Sea-Birds

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Sea-Birds
James Fisher

R. Lockley M.

Sea-Birds introduces us to the sea-birds of the North Atlantic, an ocean in which about half the world sea-bird species have been seen at one time or another. This edition is exclusive to newnaturalists.comFew of the higher animals have successfully invaded the polar regions and the sea; but those that have - the whales, seals and sea-birds - have made a wonderful success of it. There are only about 250 true sea-birds in the world (there are over thirty times as many others); yet among this select 250 are some of the most numerous and well-adapted of living species, ranging from the magnificent albatrosses, with their powers of sail-planing, to the curious diving petrels; from the penguins to the auks; from the cormorants to the gulls and terns.The arctic tern makes the longest migratory journey of any known bird, travelling 20,000 miles between the two polar regions in the course of a year. Some sea-birds species probably spend the first seven or eight years of their lives without ever touching land; and one, the emperor penguin, never touches land in its life, for it incubates its egg on the Antarctic ice!This book introduces the reader to the sea-birds of the North Atlantic, an ocean in which about half the world sea-bird species have been seen at one time or another. Sea-birds are generally more cosmopolitan and widespread than most land birds; and it is no surprise to the ornithologist to find that the communities on the American and European sides of the Atlantic are very similar, most of their member-species being common to both.The authors of this book have spent most of their active lives in research on sea-birds, Lockley specialising in Life-histories, Fisher in distribution and numbers. Each has a long record of exploration of the remotest parts of the Atlantic coast and islands. Their felicitous collaboration brings home for the first time to the general bird-watcher and sea-going naturalist what enormous strides have been recently made in our knowledge of sea-birds. We now know the world population of several soecies, and can follow with accuracy the changes in the numbers of many.

Collins New Naturalist Library

28

Sea-Birds

James Fisher and R. M. Lockler

Editors: (#ulink_6379d498-a6a2-50a8-9d93-4897decb3b2d)

JAMES FISHER M.A.

JOHN GILMOUR M.A.

JULIAN HUXLEY M.A. D.SC. F.R.S.

L. DUDLEY STAMP C.B.E. D.LIT. D.SC.

PHOTOGRAPHIC EDITOR:

ERIC HOSKING F.R.P.S.

The aim of this series is to interest the general reader in the wild life of Britain by recapturing the inquiring spirit of the old naturalists. The Editors believe that the natural pride of the British public in the native fauna and flora, to which must be added concern for their conservation, is best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research. The plants and animals are described in relation to their homes and habitats and are portrayed in the full beauty of their natural colours, by the latest method of colour photography and reproduction.

To

JULIAN HUXLEY

in gratitude for his guidance

and encouragement,and in recollection of the

many happy days we have spent together,

watching sea-birds

Table of Contents

Cover Page (#ue06afeee-71c9-5b92-b76d-5580b96341e8)

Title Page (#u2aea93dc-76da-5f42-a395-c58d0bb8448d)

Editors (#u62baa86f-402e-54e4-bc5c-360886a1b5fa)

Editors’ Preface (#u82df8f3c-0794-5426-bea5-9c8bec3c8cd9)

Authors’ Preface (#ubcca142d-8cf4-5e97-ada0-274609783747)

CHAPTER 1 (#u282085cc-6c7b-53c2-981e-2bca970f67d7) THE NORTH ATLANTIC OCEAN: ITS STRUCTURE AND ITS SEA-BIRDS

CHAPTER 2 (#u34b0cf6d-be80-5858-858c-fd2bc3293fc1) EVOLUTION AND THE NORTH ATLANTIC SEA-BIRDS

CHAPTER 3 (#uc0e757ad-758f-5b6d-97ad-6fba17dac40f) SEA-BIRD NUMBERS AND MAN

CHAPTER 4 (#litres_trial_promo) WHAT CONTROLS THE NUMBERS OF SEA-BIRDS?

