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During regular migration birds start in favourable weather but frequently meet with unfavourable weather before their arrival at the point aimed at; most of the bird "disasters" at the lighthouses and lightships, and more occasionally inland, can be explained in this way.

In his digest of the observations at lighthouses and lightships Mr Eagle Clarke shows that spells of genial weather are favourable and that during these spells migration is even flowing and continuous . Slightly unsettled conditions have little effect, but an increase of the irregularities accelerates migration. Sooner or later cyclonic disturbances interrupt regular movements, and, if these are extraordinary, act as barriers, either holding the birds in one place or forcing a hurried departure or "rush." Favourable weather immediately following a check or "hold up" often causes a rush; a sudden fall in temperature may force large numbers of birds on in autumn or retard them in spring. Temperature, he declares, is the main controlling factor in all extraordinary movements, other meteorological conditions being suitable.

In the autumn migration to Britain, the chief movements take place when a large and well-defined anticyclone has its centre somewhere over Scandinavia, with gentle gradients in a south-westerly direction over the North Sea. Coincident with this we usually find cyclonic conditions prevailing to the west of the British area, with low-pressure centres off the west or south-west of Ireland. The weather is clear and cold, with light variable airs over Scandinavia, but in Britain the sky is overcast, and the wind easterly and moderate to strong; not infrequently these conditions mean fog on our eastern coasts. If the birds leave Scandinavia under favourable conditions they may be met by the approaching cyclonic system, which usually, though by no means always, travels in a north-easterly direction across the Atlantic. Migration is thus checked, but a return of favourable anticyclonic conditions starts the birds again, often with a fresh impulse in the shape of falling temperature. When the anticyclonic area is exceptionally large, extending from the Scandinavian peninsula in a south-westerly direction and embracing the whole of the British Islands, simultaneous immigration and emigration may be witnessed.

Cyclonic spells are not always unfavourable to migration. In spring, when they are of a mild type with soft rain and warm winds following after a cold anticyclonic period, a northward movement is frequent.

Mr Eagle Clarke says that the importance of winds is overstated, but as an incentive only. The direction of the wind has no influence as an incentive but its force is an important factor; in a strong wind a bird may be blown out of its course. Birds will not start in a high wind but may pass into the influence of strong winds which may affect both progress and direction. He adds that particular winds usually prevail during the season of great autumn movements, which are not incentives but are the result of pressure distribution which is favourable to migration. These are usually north-east to south, but a westerly wind would serve as well, but it indicates a pressure distribution which is fatal to migration between north-west Europe and Britain--cyclonic areas to the north-east and east of our area.

All this, no doubt, is perfectly true. It is founded on the analysis of a huge number of carefully recorded observations, and upon a general knowledge of migration which few can ever hope to equal. Mr Clarke understands his subject. It appears, however, to me that he may put rather too much weight upon the barometric influence, and too little on one side of the wind question. Are we yet in a position to say that birds do not make direct use of certain winds? It may be that the use of the prevailing winds at migration time is far more unconsciously intentional than is at first apparent.

One or two points must be kept well to the front which are often ignored by observers. Firstly, very much visible migration is abnormal; that is to say, most of the incidents of passage which are noticeable, especially observations at the lightships and lighthouses, are during spells of weather which are described as unfavourable; it is the "hold-ups," checks, and "rushes," which attract attention far more than the even-flowing normal migration.

Mr J. Tomison, in his valuable notes on observations made at Skerryvore , shows that in ordinary clear weather birds pass at a great height, beyond the power of vision. He proves this by instances of the diurnal passage of redwings, birds which are generally supposed to migrate at night, and undoubtedly do so frequently. He heard the well-known passage-note in the daytime, but with the naked eye could see no redwings; he found them with the telescope and later discovered others which were passing above the range of normal vision. Mr Eagle Clarke, commenting upon the extraordinary numbers of rare and exceptional visitors which are noticed on many islands--Fair Island, the Flannens, the Isle of May, and Heligoland may be taken as a few examples--says that it is their detached position and comparatively small size which makes these islands so useful to the observer. The same variety of birds and greater numbers reach larger islands and tracts of land, but they are unobserved when they are thinly distributed and not massed or confined in a small area. "With all our great army of trained observers," he declares, "we in Britain see only an infinitesimal number of the migrants which visit our shores ..." and "this is especially the case on the mainland."

