Read Ebook: Mémoires de Luther écrits par lui-même Tome II by Luther Martin Michelet Jules

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Any observer may verify the assertion that birds regularly visit certain favourable food-bases by paying attention to the occurrences of birds of passage. The study of a county, for instance, shows that certain species show partiality for particular localities. Thus in Cheshire goldeneyes pass through every spring and autumn, and may be met with occasionally on any of the meres; but at Oakmere, in the Delamere district, one may be almost certain of seeing parties of this species any time during the periods of passage. The curlew may be heard or seen passing over any part of the county, but only in the Delamere fields do we frequently meet with flocks feeding in inland Cheshire. Before the winter resident golden plovers have arrived in autumn and after they have departed in spring, the favourite fields are regularly visited by passing flocks, and the lower reaches of the Mersey, where the common sandpiper is rare as a summer resident, are visited every autumn by parties of birds on passage. Chance may lead a casual wanderer to a good food-supplying spot, but the regularity of appearance suggests habit and memory.

A fact which supports the theory that birds ramble far in search of food in their winter quarters, is that in many species the winter range is more extensive than the breeding area. Thus Mr Cooke shows that the known breeding area of the Pacific golden plover has an east and west extension of some 1700 miles, but in winter it ranges over an area with an east and west extension of about 10,000 miles. The scarlet tanager, however, has a breeding range extending for some 1900 miles across eastern Canada and a winter home in north-western South America of only some 700 miles in extent.

The winter quarters, or the outermost limits of the individual but not necessarily the specific range, having been reached, the bird spends its time in seeking food, remaining in one place if food is plentiful, or wandering, according to necessity or the habit of the species. The assertion that some birds have a second breeding season in their southern home is either unsupported by any direct evidence or is the result of a mistake in identification; the bird which has been found breeding has in several instances been shown to be a southern form or a related species of the one it was thought to be.

As the northern spring approaches, the strongest of all animal instincts, on which reproduction and the very existence of the species depend, overcomes all other desires, and the bird grows restless. The hereditary instinct, the origin of which we have endeavoured to show, urges the bird to seek the breeding area which has by degrees become so far removed from the winter quarters. The bird returns home.

But here is a serious difficulty urged by some writers as a powerful argument against the sexual impulse as the great factor in the return journey. Many of the birds which migrate northwards or homewards are sexually immature, and others of them are undoubtedly to be classed as "non-breeders," which means that during that particular summer they will not be engaged in the work of reproduction; why, then, should young birds or non-breeders migrate from the winter base. Possibly in the early days of migration only the mature birds did return; that we cannot state one way or the other. But it is reasonable to argue that once a regular migration habit has become not only confirmed by heredity but a very true advantage to the species, its influence will be felt by each and every individual. Again it is clear that the sexual impulses, in an undeveloped form, are appreciated by the adolescent, and in many animals by even the most juvenile. The play of all young animals is either an imitation or reflection of the search for food--the hunting instinct--or the love-making and sexual quarrels pertaining to reproduction, the pretended competition by the young for the favours of the opposite sex. They may play at and actually perform a migration which is so closely bound up with the life of the species. That this impulse has not always sufficient strength to force them to perform the whole journey is apparent from the fact that many non-breeders, young or sexually mature, on their northward journey through our islands or along our coasts, never reach the breeding area; the food supply on the way attracts them more than the memory of home; they linger with us until the breeding season is over and the return journey has begun. Knots, sanderlings, turnstones and many other waders may be seen on passage late in June, and some remain on our mud-flats throughout the summer; in July the tide of migration has turned.

The lack of sufficient or suitable food in the winter home during our northern summer may also cause the exodus, but this is a difficult point to prove when it is remembered that the winter home of every bird is not the parched tropical land or the waterless desert. From some zones removal must be a necessity, but in others there is food for all, so far as man can tell.