CHAPTER 5 (#litres_trial_promo) SEA-BIRD MOVEMENTS

CHAPTER 6 (#litres_trial_promo) NAVIGATION BY SEA-BIRDS

CHAPTER 7 (#litres_trial_promo) SOCIAL AND SEXUAL BEHAVIOUR

CHAPTER 8 (#litres_trial_promo) THE TUBENOSES

CHAPTER 9 (#litres_trial_promo) THE PELICANS

CHAPTER 10 (#litres_trial_promo) THE SKUAS

CHAPTER 11 (#litres_trial_promo) THE GULLS

CHAPTER 12 (#litres_trial_promo) TERNS AND SKIMMERS

CHAPTER 13 (#litres_trial_promo) THE AUKS

APPENDIX (#litres_trial_promo) LIST OF SEA-BIRDS OF THE NORTH ATLANTIC AND THEIR DISTRIBUTION

Bibliography (#litres_trial_promo)

Index (#litres_trial_promo)

Plates in Black and White (#litres_trial_promo)

Colour Plates (#litres_trial_promo)

Copyright (#litres_trial_promo)

About the Publisher (#litres_trial_promo)

EDITORS’ PREFACE (#ulink_d9ac10cb-1a5d-5478-81ae-65e1194af779)

IT IS NATURAL that in a series dealing with the wild life of the British Isles sea-birds would be a subject planned for early publication; and in fact this book was announced as forthcoming five years ago. That it has not been completed earlier is not due to any want of industry on the part of its authors. On the contrary, in their researches for this book they have found their subject so absorbing that they have made the interval an opportunity to continue to publish numerous scientific papers, and two monographs, on sea-birds. James Fisher is the author of The Fulmar (1952); and R. M. Lockley, author of Shearwaters (1942), has just published Puffins (1953). There could, in fact, hardly be any other pair of authors better qualified to describe the sea-birds of the North Atlantic than these with their experience of many years of field work and visits along the coast and islands, from Spitsbergen and Iceland in the cool north, to Madeira and the Salvages in the warm south, of that great demi-ocean. They have made their visits often together, and lived much on the small remote islands where sea-birds breed.

The North Atlantic, busiest ocean in the world, is revealed in the opening chapters not as a monotonous watery plain, but as an intricately varied, densely inhabited foraging ground for sea-birds. This avian community, though remarkably homogeneous in different sections of the broad expanse of the North Atlantic, is fascinating in the variety of the species that compose it, and in the complexity of their movements and migrations. The annual migrations of some species extend the total range of the community from the arctic to the antarctic. These long transatlantic migrations, verified by ringing, take species from east to west between Europe and North America, and from north to south between Greenland and South Africa, Britain and South America.

The authors tell us of the primitive progenitors of the sea-birds, dating from over sixty million years ago, and the evolutionary adventures of their descendants, including the notorious extinction of the strange flightless great auk, the sad decline of many other fine species, also the rediscovery of the cahow after it had been presumed extinct. They have paid special attention to geographical distribution, and have provided a unique collection of maps, giving us, for the first time, the distribution of most species of North Atlantic sea-birds.

Chief among the authors’ interests has been the study of sea-bird numbers. They were largely responsible for organising the surveys of that splendid and typical North Atlantic animal, the gannet, which provided biology with the first reasonably accurate figure for the world population of any single and fairly numerous bird species. They have, from their own notes and those of many amateur and professional bird-watchers, produced interesting statistics of the total population of the fulmar, the Manx shearwater, the puffin and many others. Incidentally, such careful counts, site by site, reveal the continuous change that is going on in sea-bird populations, often directly or indirectly due to man’s influence.

The chapters on life-history are preceded by a general account of social and sexual behaviour, which throws light upon the significance of the prolonged and, to the observer, entertaining, mutual ceremonies of these strictly monogamous birds, their pair-formation, their fidelity to their mates, their nest-sites and their parental duties; at the same time problems of instinct and learning ability are discussed. The life-histories include much original field-work by the authors, who have been responsible for several discoveries concerning the incubation and fledging of a number of sea-birds.

We read of the birds’ ecology, their sharing of the wild frontiers of the land where they nest, their niches in the economy of the ocean. We learn of the contrasts between cliff-dwelling and hole-nesting species, of how the guillemot and razorbill chicks, exposed to many dangers on the open rocks, hasten their feather-growth and depart to the sea in two weeks, while the young puffins, safe in the darkness of their burrows, delay their departue for seven weeks, and are finally deserted by their parents; we learn of the strange lives of the shearwaters and small petrels which wander after the breeding season between the North and South Atlantic Oceans, living in perpetual summer—the Tristan shearwater “wintering” in our northern summer, and the Manx shearwater enjoying its “winter” in the southern summer off the coasts of South America.

But we have said enough to indicate the richness of knowledge brought together in this volume, which we confidently recommend as indispensable to everyone interested in the birds of the sea.