During an anticyclone there is a descending movement of air currents from a centre of high pressure in all directions, and these currents or winds are deflected "clockwise" in the northern hemisphere; and when cyclonic conditions prevail the air currents are directed inwards towards a low-pressure central area, rotating spirally at the surface of the earth in the direction contrary to the hands of a watch. In the southern hemisphere the directions are reversed. A cyclonic system is usually carried forward by great drift winds like eddies upon a swift stream, in the North Atlantic as a rule from south-west to north-east.

Do we really know the force and direction of the winds at a high altitude during these movements? Are we not merely guessing at the real aerial conditions by the movements near the earth at the time of the departure of the birds? Is it fair, if I am right that the meteorological observations are founded upon only those observed at comparatively low altitudes, to lay down laws as to the particular conditions which are favourable or unfavourable, or the particular winds which are used or avoided? The direction of the wind may be the same up to a great height, many thousand feet, or it may vary within 500 feet of the earth. Nearly fifty years ago, when Glaisher made his great ascents, he sometimes met with three or four currents moving in opposite directions. The more recent upper air investigations show that though as a rule the wind at various elevations is in the main from one point of the compass, its degrees vary considerably, and its force at the various heights shows remarkable differences. Generally the force rises to about 5000 feet, but there is no invariable rule. I tabulate a few examples taken more or less at random from the Weather Reports for 1908. The altitudes above the ground are measured in metres, roughly converted into feet; the letters indicate the direction of the wind, the figures its speed in miles per hour. The last one in the table, observations made at Brighton on September 20th, is particularly useful. The conditions on this date were anticyclonic, and favourable to migration. At 400 feet above the sea the wind was blowing at 5 miles an hour; at between 5000 and 6000 feet its force was 20 miles per hour. What then would happen to a bird leaving Brighton for say the Spanish Peninsula? If it flew at 20 miles an hour towards the French coast about Dieppe, it would meet the wind blowing at 5 miles an hour, and take between five and six hours to reach the coast, head to wind. If it rose to the height of 3000 feet it would meet a wind blowing at the same speed as it was flying, and it could make no headway. If, however, it flew in a south-westerly direction the more it turned westerly the farther it would drift down channel towards Normandy or Brittany, and be carried out to sea! But this is exactly what would not have happened, for on this date a feeble cyclonic system was approaching from the Atlantic and extending its area of influence over southern England. In the Channel the bird would meet westerly winds which would bring it safely to the Brittany shores, or if it missed them, to the western shores of the Bay, where the wind was actually from the north. I mention this merely to show that apparently unfavourable winds may be really favourable.

Under ordinary circumstances are we justified in saying that birds make use of the winds blowing with a certain force at the point of departure, or that they ignore them? Certainly we cannot judge by either the force or direction of the wind at the point of arrival, as Mr Clarke points out. The bird may have dropped into most adverse currents.

In Hungary, where migration has been very carefully studied, we find evidence supporting Mr Clarke's theory, and yet giving it a slightly different complexion. Low atmospheric pressure, depression very often shows the greatest rate in the arrival of the swallow. If there is a centre of depression west of Hungary, and its path is directed north or north-east, swallows appear in crowds. The fair side of the depression, with its warm southerly winds, is therefore favourable. A list of twelve other birds, which also appear in spring under these conditions in greatest numbers, is added. The "bad" side, with cool northerly winds causes delays in the arrival of these thirteen species. The depressions often have a sphere of influence extending so far as North Africa, so that birds, on the fair side, can cross the Mediterranean with southerly winds all the way .

I have endeavoured to show that often the force of wind is greater at a high than a low altitude, and there is ample evidence to prove that birds fly at a great height when conditions are favourable. Birds usually leave Scandinavia when there are descending currents flowing outwards from the centre of high pressure; is it wild speculation to suggest that it is the southward flowing currents, which are also deflected westwards, upon which the birds intend to travel? Thus the bulk of the Scandinavian birds might not touch Britain at all, but those which started upon light to moderate north-east to easterly winds from the western shores of Norway would be helped to Britain. Mr Clarke mentions that when he was at Fair Island, north-west to westerly winds did not stop migration from the north, but is it certain that the birds did travel in or against these westerly winds? May they not actually have travelled on the "good side" of the cyclonic system, with these very winds carrying them towards Fair Island? their actual visible approach from the north does not prove that they had travelled all the way in this line.