Dr J. A. Allen, a severe but discriminating critic of migration theorists, says--"Migration is the only manner in which a zoological vacuum in a country whose life-supporting capacity is a regular fluctuating quantity, can be filled by non-hibernating animals" . When in the early days of migration this periodically-supplied northern zoological vacuum was filled to overflowing by the increased numbers of avian inhabitants at the close of the breeding season, the natural food supply would be taxed to its limits; the falling temperature drove some and finally all to seek food further south, and their short migration to lands already filled with old and young birds, caused pressure and overcrowding further south. Further outward and usually southward movement was necessary and the zone of stress was gradually extended, though probably in those early days no particular species took long passages. The winter passed and the vacuum was again provided, and the rebound to fill it would create a slackening force all along the line; birds would spread from congested districts so soon as food supplying areas opened to receive them.

Mr Taverner, arguing on these lines , shows that competition would be originated in areas containing the earliest breeders, and be severest in the most productive districts. Weaker and later breeders would be driven out or prevented from colonizing by the stronger and earlier species, and the evicted ones would encroach on others, forcing them in turn to trespass on a wider circle of species. He then argues how the gradual recession of the glacial ice would increase the possible northward breeding area, and cause longer migration, and that this migration would delay breeding and conversely delayed breeding would assist the evolution of migration.

But the lengthening of the journey might surely be occasioned in another way, and the evolution of migration assisted apart from any glacial influences. Each successive increase of the length of the journey taken by the stronger and more go-ahead individuals, leading them in advance of the bulk of southward moving and competing birds, would be a distinct advantage to the individual and consequently to the species. The pioneer would arrive, like the slower movers, in a land already peopled with an avian population, but it would not have its own fellows to add to the stress of competition; it would be ahead of the greatest struggle. So the fittest would mould for the species the most suitable journey both in distance and route, and the laggards would gradually fall out of the competition.

Dr Wallace, without destroying these arguments, has shown that the survival of the fittest has a powerful influence. Those birds which do not leave the breeding area at the proper season will suffer and ultimately become extinct, and the same will happen to those which fail to leave the winter quarters when it would be a distinct advantage to the species to move into lands better suited for reproduction.

It has been put forward as a serious objection to many arguments that migration, instead of being advantageous to birds, is a danger to the race; that the perils of the journey are greater than those occasioned by more sedentary habits. It has even been suggested that migration is a habit specially created to thin down the surplus bird population. Dr W. K. Brooks, however, puts this idea, which is not entirely devoid of truths, in rather a different way. "Adaptations of nature are primarily for the good of the species--beneficial to individuals only so far as these individuals are essential to the welfare of the species" . The destruction of overabundant young, the thinning down of superfluous numbers, may be an economic advantage. It is one thing to say that migration has been caused to kill off a surplus, and another to show that, once a habit has been originated and become an advantage, it will be conducive to a greater prolificness, and that the natural sequence of an increased birthrate, when food supply and other conditions remain unchanged, must be an increased mortality. Thus the perils of migration may become a boon to the species.

The theories of C. L. Brehm and Marek that birds are living barometers, foretelling by intuition the changes of barometric pressure, may be dismissed as purely speculative. That birds begin their journeys during particular barometric conditions is certain, but what they know of forth-coming weather conditions is guess-work.

ROUTES

The migrating bird, when passing between the breeding home and the winter quarters, travels by what is termed its Route. The definition of the route has caused more controversy than perhaps any other incident of migration; the chief point at issue is whether the bird uses a particular high road, along which all its fellows from the same area travel, or if all birds move in what has been called a "Broad Front." Ornithologists have been, and to some extent still are, divided into two camps, one upholding defined routes and the other the extended or broad front movement.

After all the difference is merely one of degree. Even the widest notion of the broad front, that of G?tke, who insisted, as dogmatically as he did on most points, that the width or breadth of the migrating host corresponded with the extent of the breeding range , is of a route, bounded on the one hand by the northern or eastern and on the other by the southern or western lines of latitude or longitude which marked the limits of the range. The idea of a route may be narrowed down to the extent of a wide river valley, or to a fly-line represented on a map by a ruled line, which passes over certain ascertained places. The absurdity of G?tke's arguments are proved by the study of his truly remarkable book. According to him the island of Heligoland was only remarkable in that it possessed an observer, himself, who saw marvels unobserved elsewhere, though the same number of birds were every year passing over any particular spot in an area which, for many species, must have been many degrees in extent.