THE EDITORS

AUTHORS’ PREFACE (#ulink_8a15a433-cbd1-5745-bb84-a575364940c2)

THE HEROES of our story are rather over a hundred species of birds whose life is a sea-life, whose habits enable them to earn at least part of their living in, or on, salt water, and which have been seen in the Atlantic Ocean north of the Equator.

The North Atlantic is the scene of our book, the great ocean that is now the most travelled by man. Its two sides are provided with an almost equal variety of sea-birds: sixty-eight species, or rather over half are common to both. Of all Atlantic countries Britain, considering its size, has the greatest number of sea-bird species; with no less than eighty, it can boast on its list all but six of those that have been seen on the Atlantic coast of Europe. The British Isles therefore make a good headquarters for a survey of the sea-birds of the North Atlantic. In Britain, and from Britain, the writers of this book have explored the eastern Atlantic sea-bird stations, and enjoyed many fine islands and memorable experiences. One or the other of us has sought the sea-birds south to the frigate-petrel burrows of the Salvages, near the Canary Islands; north to the ivory-gull colonies on the nunataks that rise from the ice-cap of Spitsbergen; or from 30°N. nearly to 80°N., a distance of more than three thousand miles; west we have ranged to Iceland, the Faeroes, Rockall, St. Kilda and the Blaskets of the Kerry coast; east we have travelled to Heligoland, and as far as Laesö in the Kattegat and Gotland in the Baltic, with their off-lying islands of sea-birds. There is no coastal county in England, Wales and Scotland that has not been visited by us both, and not one in Ireland that has not been visited by one of us.

No good British sea-bird cliff or island has been overlooked in our search for what the naturalist searches for; our experience and enjoyment has been long and continuous because both of us are, each in his somewhat different way, obsessed with sea-birds and with islands. We have spent a combined total of nearly seventy years sea-bird watching.

We have seen the little crags and green island swards of the Isles of Scilly and the drowned coast of Cornwall; the granite cliffs and puffins of Lundy; the chalk of south England east from Dorset; the flats and shingles and dunes of Essex and Suffolk and Norfolk, and the sanctuaries of Havergate and Minsmere and Walberswick and Cley and Blakeney and Salthouse, with terns and avocets and many kinds of marsh-birds. One of us has spent many years of his life in the county of Pembroke, living on Skokholm, and on other islands and peninsulas of the Welsh coast; of its sea-birds he has written in many books, and on Skokholm established the first permanent coastal bird observatory in Britain; the other has spent parts of twenty seasons in North Wales, and has worked its coast from St. Tudwal’s Islands to the Little Orme. Both of us know the Yorkshire bird-cliffs most of the way from Flamborough Head to Saltburn; and we have explored the shore of Durham, where bird-cliffs and black industry mix. In Northumberland we know Cullernose Craster, and Dunstan-burgh and Bamburgh Castle, and the cliffs north of Berwick, and other places where sea-birds nest; and we have been to the Holy Island, and to Coquet Isle, and to various of the Farne Islands, where the guillemots and kittiwakes are tame. We have seen the steep cliff-hill of the south part of the Isle of Man, and the sanctuary of the Calf; and have visited the inland gull colonies of North Lancashire and the Lakes.

In Scotland we have, at one time or another, visited every important sea-bird station: in the east St. Abb’s Head, Fast Castle, Tantallon Castle, the Bass Rock, the exciting Isle of May, and many others; in the west the Lowland coast from the Mull of Galloway in Wigtownshire up-Clyde as far as Ailsa Craig, whose magnificent gannetry has been the scene of many weeks of enjoyment and experiment in efforts to improve the counting of nesting sea-birds. Our visits farther north have taken us to Fowlsheugh in Kincardineshire, and round the bird-cliffs of the Aberdeen-Banff border—Pennan Head, Troup Head and others. West along into the Moray Firth we have hunted out the bird-cliffs as far as they go, which is to Covesea in Morayshire.

In the West Highlands we have explored the mainland promontories of Kintyre and Ardnamurchan, and the islands of the Clyde and Inner Hebrides. We have searched the cliffs of west Islay closely from a slow aeroplane. The curious headland of Ceann a ‘Mhara on the lovely sunny Island of Tiree has been investigated, as have the odd-shaped Treshnishs, home of seals, and the capes of Mull. The island of Eigg, where the shearwaters nest in a mountain; the magnificent but somewhat birdless island of Skye, and some of its attendant islets and stacks; both the lonely coast of Ross and its islands—Priest, Tanera, Glas Leac Beg and many others, where Frank Darling first worked out his theory of bird sociality by studying herring-gulls.

