theories, some of which have been already referred to in connection with the points discussed.
Each theory, though it may apparently explain certain phases of migration, can be answered by some exceptional difficulty which makes it fail as a full explanation; we are driven to the conclusion that birds possess a sense of direction, which is often, very incorrectly, called Orientation. Biologically this term does not imply any connection with the East, but is simply used to describe the power of finding the way back to a certain base, or of returning home. It is a power or sense which undoubtedly exists in many vertebrate animals and in some invertebrates, though it is hard, in many cases, to separate or distinguish it from memory and impression gained through eyesight. Mr John Burroughs, one of the strongest opponents of what he calls the "Sentimental School of Nature Study," gives in his "Ways of Nature" a striking instance of this faculty which may serve as anexample, though the cases of dogs and cats amongst domestic animals, and of many wild creatures finding their way, could fill many volumes. His son brought a drake home in a bag from a farm 2 miles away, and shut it up in a barn with two ducks for a day and a night. As soon as it was released it turned its head homewards, but for three or four days its efforts were frustrated. Then Mr Burroughs decided to see what the drake would do, and to go with it to give it "fair play." The "homesick mallard started up through the currant patch, then through the vineyard towards the highway which he had never seen," and Mr Burroughs followed 50 yards behind. A dog scared the bird and turned it up a lane, but after a detour it reached the road again; it stopped to bathe in a roadside pool, then started off again refreshed. A lane, leading in the right direction off the main road, puzzled it, and it took a wrong turning, but, discovering its mistake, made for the road again, but not by actually retracing its steps. The false move seemed to put it out, for after hesitating at the next and right turning, it actually overshot the mark. Its companion, unable to spare time to continue the experiment, then headed it back, and when it reached the turning again it seemed to recognise something familiar and raced home with evident signs of joy.
The duck was a domesticated bird, but the incident is not without interest; the homing faculty was clearly exhibited, but it was not infallible; the bird made a mistake. So, inexperienced young birds, travelling instinctively by orientation may, and do, make mistakes.
Human beings, in varying degree, possess a sense of direction, and some a wonderful power of finding their way in strange places; it is most marked amongst those men we choose to call uncivilised, who, indeed, live in closer touch with nature than those of us who depend so much on compass, map, road, train and tram; we, as path-finders, are degenerate. Middendorf marvelled at the powers of the Samoyeds, but when he questioned them was met by blank surprise, and the cross-question—"How does the little Arctic fox find its way aright on the great Tundra?"
In addition to this instinctive power, the bird has eyes and brain. We can afford to put aside as purely speculative Middendorf's suggestion that the bird is impelled or dragged by magnetic force, but we cannot deny that it uses its eyes and that it has a wonderful memory; its second journey will be easier than the first, for it will recognise landmarks, just as the drake recognised something familiar when it neared home. Sight, however, cannot be always necessary, for, at the KentishKnock Lightship, Mr Eagle Clarke noticed that birds flew so low along the water that they could not possibly have seen their way.
It is purely speculative to say that the young bird which travels alone, for it seems certain that many young do travel unguided on their first journey, has an inherited memory of the actual route to be followed, but that it has an hereditary sense of direction, or an hereditary impulse to travel in a certain direction, is quite another matter. The sea, to the young bird, may be a barrier; it may wander coastwise and be lost, or, if this is the best way, find itself at the desired haven. If the shortest and quickest way is across the ocean, the young bird may brave the perils and succeed, or on this trackless waste it may wander till it sinks to the waves and be added to the long list of failures.
Certain species summer in Greenland either in the same areas or in areas comparatively near to one another; some of these travel in autumn south-east, and winter in Europe or Africa, and others go south-west into the States or South America. Most of these are distinctly eastern or western forms, but occasionally American birds are met with in Europe, and European in America. Probably at the start these stragglers joined the wrong band, and travelled for company and unconsciously by the wrong route. Birds drifted to leeward mayfind companions of quite an alien tribe; others, wind-swept in this way, may travel alone to new lands. A few pelagic South Pacific and South Atlantic petrels and other birds have reached Europe from time to time, but theirs is an error of too wide nomadic wandering. The wonder is not that these stragglers do turn up, but that so few are noticed; probably the frequent mistakes made by inexperienced or even experienced birds are speedily rectified by nature; failure to find food or exhaustion spells death.