On September 22nd, he says--"The favourable meteorological conditions of yesterday--fine weather and moderate south-east breezes,--has had a marked effect, for to-day goldcrests are swarming everywhere." But what does he mean? Favourable to him as an observer or to the goldcrests? Surely the birds did not aim for Fair Island; were not these weak-winged birds probably making for the south, when the south-east wind caught them and drifted them to the west? Fair Island was a refuge, but hardly the objective of their flight .

Compare this with Cordeaux's notes of another goldcrest immigration, this time to the Lincolnshire coast . On October 13th the wind was north to north-east in the afternoon, light but increasing in force, the weather clear and bright--a few birds arrived. They had started under favourable circumstances. Shortly after midnight on the morning of the 14th, the wind got full east, with quite half a gale and heavy beating rain, continuous to the morning of the 16th; the nights were very dark. "During this time the immigration was immense," and most of the birds were goldcrests. Cordeaux's idea that these were not normal immigrants but birds which were passing probably from north-east to south-west, when the easterly gale caught them, is probably correct.

I have referred to birds starting at a high elevation. Service says that in normal departure from the Solway, most birds mount to a high altitude, but "a strong beam wind will bring the birds--even those of strongest power--down to 200 to 500 feet of the surface, and it is interesting to see whole flocks with heads turned almost completely to wind, and yet travelling along at nearly their normal speed, at right angles to their position" . Mr Tomison mentions rooks, daws and hooded crows driven to Sule Skerry by south-east winds in March, leaving two days later in a westerly gale. They, at any rate, did not object to a strong wind which was in the right direction.

I have mentioned Mr F. J. Stubbs' paper on the "Use of Wind" , and I believe that there is much more in it than is actually proved by low-level observations. I doubt if birds always intentionally make use of strong winds, currents which would carry them for great distances at a considerable speed, but the preliminary ascent may be to search for these currents. Cyclonic and anticyclonic winds, even when at an altitude of some thousands of feet, would carry them easily, and probably it is the wind-borne individuals, parties, or even hosts, which drop for a refuge to the first island they see when carried far from their migratory path. They are carried rather than drifted from their pathway, borne in the moving current whether they wish it or not. Provided that the cyclonic winds are fairly steady in direction and force, sweeping round and inwards towards their centre, we may in imagination trace the pathway of our so-called lost wanderers to far distant islands; without many more upper-air observation stations, we cannot actually prove the route.

But even putting aside the high altitude idea, and confining our route-tracing to the known courses of air currents, we shall find immense difficulty in mapping out the actual course of any bird on any particular day. The study of some of the publications of the Meteorological Committee, such, for instance, as the "Life History of Surface Air Currents," by Shaw and Lempfert, published in 1906, shows the great variation in the pathways, speeds, and formation of these systems; a bird which accidentally entered a cyclone would unconsciously alter its actual track and speed very many times before it passed beyond the area of influence.

I am indebted to Mr Stubbs and Mr Herbert Taylor of King's College, London, for some interesting mathematically worked-out routes of birds, travelling at a given speed in a cyclone rotating at given speeds and moving at a fixed rate; these show great variation both in direction and speed according to the time and place of entering the system. The track of the bird is, of course, influenced by its own rate of progress, by the speed of the rotating currents, and by the rate at which the whole system moves in any direction. Thus a migrant passing south and coming within the influence of a cyclone which is moving north-east at a high rate of speed, say 40 miles per hour, will, if it enters towards the northern limits of the system, be at first retarded by the conflicting forces of the easterly winds, the trend towards the north-east of the rapidly travelling cyclone and its own southward flight. If it is flying faster than the speed of the cyclone it will drift westward but gradually approach the low pressure centre. After passing this its course will at once change and its speed will be accelerated towards the east.