In the North Highlands we have watched the birds of the Black Isle Coast, and those of Easter Ross where the coast continues north of the Cromarty Firth to Tarbat Ness. In East Sutherland Dunrobin Castle itself becomes a bird-cliff, because fulmars are now prospecting it—and there we have seen them; in West Sutherland we have travelled nearly the whole wild coast, in instalments spread over several years; we know the crags of Stoer; the Torridonian sandstone precipices of Handa, the best bird island in Sutherland; the lonely cliffs on each side of remote Sandwood Bay—and Eilean Bulgach opposite which only half-a-dozen naturalists have visited; the high promontory of Cape Wrath, and the higher cliff of Clò Mor to the east of it—the highest mainland cliff in Britain—where the guillemots on two-hundred-foot stacks must be observed from six or seven hundred feet above; Fair-Aird Head and the home cliffs and caves of Durness; the huge white crags and stacks of Whiten Head; the complex of islands and cliffs that stretches thence to Caithness, whose headlands too, we know, and their birds—Holborn Head, Dunnet Head, John o’ Groats and Duncansby Head, Noss Head, Berriedale Ness.

In many years, and many boats (as well as from aircraft), we have enjoyed the Outer Hebrides, from North Rona (which many call the loneliest place to have been inhabited in Britain) to Barra Head. We have seen the seals and birds of Rona, and counted the gannets of its lonely neighbour Sula Sgeir; and have hunted out the coast of the Lewis, and much of Harris. One of us has slept some nights on the Shiants, among the rats that may be affecting the population of that vast remote puffinry; and has several times threaded the maze of the Sound of Harris, and eight times has been to St. Kilda, whose unsurpassed cliffs and towering stacks have to be seen to be believed (and are sometimes then not believed). We have traversed the Long Isle—North Uist, Benbecula, South Uist and Barra—and many of its attendant isles, and carried on to sail close under the cliffs of Mingulay and Berneray, which for remoteness, grandeur and personality are rivals—much overlooked rivals—to those mighty precipices of St. Kilda, Conachair, Soay and Boreray.

One hundred and ninety-one miles west of St. Kilda, and about three hundred miles from the mainland of Scotland, lies a tiny rock which has been a magnet for us both—not only because of its bird-problems, but because it is a tiny remote rock! Fisher flew over Rockall in 1947. In 1948 Lockley spent twelve days in a trawler fishing within sight of, and on occasion very close to it. In 1949 Fisher sailed there in H. G. Hasler’s sixteen-ton yawl Petula, and spent some time investigating it at close quarters.

One of us has visited Sule Stack, the lonely gannetry thirty miles west of Orkney; and we have enjoyed nearly every island, from North to South Ronaldsay, from Eynhallow to Hoy, and have seen sea-birds in a great range of surroundings. Neither of us is a stranger to the well-named Fair Isle, a great migration and sea-bird station. We know the Shetland gannetries of Noss and Hermaness, where thousands nest—though forty years ago there was none. We have stood at the top of Foula’s Kame, and gazed twelve hundred and twenty feet to the auk-scattered sea below. We have sailed in and out, and round about, the stacks and rocks and skerries, and voes and geos of straggling Shetland, and seen many a fine cliff, from Sumburgh in the south to Saxa Vord in the north; from Noss on east to Papa Stour on west. We are no strangers to Fitful Head, or Hillswick, or Ronas Voe, or Burra Firth; or to Hascosay, the bonny isle of Whalsay, Fetlar, Bressay or Mousa; or to the Out Skerries, nearest British land to Norway.

Perhaps in Ireland we have not seen all we should; but one of us knows the windy corner of Kerry, the end of the world, where the pure Irish survives on the Blasket, and where the fulmars now glide and play round Inishtearaght, Inish-na-Bro, and Inishvickillaun; and where the gannets mass white on the serrated pinnacles of the Little Skellig, second gannetry of the world. He knows, too, the little gannetry of the Bull, and its neighbour the Cow, and other crags of Cork from Cape Clear Island and Dursey Island east to Great Newtown Head. In Clare the cliffs of Moher bring sea-birds to nest among many beautiful flowers. We have seen the bird-colony of the Great Saltee in Wexford, and that of Lambay not far from Dublin. One of us knows the many fine, high cliffs of Mayo and Sligo, and some headlands of the maze-coast of Donegal; the other has watched fulmars haunting the curious inland cliffs of Binevenagh in Derry, and hunted out the basalt coast of Antrim and the Giant’s Causeway.

