STUDIES IN BIRD MIGRATION.*
THERE probably exists no one in the world better fitted than Mr. William Eagle Clarke to write this book. In the first place, the British Isles afford unrivalled advantages for studying the migration of birds. In the second place, he has enjoyed experiences which few can match. He prepared the reports of the great inquiry instituted by the British Associa- tion. This was completed in 1903. In 1901 he spent a month in the autumn on the Eddystone lighthouse. Two years later he spent what must have been a most disagreeable month tossing in the Kentish Knock light-ship. In 1905 he spent in autumn holiday on Fair Isle; and, having discovered that it equals Heligoland as an observatory for studying migration, he visited the island -for five consecutive autumns, staying five weeks ` each time, and also thrice in the spring. He has also visited St. Kilda ; and in the two chapters devoted to,the most remote island of the British archipelago we have a nseful summary of - all that in known about the resident.and migratory birds. - In the autumn of 1904 he spent sixteen days • Studios in Bird kigration. By William Eagle Clarke. With Map's, Weather.leharte, sae other Alltudrationa.T. 2 - vole. London.: Gurney_ Jackson. L18s. net.] 7,4 ..sq
on the Flainian Islands, living in thmlighthouite. A visit to .:ITshant, which is one of the most promising points of atter- • 'ration, proved an unfortunate failure; owing to the suspicions 'which the Fashoda affair excited in the minds of the French authorities.
It is almost unnecessary to say that, equipped in this fashion, Mr. W. Eagle Clarke has written a book which is of great -interest to ornithologists. It also adds much to our know- ledge of what still remains a somewhat mysterious matter. , Having -touched shortly on the views of the prophet Isaiah .and the philosopher Aristotle, he devotes a chapter- te modern' • aspeets of the problem which will help the less learned , readers to follow the special studies which follow. If we ask . what birds migrate, we are told that tropical birds are essenti- ally sedentary, but that most of the birds in northern regions' ..are migratory. The best explanation that we can give of the „causes of migration seems to be that want of food drives birds, -southwards in autumn and that a sort of patriotism impels. them back to breed in the place where they were bred. Time ; has, however, modified the primitive state of affairs ; and .many birds now depart from the northern lands as soon as the young are able to take care of themselves. Birds migrate ,across-the sea by night in order that they may search for food by daylight as soon as they arrive. The stomachs of newly- „come migrants, at points on the coast, are always empty. . The fact that birds travel during the hours of darkness leads to the belief that they are endowed with what the late Pro- ; fessorNewton called "inherited but unconscious experience,”
• which means that a special sense of direction guides them. It :has long been clear that the movements of migratory birds are .far more complicated than a mere direct north and south ,journey.
There are various groups of British migrants ; and one may :nummarize their movements during the English spring some- , what as follows : (1) Local movements from British winter retreats to British summer haunts; (2) the return from their . Continental winter quarters of the partial migrants ; (3) the „arrival in our islands from their southern winter retreats of :summer visitors; (4) the departure from our islands of the winter visitors for their summer haunts ; (5), the appearance , on our shores of birds of passage from the South bound for snmmer haunts beyond our isles; (6) the return of the birds !evicted by the severe weather of the previous winter; and J7) the appearances of casual visitors. We now know that birds which were once believed to be sedentary regularly move their haunts.; so the familiar song-thrush figures as a summer visitor, a partial migrant, a bird of passage, a winter visitor, and a winter emigrant through evic- etion. Some thrushes, moreover, are residents. The move- .-ments of autumn are less defined and more difficult to .observe than those of spring. The migrants slip away, or :arrive, unnoticed in September and October. It is at this season that the " rushes " at most lighthouses and at points on ,the east coast enlighten us as to what is going on. At the Eddy- stone, and elsewhere, the lantern; however, only acts is a decoy in certain atmospheric conditions. There must be moisture, :rain, haze, or cloud present in the air. Then the revolving .rays present a mystifying appearance, and thousands of birds :strike the glass lantern and fall dead or flutter wounded to drown in the waves. Their roast bodies form a welcome :relief from the diet of salt beef ; and the records that have been kept by lighthouse keepers all round our coasts for :many years past have added enormously to our knowledge of- the movements of birds. At the Kentish Knock light- -ship, which lies near the centre of the migratory stream that proceeds from the Continent to our coasts, there ;is evidence of extremely complex