Mr C. Dixon emphatically declares that each party of young birds is accompanied by one or more older ones to act as guides—"The many winter'd crow that leads the clanging rookery home." This is as emphatically denied by other observers. Probably there is no regular rule for many species, as there certainly is not for swallows, in which the old and young birds can be easily distinguished. I have seen bands trailing south in autumn in which all the birds were immature, and others in which a number of young were accompanied by a few mature birds, though, certainly, these old birds did not appear to lead. Another assertion, that the old and young do not even travel by the same route rather supports the idea that the young birds find their way simply by a sense of direction, a sense liable to blunder, whilst the old birds travelby the perfected or best route which their experience has taught them. Martorelli wisely asserts that orientation is not infallible, but develops with age.
Mach-Bruer attempted to localise this sense of direction in the semicircular canals of the ear, which are so highly developed in birds, but Dr Allen contends that the theory has been refuted by experiments on pigeons. Möbius urged that birds were guided in their journey by the direction of the roll of the waves; Newton replies that though this may be a constant direction in certain parts of the Pacific, it is most inconstant in the stormy North Atlantic(37).
There is a very generally accepted idea that birds prefer to travel with the wind striking them diagonally—the "beam-wind theory," a theory, which so far as I can see, has absolutely no sound foundation. When on the Kentish Knock Lightship, Mr Eagle Clarke noticed, on a bad day, east to west migrants hurrying past "as if to avoid as much as possible the effects of the high-beam wind."
Mr A. H. Clark worked out the long oversea and overland course followed by the American golden plover, and showed to his own satisfaction that the birds always travelled at right angles to the prevailing winds; therefore, he argued, theywere guided by the beam-winds; always keeping the wind on their flanks led them aright(14). He says that if they fly at 100 miles per hour, with a beam-wind of 30 miles per hour, they will reach a spot 100 miles from whence they started, but 30 miles to leeward of a line drawn at right angles to the wind. Thus, if they rely upon the wind, their course is more or less diagonally across it according to its strength. He maps out the supposed route according to prevailing winds, but fails to notice that the very route he maps may be caused simply by the leeward drift when flying on winds which are not with them. One portion of the journey is enough to illustrate what I mean. From Labrador to the east of Bermuda the birds fly south-east, so, he argues, as to cross the south-west wind at right angles. But supposing the birds headed due south, meeting the south-west wind on their right front, they would of necessity, if the wind was strong, drift away to the east. It is improbable that they actually aim to strike the Bermudas, for it is only during certain weather conditions that they visit these islands. In favourable weather the birds do not touch the Bermudas, but continue their flight direct to South America.
The leeward drift of birds in a strong beam-wind may be noticed during ordinary flight; it has occasioned one of the most remarkable of Gätke'sstatements. Referring to hooded crows, he says—"To escape the disagreeable experience of having the wind (south-east) blowing through their plumage obliquely from behind, they turn their body southward, and appear to be flying in this direction. This, however, is not the case. They do not make the least forward progress to the south, but their flight is continued in as exactly a westerly course, and with the same speed, as though the birds were moving under favourable conditions straight forwards,i.e., in the direction of the long axis of the bodies. This is shown in the most convincing manner by such bands as happen to pass immediately over the head of the observer.
"Besides hooded crows, many other, indeed perhaps all species, are capable of executing a laterally-directed movement of flight of this nature, not only under such compulsory conditions as they may encounter during the flight of migration, but also during the ordinary activities of their daily life"(29). He admits that he once thought it was a drift to leeward, but that he is now convinced that it is intentional, and is sure proof of his East to West flight. In the face of such absurd statements as these, how can anyone quote Gätke as an authority on migration! Yet, in recently-published books, this east to west flight across Heligoland to Yorkshire is stated to be a proved fact, though Mr EagleClarke, so long ago as 1896, showed it to be unsupported by British evidence.