Between us, then, we have seen much of the coast of our glorious islands; but we have not seen nearly enough, and we hope to see what we have already seen, all over again. And we would see the west side of the ocean we have grown to love, and compare it with the Britain we know, and other sea-bird countries we have seen—the tuff and lava and basalt of Iceland, the basalt crags of Faeroe, the dissected plateaux of Spitsbergen, the misty cliffs of Bear Island, the drowned coast of Norway with snow-coated Lofotens and dark fjords like corridors, the friendly limestone of Sweden’s Gotland, the skerry-guard of Stockholm and Uppland, the dunes of Denmark and the Dutch islands, the red sandstone cliffs of Heligoland (the only cliffs in western Germany), the chalk and granite of north France, and the islets of Brittany; the benign, sunny slopes and little scarp-precipices of the Channel Islands where one of us lived for a while; the warm, shearwater islands of the Portuguese Berlengas, the Madeiran Desertas, and the Salvages; and the gulleries and terneries of the Camargue, within the Mediterranean.

This book is not a comprehensive survey of a problem based upon a lifetime’s experience nor yet a full bibliographical compilation. We have paused in field-work simply to offer this book as a stimulant, which we hope very much it will be. We intend it as no more. It is a statement of some of the facts concerning the wonderful sea-birds of the North Atlantic, and of some of the interesting problems connected with their lives and their evolution. It is intended to exhibit the ignorance of ornithology as much as its knowledge, and to draw attention to what needs doing as much as to what has been done. It is our wish, we must also add, not only to take the reader with us—if he will come—to the east side of the North Atlantic where the sea-birds are more in our personal experience, but also to the western seaboard, which is zealously worked by the ornithologists of the United States and Canada and described by them with such enthusiasm and thoroughness in numerous books and journals. One of us has corrected the galley proofs of this book in an aircraft bound for North America, on the beginning of a journey among the sea-birds of that continent; as he left Britain, Ailsa Craig flashed white with gannets in an April evening sun, and the first bird he saw in the New World, through Newfoundland clouds next morning, was a gannet.

For help, encouragement and information we have more friends to thank than we can mention. Our search of the literature has been chiefly pursued in books belonging to the Zoological Society of London, the Alexander Library at Oxford, the Royal Geographical Society and the London Library, and we thank G. B. Stratton and W. B. Alexander particularly. Among those who have given us valuable help or information (they have no responsibility for the use we have made of it) are B. M. Arnold, R. Atkinson, J. Buxton, T. Cade, F. Darling, E. A. G. Duffey, A. Ferguson, Finnur Guðmundsson, H. G. Hasler, P. A. D. Hollom, J. S. Huxley, the late P. Jespersen, G. T. Kay, Miss J. Keighley, T. C. Lethbridge, H. F. Lewis, C.–F. Lundevall, S. Marchant, R. C. Murphy, E. M. Nicholson, R. S. Palmer, R. Perry, R. T. Peterson, L. E. Richdale, M. Romer, F. Salomonsen, H. N. Southern, D. Surrey-Dane, N. Tinbergen, L. Tuck, L. S. V. Venables, H. G. Vevers, K. Williamson and V. C. Wynne-Edwards. Mrs. E. Marshall patiently typed several drafts of most of this book. J. F. Trotter prepared the final copies of most of the maps. One of these is on a mapnet invented by the late Professor C. B. Fawcett and is used with his permission and that of the Royal Geographical Society (e.g. Fig. 24 (#litres_trial_promo)). Another mapnet, devised by one of us (J.F.) is used for the first time in this book; it is based on the South Pole with the oceans in three petals, and is useful for showing the range of the many sea-birds that have a primarily southern distribution (e.g. Fig. 22 (#litres_trial_promo)). J. Fisher’s fellow New Naturalist editors have been encouraging; and Eric Hosking in particular has found us many unique photographs. R. Trevelyan, of Messrs. Collins, has been most ingenious and helpful. The American Ornithologists’ Union, who published our frontispiece first in the Auk, have very kindly allowed us the use of it; this fine painting by Roger Peterson of the interesting cahow, long thought to be extinct, embellished the paper by R. C. Murphy and L. S. Mowbray on their recent rediscovery of its breeding-grounds.