migration where many lines rof flight probably cross. Vast Numbers of birds pass during ;the autumnal east-to-west movements. It would seem that they are mostly birds of Central European -origin, whodeave the Dutch poset nt the:mouth bf the Rhine having poured -.down the:river valleys• from distant inland districts. . AC:Fair Isle, midway between the Orkneys and Shetlands, ave-haenanother,side of. migration presented to ue. Here the ,tectet tatounding-variety of :birds stop to reatand- feed. !.As resnit- of six, and half years' investigations this -insigni- &ant island has beetevieited by. no. less thait 207 species, or ?bout one-half of the birds that have ever been known to have .2c4itrresriliAilViti4 Mee Nor are ifs reaceUrC'ea..iii.this respect by any i theatti exhatiated -On the ettCh
Year adds its quote tif important records and novelties, and it is intended to continue the investigations for some 'time to come." The portions of Mr. W. Eagle Clarke's book which deal with the observations that have been made on Fair Isle are among the most instructive that we know of. There are twenty-eight species breeding on the island, of which ten are summer visitors. All the remaining 179 species out of 207'are migrants of the most miscellaneous description. One of the latest is a Pine-Bunting, a native of Siberia, not pre- viously known to have visited the British Isles. The real interest of these rarities is that many, once supposed to be casual stragglers, are now discovered to be more or less regular birds of passage. In this way fresh light comes on the many unsolved problems Of migration that remain.
Gatke believed that migrating birds flew at enormous speeds, even attaining two hundred miles an hour. This view is now generally discredited. Mr. W. Eagle. Clarke's own experi- ences at the lightship and on his return journey, when he had many opportunities of observing parties of birds crossing the sea by daylight, do not lead one to believe that they fly very fast. Nor does it seem that most of the Passerine birds, at all events, fly at enormous heights, as was once suggested. Lastly, we come to the meteorological conditions under which migration mostly takes place. On this new light also has re- cently been thrown. Birds, like ourselves, prefer fine weather for travelling ; but it often happens that they may leave Norway in a calm and sunshine and fly into ,a gale and rein. The mortality at lighthouses and in bad weather must be beyond all possible reckoning. Thousands and , thousands perish. Birds are extremely sensitiye to atmospheric condi- tions, and they discern approaching weather changes. Year after year it has been proved by experience that, the type of weather which prevails when the winds range from south to east is the most favourable for migratory movements both in spring and autumn. It is not the direction of the wind that affects the birds, but the type of weather that ac- companies the wind. There are eharts in this volume, with isobars and wind-arrows, which merit eareful study. The great migratory movements, which observers call "rushes," are largely performed when a large Continental anti-cyclone lies to the east of the British Isles and extends to our shores. It often happens in the autumn that fog accompanies great migratory movements; then birds are sometimes killed at lighthouses in great numbers; and there is" some slight evidence that birds lose themselves in fog, which is rather contrary to the theory that they have a special sense of direction. There are eight chapters in this book devoted to the particular migrations of the swallow, the field- fare, the white wagtail, the song-thrush, the skylark, the lapwing, the starling, and the rook. In these one can follow out in the case of each species the general considerations which affect migration in these isles .throughout the year. Numbers of unobservant people note the arrival of the cuckoo and hasten to write to the local paper;, but few notice or perceive the :complier—movements that are going on during almost every month of the year from February to November among a hundred or, more : other species. It is, in fact, extremely difficult to observe migration inland and in a wooded country, even for a, man who devotes his whole time to ornithology. It is diffeeent. where a few hundred acres of treeless country can .be daily completely perambulated and a diary kept. For this reason Fair Isle promises to be at least as good a station as Heligoland. But even there species which are birds of passage at certain seasons exist as resident species throughout the year. Every bird breeds at the most northern limit of its wanderings and, so far as we know, no bird has a second brood,in its southern winter quarters. That birds travel by defined routes is clearly established. These we are roughly ahle to map out. But the subject of migration is far more complicated than the older naturalists believed. After what we have said it is almost needless, in conclusion, to recommend Mr. W. Eagle Clarke's work to the .attention of "oril t ologists. These studies in migration are of absorbing interest.
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