Even violent storms move at varying rates, and it is conceivable that a bird leaving Scandinavia on favourable anticyclonic winds might at once come into the influence of a large, slowly-moving, circular storm, with a low-pressure centre to the west of Ireland, and might, if the air currents were strong, be carried westward at first, then south and finally eastward, so that it would actually pass round the British Islands. I have taken this exceptional case from the actual course of a storm, which varied between forces 9 and II on the Beaufort Scale but only travelled slowly eastward at about 17 miles per hour. In some cases the storm centres are nearly stationary for many hours.

It is easy to appreciate Herr Herman's statement that spring immigration in Hungary is accelerated on the good side of a mild cyclone; the direction of the bird, of the circulating air currents and of the whole system may be coincident. Given a low-pressure centre west of the Bay of Biscay, spring migration would be accelerated through Spain and France towards Britain.

Mr Stubbs points out that the pathways of several birds, or parties of birds, which started at different hours, would be divergent, for they would come within the influence of winds blowing in various directions according to the position of the system; this he argues is contrary to the accepted idea of routes. This, however, entirely depends upon what we mean by a route, as I endeavoured to show in an earlier chapter. The journey from point to point is a route, although the bird may be drifted many miles in one direction or another on the way; it is only when the bird fails to reach its objective, a suitable breeding place or winter station, that the route is a failure.

The frequent occurrence of rare birds, some of them almost or quite unknown elsewhere in Britain, on out-of-the-way islands, has led to strange theories. One is that there are regular fly-lines over Fair Island, the Flannens, St Kilda and elsewhere, similar to the one which is said to pass over Heligoland. Mr Eagle Clarke's long expected book will contain the ideas of the man who is best able to theorise on this point; I write, now, with the feeling that his knowledge may lead me to alter my ideas. The suggestion I can offer at present is that there are ornithologists directing their attention to these spots which, through geographical position and isolation, are the likely refuges for wind-borne migrants. Also that the accidental departure from the directions aimed at by the birds is, where wind and barometric systems are so variable, far more frequent than is usually suspected. Direct routes are doubtless aimed at, but only accomplished under favourable conditions for the whole journey; migration is less infallible than we have been led to think. It is, too, an evolving habit, strengthened by those which survive its perils, now as it was in its early days.

During a long overland journey, winds will probably have less influence, though for rapid passages high flights certainly appear to be not uncommon. There is, however, another aspect of the connection between migration and weather which we have hardly touched, migration synchronal to the change of season. Mr Cooke shows that in North America the push forward in spring is not in most species so soon as the weather permits; they do not actually move on the spring wave. Many warblers which nest in the Great Slave Lake region in an average temperature of 47?, linger in the Tropics, and reach New Orleans when the temperature is about 65?F. Then they hasten northwards, outstripping the advancing spring, finding in Minnesota a temperature of about 55?, and 52? in Manitoba, and gain another 5? on the season by the time they reach their home. Thus they continually reach colder weather as they travel north.

Instances worked out in America and elsewhere might be quoted to show how some species forge ahead and others lag behind the vernal wave. Each species needs separate tracing in its routes and times and habits, but on the whole the movements have relation to the changes in seasonal temperature. In autumn the journey varies according to the time of starting. Early fall migrants, and indeed the majority of autumn migrants all the world over, travel more slowly than in spring; they are neither impelled by sex-impulses nor the need to escape from failing food supplies. A little later the supply does slacken and with it the temperature cools, and if the changes are sudden southward migration is accelerated. Migration, however, is such an advantageous and well-established habit that it usually begins before hurry is necessary, and the birds loiter southward, feeding as they go.

THE PERILS OF MIGRATION

During normal migration birds may be brought to a lower elevation by strong contrary winds, or they may be bewildered by fogs and cloud and dropped nearer the surface; it is then that the travellers meet with disaster at our coastwise lights.