Ornithologists’ wives do many (if not most) of the chores that husbands normally do. We thank ours for more things than they probably remember.

JAMES FISHER

R. M. LOCKLEY

CHAPTER 1 (#ulink_c75d4537-b570-52e6-8b7c-2d48120592c0) THE NORTH ATLANTIC OCEAN ITS STRUCTURE AND ITS SEA-BIRDS

THE ATLANTIC OCEAN is a big broad blind alley, kinked like a zig-zag, its jagged north end blocked with ice, its broader south butt cornered by the cold stormy narrow eastern entrance to the Pacific Ocean, and by the warm, windy and wide western gate to the Indian Ocean. It resembles two wedges, their apexes towards the North Pole, one of them truncated midway and at that point connected sideways to the base of the other.

The birds inhabiting the more northerly of these wedges, the North Atlantic, are the birds of this book. Two of these birds have become extinct in historical times: the great auk was never seen alive after 4 June, 1844, and the last Labrador duck was shot in 1875, though some say one was shot in 1878. The number of living species that remain is about one hundred and eighteen, of which eighty-six have been seen on the western seaboard of Europe (which includes Iceland), and ninety-three on the eastern seaboard of the New World (including Greenland).

However, for an understanding of the environment to which the North Atlantic birds are adapted, a description of the whole ocean is necessary, and to this we must proceed.

The extremely simple fundamental shape of the Atlantic invites diagrammatic caricature (Fig. 1 (#litres_trial_promo)). It is the second largest ocean in the world. It is, on an average, over two and a quarter miles deep, and in some places nearly six. It is, on an average, three thousand five hundred miles across (maximum about five thousand); and is nine thousand miles long. Its area has been estimated as thirty-three million square miles, and its volume as seventy-five million cubic miles. It is a vast place, with many miles of coast, upon which much of civilization depends: considering its size, it has few islands. In comparison, the Indian Ocean is not quite as large (about twenty-eight million square miles); but the Pacific (about sixty-four million square miles) has nearly twice the area, and is ten thousand miles across its widest part. The Arctic Ocean (about five and a half million square miles) is small and nearly full of ice at all times of year; in spite of this it is at times very full of life. Finally, it is usual to describe the cold waters round the Antarctic Continent (itself the same size as the Arctic Ocean) as the Antarctic Ocean.

South of the normal steamship route from Britain to New York the Atlantic is almost everywhere over two miles deep, and in large areas more than three. But down mid-ocean, following the tropical kink in the zig-zag, runs a very long submarine ridge, above which is less than two miles of sea; it is only broken by deeps for a short distance on the Equator, and it rises to the surface in places—in the northern hemisphere at the Azores and St. Paul Rocks, and in the south at the lonely isles of Ascension, Tristan da Cunha and Gough. Other oceanic Atlantic islands, such as Bermuda in the north, and South Trinidad and St. Helena in the south, rise abruptly from very deep parts of the ocean. A sketch-chart will be found in Fig. 2c (#litres_trial_promo).

It will be seen that there are prominent shallows along the east coast of southern South America, north of the mouths of the Amazon and along the Guianas, in parts of the Caribbean Sea and the Gulf of Mexico (there are also marked deeps in these tropical waters), off the New England States, Nova Scotia and (most particularly) Newfoundland, and round Britain, the Channel and the North Sea, and round Iceland. A submarine ridge, over which the sea is five hundred fathoms or less, cuts the North Atlantic entirely from the Norwegian Sea and the waters of the Polar Basin; Shetland, the Faeroes and Iceland lie on this ridge. Davis Strait is shallow, and the waters of Labrador and Hudson’s Bay very shallow. Where the waters are less than a hundred fathoms deep, what they cover is usually described as the Continental shelf. This has its own particular community of birds.

For practical purposes, and because all charts and maps mark the Arctic Circle and the Tropics, we have classified the North Atlantic and its birds into arctic, temperate and tropical areas based simply on latitude. In our analysis of breeding-distribution, for instance (see here (#ulink_58a5bd34-f8d4-527c-a3a3-7b3cb232d93b)), we regard birds nesting north of the Arctic Circle as arctic, those nesting south of the Tropic of Cancer as tropical, and those nesting between as temperate. However, the temperature of neither air nor water arranges itself, in the Atlantic, according to latitude.