Had not so much weight been placed upon, and so many arguments based on G?tke's extraordinary statements by, unfortunately, many of our leading British ornithologists, his theories might have been ignored. Unfortunately he is looked upon as an authority, even an oracle, whereas, as Dr Allen pointed out, on many points which he treats with great positiveness his knowledge is obviously as limited as the little field which was the scene of his life-long labours . Glibly he tells of hooded crows "in never-ending swarms of hundreds of thousands" passing across and for many miles on either side of the island; of "every square foot of the island" teeming with goldcrests, and of "dark autumn nights" when "the sky is often completely obscured" by the migrants, which pass thousands of feet overhead. How did he observe the obscured sky? Indeed he again and again declares that migration passes unseen yet calculates the numbers observed on the darkest nights; the illumination of the lighthouse could not be sufficient to enable him to even guess at the numbers he mentions. After stating that "the whole vault of heaven was literally filled to a height of several thousand feet with these visitors from the regions of the far North," and that a certain east to west passage extended from the Faroes to Hanover, he concludes that "the view--that migrants follow the direction of ocean coasts, the drainage areas of rivers, or depressions of valleys as fixed routes of migration can hardly be maintained."

Coasting undoubtedly exists; birds, day migrants especially, may be observed following coast-lines in steady flight, though a mile or less inland no passage is visible. On the North Norfolk coast I have seen little parties of swallows passing along the shore in spring, coasting slowly but steadily from east to west. All day long and almost every day for more than a week this steady flight was continued, though I never saw any passing within sight more than a few yards out at sea, nor any at all more than a few hundred yards inland. Evidence which cannot be refuted shows that this habit of coasting is general, though a deeply indented bay, an estuary or strait, is usually crossed, and by no means always at the narrowest point. The same careful observations prove that both narrow and wide river valleys are followed by migrating birds in greater numbers than are ever observed passing beyond the limits of these valleys.

Seebohm's experience in Siberia led him to doubt the existence of routes, but his later studies of migration in autumn at Arcachon and in spring at Biarritz, caused him to modify his ideas. He found a gentle but continuous stream of migrants following the coast of the Bay of Biscay, arriving from over the Pyrenees on their northward journey, but moving "only within a mile or two of the coast." He contrasts island and coastwise migration; in the latter the travellers can rest at night or take short journeys during bad weather, but in the former they must await favourable conditions before attempting a perilous passage .

On the other hand many birds undoubtedly pass over inland localities independently of any river valley or mountain range which might indicate a route. Even such typical coast-lovers as the maritime waders constantly cross or pass through inland England. They are heard at night, or met with resting or feeding on inland waters, or their bodies are found when, on a dark night, they have collided with telegraph or telephone wires.

So long ago as 1886 Mr W. Brewster maintained that the breadth of the fly-line varied according to the character of the country which was being crossed. The migrating column, he said, might be hundreds of miles in length, "a continuous but straggling army," which only became a "solid stream" when travelling through some narrow pass . This solid stream or army passage is, however, seldom observed when the birds are crossing continents, especially if they are traversing a wide area in which food is equally plentiful for miles on either side of the direction of flight. The consolidation of their numbers appears only to take place when, either on account of the indifferent food-supply or of unsuitable weather conditions, the speed is accelerated.

In America Mr Cooke proves that the Mississippi Valley is undoubtedly utilised as a fly-line by a large number of species, but by no means all, and his evidence, though proving the use of routes, is that these are seldom constricted pathways but broad areas crossed in a generally coincident direction by the birds which make use of them. This main fly-line is however formed in America as in other places by the convergence of subsidiary streams, and it is these tributaries, as Herr Herman points out, which have in many instances led to error; they have been mistaken for main routes. The main route may be compared to the trunk of a tree, the birds following the roots from the area in which they have been nesting or wintering, and at the end of the journey splitting off in various directions, like the branches, to their temporary winter or summer homes.