Mr Tomison records some of his experiences of migration at Skerryvore . He never saw a bird at the windows when the moon was shining, and on clear nights the passing crowds go on without a pause. But on hazy nights, with an easterly wind and drizzle, or during fogs, if large numbers of migrants are passing, hundreds may be seen flying in all directions, "all seemingly of the opinion that the only way of escape out of the confusion--is through the windows of the lantern." On one September night, when he was standing on the balcony, he likens the appearance of the birds to a heavy fall of snow. "Thousands were flitting about; hundreds were striking against the dome and windows; hundreds were sitting dazed and stupid on the trimming paths; and scores falling to the rocks below, some instantaneously killed, others seriously injured, falling helplessly into the sea." On the following night when many fieldfares, redwings, thrushes and other birds were passing, he says--"Sometimes we use the terms hundreds and thousands without thinking what these figures mean but on this occasion when I say thousands were killed I do not exaggerate in the slightest."

Mr W. Brewster's account of his experiences at the Point Lepreaux lighthouse , shows that similar disasters occur in Canada and the States, as indeed they do wherever there are passages of birds. On a foggy evening in September 1885 "as soon as the sky became overcast small birds began to come about the light--with the advent of the fog they multiplied tenfold in the course of a few minutes" and many struck. "About the top of the tower, a belt of light projected some thirty yards into the mist by the powerful reflectors; and in this belt swarms of birds, circling, floating, soaring, now advancing, next retreating, but never quite able, as it seemed, to throw off the spell of the fatal lantern.... Dozens were continually leaving the throng" of birds which had flown to leeward, "and skimming towards the lantern. As they approached they usually soared upward, and those which started on a level with the platform usually passed above the roof.... Often for a minute or more not a bird would strike. Then, as if seized by a panic, they would come against the glass so rapidly ... that the sound of the blows resembled the pattering of hail." During his stay no birds came to the light except during dense cloud or fog, and they came in greatest numbers when an hour or two before the fog the sky was clear.

The experiences of Eagle Clarke, Seebohm and others who have spent migration seasons at lighthouses might be quoted, but these two give a vivid description of what regularly takes place when weather conditions are unfavourable. Steady white lights are the most fatal to migrants, revolving lights, if white, are struck by some birds, but red lights seldom attract the passers. Mr Eagle Clarke thinks that birds are actually decoyed from their path and arrested in their course by the action of the lights; he says that a change from white to red lights at the Galloper Lightship stopped bird attraction.

On the mainland a new high building or tower, new telegraph wires or other erections, until their presence is familiar, take toll of passage birds.

Mr R. M. Barrington has for years collected information from the Irish lighthouses and light-vessels; some of his results were added to the work of the British Association Committee, and some he published himself . He emphasises the fact that these phenomena depend largely upon weather, and therefore are not trustworthy indications of the density at any time or place of migration. Out of 115 song thrushes killed at the lights and sent to him, 80 per cent struck during the fourth and first quarters of the moon, and the same rule holds good for other species. The intimate relation between the lunar phases and the number of examples killed was shown by statistics from 1888 to 1894. Out of 673 specimens received only 116 were killed when the moon was more than half full.

Apart from fog or cloud, birds may fail to hit the land aimed at, either through accidental divergence from correct direction or wind drift. In November 1884 Mr Barrington received information of large numbers of rooks passing simultaneously at the Tearaght and Skelligs Lights--island stations 20 miles apart and each 9 miles off the Kerry coast. The birds arrived in continuous flocks from the westward--the open Atlantic--and passed in an easterly and landward direction; they came in small parties and in flocks numbering two or three hundred, on many days between the 2nd and 25th of the month. A few birds were noticed at the same time at stations on the south and east Irish coasts, and all alike making for the land. From similar observations made in other years he concludes that these were portions of hosts which had overshot the mark, and failing to find land had turned back. The weather charts, he adds, show no sufficient reason for the birds to have been blown out of their course by storms.

The weather charts, as I have pointed out, do not indicate the force or direction of the wind at high altitudes; I suggest that these birds were carried rather than blown out of their way by strong currents at a higher altitude than recorded on the charts, and that having left the air currents they descended to the elevation of about 700 or 800 feet at which most of them were flying when they were observed making for the land.