(#litres_trial_promo) For instance, if we examine the July air isotherms over the world north of the Tropic of Cancer we see that that for 45° F. runs well south of the Arctic Circle in the areas Greenland-Baffin Island and Bering Strait, and well north of it off Scandinavia, avoiding Lapland altogether.

FIG. 1

Diagram of the Atlantic Ocean

If we examine a map of the world (showing particularly the lands between the Tropics), we see that the summer isotherm for 80°F. (July in the northern hemisphere, January in the southern) runs well north of the Tropic of Cancer in Mexico and the southern States, and in Africa and Asia, and south of the Tropic of Capricorn in Africa and Australia; yet large parts of the tropical Pacific and Atlantic Oceans never reach an average summer air temperature of 80°F.

In the North Atlantic there is not only relatively little direct correspondence between isotherms and latitude, but there is a good deal of difference in position between the same isotherms under the surface, on the water surface and in the air.

The primary cause of the ocean currents, and of the prevailing winds which are associated with them, is the rotation of the earth. The plot of the Atlantic currents and Atlantic winds is almost, though not quite, coincident. To a very large extent the distribution of Atlantic water temperatures, and to a large extent that of air temperatures, is a consequence of these currents and prevailing winds. However, in parts of the Atlantic evaporation and the melting of ice produces temperature and salinity gradients which themselves produce consequent currents. Hence the web of sequence and consequence, of cause and effect, becomes complex. We must examine the great equatorial current first, for almost every one of the more important sea masses in the Atlantic owes its existence to it. It is quite justifiable to write in terms of sea masses, for, as we shall see, the Atlantic waters are by no means homogeneous and can be divided, sometimes with strikingly sharp boundaries, into volumes possessing very diverse properties.

We need scarcely remind the reader that if he faces a globe, poised in the ordinary way with North at the top, and spins it as the earth naturally rotates, the points on its surface will travel, as they face him, from left to right. The points travelling with the greatest velocity will be those on the equator, and the two points represented by the Poles will travel with no velocity relative to the earth’s axis.

In general terms it is true that, as the earth rotates, its atmosphere rotates with it. However, there is a certain effect due to inertia or drag; and this effect, obviously, is greatest at the equator, where the surface velocity is greatest. The effect operates on all objects but can put only liquids and gases into a dynamic state. Upon these Corioli’s force—the deflecting force of the earth’s rotation—acts in a simple manner. It sets them in motion in a direction which, at the equator, is opposite that of the rotation of the earth. Thus if we examine a map of the prevailing winds and ocean currents of the world, we find pronounced positive east-to-west movements in all equatorial regions. The liquids and gases thus displaced circulate into the temperature regions and perform return movements in the higher latitudes where the Corioli’s force is less. Consequently, in the northern hemisphere water and wind currents tend to turn right-handed, whereas in the southern hemisphere they turn left-handed. (Exceptions to this rule are mostly found in minor seas, where the impact of the currents upon coasts may cause contra-rotation.) The main clockwise movement of the northern hemisphere wind and currents is very obvious.

The Atlantic equatorial current can be traced from the African coast south of the equator westwards as far as the sea reaches. Approaching the coast of Brazil it attains a remarkable speed. It sets past the isolated oceanic island of Ascension so that even in calm weather it leaves a wake of turbulence which must make that island unusually visible from far off by its numerous bird inhabitants.

Just north of the equator the lonely St. Paul rocks, which represent the pinnacles of a submerged, steep-sided mountain over thirteen thousand feet high, face the full strength of the great equatorial current, especially in August, when the associated south-east trades are blowing their hardest. During the cruise of the Challenger in 1860 H. N. Moseley saw the great ocean current “rushing past the rocks like a mill race.” A ship’s boat was quite unable to pull against the stream.

The equatorial current divides when it impinges on the corner of Brazil at Cape São Roque. The northern element—the Guiana coast current flows past the mouth of the Amazon with sufficient rapidity to displace the outgoing silt 100 miles or more in a northerly direction; and it continues steadily past the mouth of the Orinoco and Trinidad to flow with scarce-abated force into the Caribbean, mainly through the channel between Trinidad and Grenada in the Windward Islands.