Dr Allen, reviewing Dr H. E. Walter's "Theories of Bird Migration"(3), cites the following experiments as strong arguments in favour of orientation. Dr J. B. Watson took fifteen sooty and noddy terns from Bird Key, Tortugas, and liberated them at intervals after they had been marked. The shortest distance was 20 miles from the Key, the farthest, Cape Hatteras, 850 miles; thirteen returned to the Key. Neither sooty nor noddy terns range, as a rule, north of the Florida Keys, so that it is unlikely that any of the birds had been over the route before. They could have gained no experience, or hereditary knowledge, and as they were released during the breeding season, there would be no marked movement southward which they might follow, nor would they at that time be impelled by any desire to migrate. The change of direction from the Florida Keys, westward, to the Tortugas, occasioned by the water course which feeding habits would force them to follow, "removes the direction of the wind as a guiding agency, whilst the absence of landmarks over the greater portion of the journey makes it improbable that sight was of service in finding the way."
Not only do the distances of the migration paths of different species vary considerably, from a trip of a few miles to a voyage from the Arctic to the Antarctic, but the individuals of one and the same species do not all travel to the same degree.
The familiar swallow,Hirundo rustica, though subject to certain geographical variations, is found throughout the Palæarctic and Nearctic regions, nesting throughout Europe to between 63° and 70° north and in Africa north of the Sahara, where, however, Canon Tristram found it also wintering in the oases. South of the Sahara to the Cape it is a winter visitor. In Asia it breeds, according to Seebohm, in Asia Minor, Persia, Afghanistan, and western Siberia, and winters in Scinde and western India. One form breeds in and north of the Himalayas, eastward to China and Japan, and winters in India and Burma, and another ranges from eastern Siberia across Behring Strait throughout North America, so far south as Mexico. This form winters in Burma, in Central America andBrazil, but the Mexican birds are more or less stationary at all seasons.
Our swallow and its congeners have an almost cosmopolitan range, summering in the Northern and wintering in the Southern Hemisphere or comparatively near to the Equator in the Northern. Towards the centre of its range its migrations are either short or the bird is non-migratory.
Mr W. L. Sclater, addressing the South African Ornithologists' Union(42), stated that the swallow arrives at Cape Town at the end of October, and is common from November to March; practically all have left by the middle of April. Swallows begin to arrive from the south in Africa north of the Sahara in the latter half of February; early in March they reach southern Europe, later in the same month they are in Central Europe and by the middle of April large numbers arrive in England. Thus swallows leave South Africa actually after they have arrived in England; the South African birds cannot be the same which are in North Africa a month earlier! The swallow supports Seebohm's thesis that the individuals which go farthest to the south in winter, breed farthest north. A day-migrant and by no means a rapid one, the swallow may be timed from place to place, and it is not presumption to suggest that the birds which reach Britain to nest came from lands little south of the Sahara and well north ofthe Equator, and that those which pass through England and along our shores in May and even June are on their way from Southern Africa to the northernmost limits of their range. Mr Sclater points out one very interesting fact; when the swallow reaches South Africa it is in ragged worn plumage, before it begins its northward journey it passes through its one annual moult.
Waders and shore birds which reach South Africa in autumn—the spring of the Cape—are moulting into winter dress; before they leave they have often assumed or partially assumed the breeding dress. When they arrive the native South African birds are breeding, but though Mr Sclater thinks that some nest a second time in the south, no satisfactory evidence has ever been brought forward to support the suggestion. These long-distance travellers not only move from a zone of moderate temperature to a warmer one, but many of them pass through the hotter zone to a country having a similar temperature to the one in which they bred, thus enjoying summer but not torrid heat all the year round.
There are birds in which the northern and southern forms are distinct. The wheatear,Saxicola oenanthe oenanthe, reaches us early, sometimes during the second week in March, and speedily settles down to nest. Towards the middle or end of April a brighterlarger bird appears, the Greenland wheatear,Saxicola oenanthe leucorrhoa, which was recognised in Greenland, Iceland and eastern North America before it was seen that both forms occurred in Britain. This larger bird loiters through Britain, for its northern home is not ready for it until the Arctic spring. We know it breeds farther north than our wheatear, but its winter range is not fully worked out. The smaller bird is found in north and north-western Africa, and the larger form farther east, even south of the equator on the eastern seaboard, and probably, when we know more about the range of the two we shall find that the form breeding farther north, winters farther south.