On the night of March 29th to 30th, 1911, the south-eastern extremity of Ireland experienced a remarkable rush of migrants, and the local papers were full of the avian disaster, for large numbers of birds struck the lights as well as buildings and other objects in inland towns. Mr Barrington collected information , and found that most of the birds were starlings, though thrushes, blackbirds, and redwings were numerous. He received specimens of woodcock, water-rail, snipe, dunlin, meadow pipit, wheatear, goldcrest, starling, song thrush, redwing, blackbird, black redstart, robin, skylark, and stonechat, whilst some thirteen or fourteen other species were said to have been recognised, amongst them oyster-catcher and wild duck. The area affected lay south-east of a line drawn across country from Balbriggan to the Old Head of Kinsale, with a coast line of some 200 miles; most of the birds noticed inland were at towns on the rivers Suir, Barrow, and Nore. The flight was mostly north-east, and at the lights offshore, towards the land. Mr Barrington gives the following explanation. After crossing the Channel the coast of Wexford was reached and the stream divided, some going north along the east coast and others westward along the south coast, but changing their direction when they reached the wide mouth of the Barrow. The flocks which passed Lucifer Shoals, 10 miles offshore, proceeded north without touching Wexford. Northerly and easterly winds had prevailed for weeks prior to the 29th over France and the British Islands, and birds would be held up in southern Europe; the milder coastwise temperature of western France, he thinks, would cause them to take a more westerly course than usual. On the morning of the 29th the wind changed to the south at Valentia, Pembroke and the Scilly Islands, and there was an average rise of 7? in temperature at French stations. This rise and the southerly wind liberated the birds, but as the wind continued north-east or east in England they "decided" to take a longer and more exhausting course than usual, pass to Ireland and then turn north-east. The change took place exactly on the last day of the last quarter of the moon--the darkest night for travel. A bank of fog and drizzle met them off the Irish coast, and baffled and weary they were attracted by the lights, not only on the coast but in the inland towns they passed.

In the main I think Mr Barrington's explanation is correct, but even if the birds were gathered farther west than usual, which I doubt, it was the north-east wind which had drifted them, and the word "decided" is a bold one to use when dealing with the behaviour of birds. Easterly winds would drift them westward, and the striking Ireland was accidental; it was the safety of the many, as well as the deathblow to the comparative few. On the night of the 31st I received news of this visitation, and later found that similar movements, without disaster, were noticed on the north coast of Wales and in Cheshire. On the nights of the 30th and 31st birds in large numbers passed over Bangor and the Menai Straits; amongst them were golden plover, and the next day these birds with fieldfares and redwings were more abundant than before in the mid-Cheshire fields. On the night of April 2nd, from dusk to midnight, a large passage occurred over Mere in Cheshire, where curlew, golden plover, oyster-catcher and wild duck were recognised by their calls, and at the same time a passage was observed at Old Colwyn on the Welsh coast. I do not even suggest that these were the same birds which passed over south-eastern Ireland, but their presence within so short a time, indicates the volume of the movement.

Welsh papers recorded an "Extraordinary feathered catastrophe" at Pwllheli in Cardigan Bay which occurred on the night of March 17th, 1904, in which "thousands" of birds fell dead and dying upon the town and shore. The journalistic description was lurid, but I am able to give the explanation sent to me by a friend who was an eye-witness. The night had been dark and foggy, and in the morning he found "scores of dead starlings, redwings, thrushes and blackbirds lying on the beach at high-water mark." During the night a steamer had been loading setts at the quarry at the Gimlet Rock, a large outcrop outside the harbour, and the artificial light used had been one of the powerful oil flares. The fog-bewildered birds were led astray and had struck masts, rigging, and rock in their confusion.

During a big fire in Philadelphia on March 27th, 1906, Mr W. Stone saw large numbers of birds passing in its illumination, and many passed too near and fell into the blaze; he picked up a few half-burnt song sparrows and juncos.

Blizzards on continents, and to a less extent snow-storms in our islands, account for the death of thousands of travellers. And even in most favourable weather birds fall exhausted. During a stay on the Yorkshire coast in autumn, when migration was even-flowing and unchecked by adverse weather, I found several goldcrests which had reached land only to die, and though most birds came in without showing signs of fatigue, a few larks and starlings were so tired that they made little effort to escape when approached.

Ornithological literature supplies many accounts of more or less similar disasters to migrating birds, but these are enough to show that the perils of migration are not exaggerated.