Through the Caribbean the current flows from east to west, turning northerly and entering the Gulf of Mexico through the fairly narrow channel between Yucatan and Cuba. It is no doubt aided here by the climate, for this part of the world is very hot, and not excessively wet, and there is much evaporation of the waters of the Caribbean and the Gulf of Mexico, which has to be replaced. The current finally comes up against the coast of Louisiana and Texas and proceeds to mill right-handed, escaping finally through the narrow gap between Florida and Cuba, into the Bahama Seas.

Here the Gulf Stream is formed, not only by the waters escaping from the Gulf of Mexico but by more northerly elements of the equatorial current which impinge upon the outer shores of the West Indies and are deflected northwards. This north equatorial current flows across the ocean from the Cape Verde Islands and the joint product swings quickly east again, narrowing in width but probably gaining in velocity, to sweep past the tail of the Great Bank of Newfoundland and thence to carry on as what is now called the West Wind Drift (because of its associated air currents). The most direct continuation of this drift flows northwards and eastwards past the west coast of Ireland (giving off a branch towards Iceland), between Rockall and the Hebrides, through the channel between Shetland and Faeroe, north-eastward up the coast of Norway, whence elements strike east into the Barents Sea and north to reach Spitsbergen. It is because of this warm drift that, of all lands reaching latitude 80°, Spitsbergen has been the most accessible. If it was not for the Gulf Stream, many Oxford expeditions could never have explored there in the Long Vacation and got back in time for the Michaelmas Term.

So far we have described the simplest and best-known currents of the North Atlantic. The fate of the waters in their return circulation is more complex. Much of the return circulation is below the surface, for cool water is denser than warm water. In the lower latitudes of the North Atlantic, between the westward-flowing north equatorial current, and the eastward-flowing Gulf Stream and drift, there is an area of clock-wise milling. The centre of this area is the part of least water-movement, and bears some resemblance to an oceanic desert. This is the Sargasso Sea, usually windless, too, with masses of the floating Sargasso weed, which has berry-like air vessels, and is used by sea-birds as a resting-platform; but on the whole this stagnating area is as devoid of animal life as it is of movement.

There is a corresponding and not dissimilar area in the South Atlantic, which also has calms. It has never been named, though it could well be called the Southern Sargasso. These Sargasso areas contain fewer plants and animals than any other part of the ocean. In both there is a rather fluctuating and not very well marked line or lines of convergence between the warm equatorial waters and the comparatively cool temperate waters.

We must now return to the temperate waters, which, as we have seen, form a drift right across the Atlantic and into the Polar Basin. starting on the west below latitude 30°N. and reaching latitude 70°N, or more on the east side. The counter-movements and mills consequent on this great temperate drift are mostly in an anti-clockwise direction. Thus the waters of the North Sea tend to rotate anti-clockwise, running south down the British coast, east and north round the Heligoland Bight, and north-west from southern Norway. In the Norwegian Sea two major and several minor anti-clockwise mills can be detected, and the waters of the Barents Sea also tend to revolve anti-clockwise.

But the greatest counter-movement in the North Atlantic is composed of the Greenland and Labrador currents, carrying cold, heavy water south past Labrador, past Newfoundland and far down the United States’ eastern seaboard. This great counter current sets south along the east coast of Greenland down the Denmark Strait between Greenland and Iceland, round Cape Farewell, the southern tip of Greenland carrying with it many bergs tumbled from the sliding glaciers of the inhospitable east Greenland coast, runs north some hundreds of miles up the west coast of Greenland, then west and once more south, collecting the ice of Davis Strait and Baffin Island, and pursues its final course down the Labrador shore. As it turns the corner of Newfoundland and passes over the great shallow Banks, it deposits its last icebergs and suddenly impinges on the northern boundary of the Gulf Stream or West Wind Drift. Here a long, well-marked line of convergence extends for many hundreds of miles. The cold water sinks rapidly under the warm, and much turbulence is the result. Many organisms are brought to the surface. There is a steep temperature-gradient and frequent climatic upheavals, including fogs. It is largely because of the cold Labrador current that New York, though a full ten degrees farther south, enjoys a climate similar to that of London though with greater extremes of temperature.

The Atlantic thus is a mosaic, not a homogeneous area. Each patch in the mosaic is characterised by some peculiarity of climate. In practically all areas the water, the prime constituent, is in a state of continual movement. The fortunes and distribution of our sea-birds depend on this environment, so continually in turmoil. We must beat the bounds, then, of the North Atlantic and discover how our birds and their lives are interlocked with this climate and scenery.