The folly of laying down the law on the strength of the knowledge of the habits of a few species is shown by the study of the movements of American birds. Mr Cooke shows that as a rule "the migration is a synchronous southward movement of the whole species" in autumn, "the different groups of individuals or colonies retaining in general their relative position." The black and white creeperMnistitta variabreeds from South Carolina to New Brunswick, nesting in the south in April and reaching the northern limits in the middle of May. In the middle of July old and young birds have been seen at Key West, 500 miles south of the breeding range, and towards the end of August they havereached the north coast of South America. The New Brunswick birds cannot be ready to leave before the middle of July, and Mr Cooke allows them fifty days for the trip, bringing them to the Gulf States in September; he argues that this is proof that the earlier migrants must have been birds from the southern part of the range. Black-throated blue warblers,Dendroica coerulescens, reach Cuba at about the time that others of the same species are arriving in North Carolina; the first, he concludes, are birds from the southern Alleghanies and the others from northern New England or beyond(20). Other species illustrate the same order which he calls "normal," but show that it is not an invariable rule.
Southern-bred Maryland yellow-throats,Geothlypis trichas, reside throughout the year in Florida; those in the middle districts of the range migrate for a short distance only, whilst the Newfoundland birds pass over the winter home of their southern relatives to the West Indies. The palm-warblers of the interior of Canada travel 3000 miles to Cuba, passing through the Gulf States early in October; those from north-eastern Canada travel later and slowly and settle in the Gulf States, after a journey of only half the distance. He sums up wisely—"No invariable rule, law, or custom exists in regard to the direction or distance of migration....Each species presents a separate problem, to be solved for the most part only by patient, pains-taking observation and by the recognition of sub-species."
The order in spring is yet unproved. "With many birds ... the first individuals to appear in spring at a given locality are supposed to be old birds that nested there the previous year." These are followed by those which nested a little farther north, followed later by those whose homes are in the most northerly part of the range. "If, then, for any species, the southern nesting birds lead the van in both fall and spring migration, and the near guard in each case is composed of northern breeding birds, it follows that some time between October and April a transposal of their relative positions occurs; and that the more southern birds pass over the more northern ones, which delay their migration, knowing that winter still holds sway in their summer dominions." It is not known where this transposal takes place, nor whether the northern birds remain in winter quarters till the southern birds have passed, or start a slow migration, during which the southern birds pass over them. Later another transposal occurs; the northern birds cross the southern part of the range, passing birds which are already nesting. "Spring migration seems to be therefore for some species a game of leapfrog—thesouthern birds first passing the northern, and the northern passing them in turn"(20).
The custom, now fortunately becoming widespread, of marking birds by affixing a numbered metal ring to one leg, may help to elucidate this and many other problems, but until a large number of results are collected it is unwise to draw conclusions. Almost every month the recovery of some of these marked birds is noted in the scientific journals, but so far, beyond indicating the minimum distance travelled by individuals, little can be proved. It is, however, plain that birds do not invariably act as they ought to do if they obeyed all the laws which have been invented for them. A few records or results may be quoted, but any suggestions from these must be treated as suggestions only; many more must be forth-coming before we can say, proved.
The white stork,Ciconia alba, has been systematically ringed in Rossitten in East Prussia, in Denmark and in Hungary for some years, and Mr A. L. Thomson gives a brief summary of the interesting results up to date, in "British Birds" for May, 1911. Ten young birds, taken during their first autumn journey, show a general south-easterly trend through Europe. Three east Prussian storks were obtained in Syria, one in the April after it was marked, the other two in April and July of the second year; another wastaken in Palestine, and one Hungarian stork in Syria. In the May of the year following that in which it was marked in Prussia one was obtained at Alexandria. In their first autumn Prussian storks have been recorded from near Lake Chad in October, from Rosaires on the Blue Nile in the same month, and from the Victoria Nyanza at the end of November. A ringed bird is reported from German East Africa, but full details are wanting, but one shot at Fort Jameson in north-east Rhodesia in December 1907 had only been hatched in Pomerania a few months before; it left the nest on August 19th and began its journey south on or about the 26th. In its first winter a Prussian bird was shot in the Kalahari Desert.
Seven Prussian and about a dozen Hungarian birds have been obtained in winter quarters in the Transvaal, Natal, and other parts of south Africa, and one in German south-west Africa; one, recovered in the July following the summer in which it was marked, was possibly a weakling bird which had failed to make the return journey. Storks which had returned are recorded in their first, second and third summer; most of them having been found within a few miles of their birthplace. One bird, marked as a nestling near Brunswick in 1906 was reported in June 1908 from Sorquitten in East Prussia, over 430 miles away. Mr Thomson, fromhis reference to this being roughly in the same latitude but reached by a different "line of flight," suggests that it is an exception; this will be shown when some hundreds more cases have been collected. It may, however, be found that some birds deliberately launch forth in search of new homes, though at present it looks more like a bird which on approaching the breeding area had banded itself with the wrong local body of travellers.