EARLY IDEAS OF MIGRATION

The evolution of the study and knowledge of migration is an interesting subject, dealt with more or less completely by several writers. In a manual it is impossible to treat it fully. That the Greek poets--Homer and Anacreon for instance, and the writers of Jeremiah and Job, knew something about the regular movements of birds is evident, nor is it surprising that in lands like Greece, Egypt and Palestine the passage of birds should be noted and directly connected in the popular mind with the seasonal changes.

British, and many Continental observers too, saw when birds had come and in autumn that they had gone. Early swallows and martins were always met with near water, and were watched dropping to roost in the reed beds, as they always do in autumn before departure. Next morning none was visible. Certainly then they had vanished to hibernate in the water. The discovery of masses of torpid swallows, dead or dying, by no means an unknown thing when birds are overtaken by sudden falls in temperature in autumn or by a severe setback in the spring, was to these puzzled men confirmation of their theory of hibernation. Other details of the many stories of swallow hibernation are due to exaggeration or to misconception. In the second half of the eighteenth century a fierce discussion waged for and against hibernation, and many, including Geoffroy St Hilaire and Montagu, sat on the fence, admitting that it might be possible with some species and probably was with swallows. Later some Americans produced "evidence" in favour of avian hibernation, and even Mr Charles Dixon, in his earlier book at any rate, did not think it impossible . The only argument in favour of hibernation is that it is a habit resorted to by other vertebrates to escape the consequence of exposure to severe temperatures. The arguments against it are that not a single instance of avian hibernation will stand the light of reason and investigation, and that birds are provided with the means of escaping from the cold zone and certainly use these means. There are flightless birds, but they all live in climates in which they can exist at all seasons. As Seebohm puts it--"The hibernation of birds is a theory, the evidence in support of which has completely broken down. The migration of birds is a fact, as completely authenticated as the fact of their existence."

All this time, from 1736 onwards, the family of Marsham in Norfolk, had been quietly recording observations on the arrival of migrants, each generation continuing the work. The accumulated results have been used, and will be used again, in studying the science of "ornithophaenology."

A myth, founded on mistaken observation as well as upon mere speculation was, and to some extent still is, that the larger migrants assist the passage of the weaker ones. How else, is still asked, can weak-winged species cross the sea? It was an old legend when J. G. Gmelin heard it from the Tartars in 1740; each crane they told him took a corncrake on its back. There are men who know the corncrake well, who believe to-day that the bird must skulk unseen through the winter, for they assert it is quite incapable of lengthy flight. It is useless to argue with them; the only answer is that it not only can, but regularly does perform a long double journey; its range extending from northern Europe to South Africa. In 1911 I handled a water-rail, a bird with short rounded wings like those of the corncrake, which had struck the lantern of a lighthouse with great violence. Its smashed head was nearly severed from its body.

Herr Otto Herman's "Recensio critica automatica" supplies much information about the literature on bird migration, and the strange divergence of opinion on nearly every point. It is carried up to the beginning of the twentieth century, but much of the valuable work done in America is altogether neglected.

A short bibliography is given at the end of the present volume, including the more important works on the subject and a few of the papers in periodical publications referred to in this manual.

SUGGESTIONS AND GUESSES

Several important migration phenomena have hardly been touched upon in the previous pages; a few words about these may not be out of place.

There is no doubt that now and again American species are met with in Europe, and European in America, though there is no evidence of direct regular trans-Atlantic passage, except from Greenland. The appearance of these birds has been explained in several ways, the general notion being that it is impossible for a bird to fly unaided across the Atlantic, say over 3000 statute miles, without rest. In considering the question we are met with various points on which we still lack knowledge.

We know that strong-winged waders can accomplish 2500 miles, apparently without a rest, and that if rest is necessary these birds can swim and rise from the waves. We know, too, that there is regular passage between Greenland and Europe. We do not know how long a bird can, without rest and food, sustain flight; we do not know the speed it can travel when aided by favourable winds, nor to what extent even passerine birds may rest upon the water. My friend Mr J. A. Dockray, when punting in the Dee estuary, has often seen birds alight to rest on his punt, and once saw a tired thrush settle repeatedly on the water and finally safely cross the estuary. There are several instances recorded of passerine birds alighting upon and rising again from the water.

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