The contrast in the method of travelling of different species or of the same species under different conditions, may be realised by taking two examples. Firstly, Mr Eagle Clarke's experiences at the Eddystone and Kentish Knock Lightship, when birds passed during the daytime at varying elevations, sometimes close to the waves, in twos or threes or scores, and at night in large numbers. The other is an observation of a "bird wave" by Mr P. Cox, during a snow storm in 1885 at Newcastle, New Brunswick. The birds passed eastward in a column about twenty-five yards wide, some just above the trees, others hardly visible, but the bulk in a massed column directly over the margin of the shore, and not over the river or meadow on either side. The movement was continuous for about two hours.

Von Middendorf collected statistics of the passage of birds in the Russian Empire, and by reckoning the average date of arrival of a few species at certain points of observation, worked out a number of curves or lines which he calls "isepipteses," or lines of simultaneous arrival . The result was, according to his argument, a general convergence northwards; the birds passing through Central Siberia travelled roughly in spring from south to north, in Eastern Siberia from south-east to north-west, and in Europe from south-west to north-east; they converged, in fact, upon the Taimyr Peninsula. This to some extent is doubtless true, but Middendorf goes on to prove that the magnetic pole is situated in this Peninsula and that the birds are drawn thither by magnetic influence, "in spite of wind, weather, night or cloud." He calls them "sailors of the air," possessed of an internal magnetic influence. He supports his argument by the statement that there is a similar convergence in North America towards the magnetic pole of the western hemisphere.

But all birds do not go in the direction of the magnetic poles, and many of those which do, stop short at suitable breeding places long before they have travelled so far north. The Taimyr Peninsula, a vast area in the extreme north of Siberia, is each spring a "zoological vacuum"; towards this desirable spot migrants will stream.

Herr Otto Herman cleverly shows the absurdity of many of the reputed routes by cartography; his map is crossed in all directions by the routes upheld by various theorists. Birds could not possibly follow all the directions "which authors invented for them," most of which he adds are founded on mere supposition . Dr Palm?n, he shows, usually managed to avoid districts where there were no observers, but Mr Dixon and M. Quinet made their routes follow rivers and coast lines, whether there was evidence to support this idea or not.

Only to a certain extent can it be safely contended that the present route of a species is an indication of its earlier journeys, or that the direction of original dispersal is recapitulated in the present line of migration. Heredity, experience, and imitation would certainly tend to preserve and confirm the general direction; the shortest and easiest passage from food-base to food-base would become an hereditary route, unless circumstances arose which caused a change. Mr Cooke shows how there has probably been evolution of the route as well as of everything else concerned with a mutable animal. The fly-line across an arm of the sea may be lengthened if this lengthening means a corresponding advantage in reaching the desired haven. Thus the birds which now cross the Gulf of Mexico at its widest part, at one time probably coasted round the Gulf, as many do still, by the land-bridge of Mexico and Central America. The gradual straightening of this curve would shorten the journey both in time and distance, though lengthening the actual single flight across a portion of the sea. We can imagine a bird arriving in autumn at the mouth of the Mississippi, at first passing from Louisiana to Mexico, so as to save the time of travel through Texas. Generations later the shortening of the journey, through lengthening of the short cut, would lead the birds to Vera Cruz and later still to Yucatan. It may be questioned, what object could the birds have in risking an oversea voyage, away from chance of food and hope of rest, when the land-bridge remained open for them? Each individual or group of individuals which arrived at any particular place a little in advance of the migrating multitudes of its own species, or others which fed upon the same kind of food, would certainly gain advantage, and would be the most likely to develop strong flight and the power of endurance in its descendants; it would indeed be a winner in life's race.

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 land and 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 . 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 nightingale from 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 the lines of an army of migrants--say 500 miles in width; yet not a sign did 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 .

THE HEIGHT AND SPEED OF MIGRATION FLIGHT

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.

G?tke 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 . 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.

Mr F. W. Carpenter, reviewing these astronomical calculations, says that Verey compared the apparent size of birds with lunar features, and considering that 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 .

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.

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