A bird marked at Cassel in western Germany has been recorded from Barcelona in Spain; this so far single record may indicate that storks get lost, that there is no regular direction, or that there is more than one, a south-westerly as well as a south-easterly route. That too, we hope, will be shown in the future.
That much will be learnt from storks is evident, but the lessons will be only general; each migratory species must be treated separately, and to some extent this is being done. In the opening chapter I mentioned the uncertainty about the behaviour of any individual song thrush, merely as an example. So far, the few records of marked song thrushes add to rather than solve this problem. Years ago the song thrush was looked upon as a permanent resident so far as Britain was concerned. Then it was found to be migratory even in Britain, and it was suggested that each song thrush performed a shortmigration, southern British birds leaving the country, northern British retiring to winter in the south of England, and northern European birds replacing these as autumn immigrants. The study of local races quickly altered this opinion; it was guessed that in Britain there was a sedentary insular race and a migratory passage race; others, however, saw that some of our home-bred birds left in autumn. What do we find? A song thrush, marked as a nestling in July in Northumberland, is found in November in Durham; another one, marked in Berkshire travels to Norwich and is recorded in November, but a third, born in Aberdeen takes an autumnal flight of at least 1500 miles and is found in Portugal. Evidently we cannot yet frame any rule for our British-bred birds.
It is said that home-bred lapwings are somewhat sedentary, and that the large winter flocks are composed of Continental immigrants. The frequent westward migration of lapwings during exceptionally severe winter weather has led to the supposition that these birds fly for refuge, under these circumstances, to Ireland. This is true, so far as it goes, but a lapwing marked as a nestling near Stirling has been found in the south of France, and two others in Portugal, whilst five have been recovered in Ireland.
The results of marking sea-birds are interesting, showing that the young birds often wander northwardin search of food before there is any marked autumnal southward migration. Terns and black-headed gulls have been found a month or more after they have left the nest to the north of their breeding colonies in Cumberland and mid-Wales. A bird from Ravenglass was taken in its first January in Brittany. Rossitten black-heads have been shot in the Isle of Wight and in Breydon in Norfolk.
This may only mean that the young blackhead is a confirmed wanderer in search of food, but the few results with woodcocks, marked as British-bred nestlings, are puzzling. They have been known to linger in the neighbourhood of their home until November, and have been found in Portugal only a month later. Birds marked at Tyrone have been found so far apart as Cornwall, Harrow and Inverness; what route for the Irish birds can be guessed at?
Birds marked as adults present further problems, but also provide interesting evidence. Hooded crows, captured on migration in spring at Rossitten and then released, have been recovered in autumn actually in the same place and in other localities in Germany, and one marked in October was taken two years later, in spring, in Finland. The sum of these records of crows proves one thing conclusively—the fallacy of Gätke's due east to westand west to east flight, and supports a coastwise migration for this species.
Adult teal, captured in decoys, ringed and released in South Denmark in September and October, were taken in November and December in Hampshire, Suffolk and the Moray Firth, whilst others from the same place were recorded from other parts of England and Ireland, from western France, Holland, the south of Spain and the north of Italy. Fly-lines, if followed, are divergent and complicated. Four young herons were marked in one nest in Denmark; one was recorded in Holstein in June, and another in Mecklenburg in July; the third was killed near Salisbury in Wiltshire in October, and in the following February the last was obtained in the north-west of France. Two from another nest were recovered in Denmark, one in July and the other in February, twelve months after birth. Another heron reached Andalusia by August. In each case where there was indication of a direction it was south-westerly. Many more records might be mentioned, but these are sufficient to show the value of the method and the present insufficiency of results.
Many of these records show that the speed of the migrating birds, even in spring, is not great. Mr Cooke proves that most species in North America travel slowly through the districts where food isplentiful and during the earlier part of the journey northwards only a few miles are covered per day; they travel with the slowly advancing vernal wave, but, as we shall show in the next chapter, many species actually outstrip it, and travel from warmer to colder climates.