Kitabı oku: «Sea-Birds», sayfa 2

CHAPTER 1 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). 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.

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), 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.^* 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.

A suitable place from which to begin our tour of the North Atlantic is the St. Paul Rocks. Only three species of sea bird nest on them—the brown booby Sula leucogaster, and the noddy terns, Anoiis stolidus and A. minutus. The islands have been visited by many naturalists, including Charles Darwin, who spent some hours of the afternoon of 15 February 1832 obtaining bird specimens with his geological hammer!

From here we move to the coast of South America between the Equator and the Caribbean: this is a mud-coast and not, as are many tropical coasts, a coral coast. Indeed, there is no sign of the coastal coral barrier-reef off Brazil until some distance south of the Equator. If we start at the Equator, on the islands in the mouth of the Amazon, we find a typical river bird-community. The water is fresh for some considerable distance outside into the ocean and the birds consist of skimmers (Rynchops nigra) and various river-loving terns such as the gull-billed tern Gelochelidon nilotica, the yellow-billed river-tern Sterna superciliaris, and the large-billed river-tern Phaëtusa simplex. Off-shore the true sea-birds come in, and Murphy records species such as Leach’s petrel, Wilson’s petrel, the Tristan great shearwater, the great skua, boobies and tropic-birds. North of the Amazon mouth the Brazilian Guiana coast is forested down to the muddy shore. Many small rivers, often choked with the debris of tropical forests, flow into it.

In French Guiana, however, rocky promontories and islets appear, and they are inhabited by some sea-birds; regrettably little is known about the species involved, but they probably include boobies and tropic-birds. Along the coast of Dutch and British Guiana we are once more in a muddy coast with no headlands or islands. North-west of the mouth of British Guiana’s main river, the Essiquibo, there are some shell-beaches, but most of the coast is of mangrove-swamp jungle, in which the only animal resembling a sea-bird is the Mexican or bigua cormorant Phalacrocorax olivaceus. Over the Venezuelan border we are at once in the delta of the great river Orinoco. It is a land of dense mangrove forest and a very large number of low wooded islands. Off-shore the immense tonnage of mud and silt is seized by the equatorial current and driven northwards towards Trinidad, which it thus provides with a very wide continental shelf. As Murphy (1936, see here) writes, “The delta of the Orinoco is not the home of birds that can be called marine … Only our adaptable old friend the Bigua cormorant seems … at home.”

Generally speaking, from the mouth of the Amazon to the mouth of the Orinoco the coast scarcely harbours a breeding sea-bird. However, the British islands of Trinidad and Tobago, off the northeast corner of Venezuela are provided with rocky promontories and many islets on which sea-birds nest. The brown pelican Pelecanus occidentalis, the red-footed booby Sula sula, the man-o’-war or frigate-bird Fregata magnificens, nest on low trees or on mangroves. On the bare Soldado rock the sooty tern Sterna fuscata, and the two species of noddy, nest. One tubenose, Audubon’s shearwater Puffinus I’herminieri, nests on Tobago, which is its southernmost breeding place on this coast. The gull-billed tern nests in fresh water marshes.

West of Trinidad we are in the Caribbean Sea and following the coast, which for 250 miles more has a wide continental shelf, with small islands dotted in it. Opposite the western part of Venezuela, however, the water is much deeper close in-shore, and the off-coast islands of Curaçao and other Dutch possessions rise from a deep sea. Both the islands of the shallow shelf, such as Los Hermanos and the Testigos, and these Dutch islands, have many sea-birds, including three kinds of boobies, man-o’-war birds, tropic-birds, noddies and sooty terns. At least eight species of terns are found at Aruba, the westernmost of the Dutch islands. But there are few species which can be described as oceanic, though the boobies are marine; many of the sea-birds probably nest on the islands rather than on the mainland because of the additional safety and the existence of outcrops of rock such as are not found along the interminable mangrove coast.

Of all coasts that we have so far considered, those of northern Venezuela are the driest, and the Caribbean is the hottest part of the North Atlantic region. The western Caribbean, however, has intense summer rain; in spite of this, evaporation is great and the equatorial current is boosted along, flowing into the Gulf of Mexico with some rapidity.

In the Antilles, which form the eastern and northern boundaries of the Caribbean Sea, we find islands clad still in fairly thick jungle vegetation, with coastal mangroves, but also many sandy islets and bars and real coral reefs. Though the Guiana coast was too muddy to support coral reefs, these are found fringing the islands north of Venezuela, such as Curaçao. There are also many reefs along the western shore of the Caribbean, particularly at the corner of Nicaragua and Honduras, at the end of the shallow Mosquito Bank. Throughout the West Indies the distribution of sea-birds is linked primarily with available food, but that of the breeding adults probably also with available nesting-sites. Islets where there are exposures of rock or sand are much favoured, but some species as we have seen, including the red-footed booby, the brown pelican, the bigua cormorant, the darter Anhinga anhinga, and some terns, nest in trees. One very rare petrel Pterodroma hasitata (see here) nests above the tree-line on some of the West Indian islands, among the rocks of steep mountains.

A typical sea-bird islet in the West Indies is Desecheo, described by Alexander Wetmore. This lies in the hot dry zone west of Porto Rico. It is a rocky islet with cliffs and a gravel beach, and a thin top-soil covered with a dense thicket of cacti and the curious West Indian birch. Here brown boobies nested on the ground among the thickets and floundered through the prickly pear and cactus. Sooty terns nested on ledges, on shelves on the limestone cliffs, and B. S. Bowdish found a few bridled terns Sterna anaetheta, nesting on flat ledges. This species also breeds on the little islets or cays of the Barrier Reef south of Jamaica, among the broken coral rock and the mangroves.

North of the Antilles the low-lying British islands of the Bahamas occupy a large area of the west Atlantic. The blue Atlantic beats directly against steep east-facing limestone cliffs, while to the west there are shelving beaches. Many of these islands are covered with cacti, and the sea-grape Coccolobis, which forms low, thick vegetation in which brown boobies nest, scraping slight hollows in the ground and lining them with grass. In some Bahamas the man-o’-war bird builds its nest quite on top of the prickly pears, though more normally on the mangroves in the swamps, together with brown pelicans and the double-crested cormorant of Florida Phalacrocorax auritus floridanus. Upon the more exposed sandspits in the Bahamas several kinds of tern breed, including the gull-billed tern, the little tern Sterna albifrons, the roseate tern S. dougallii, Cabot’s tern Thalasseus sandvicensis, and the sooty tern.

The coast of the Gulf of Mexico is low-lying, with coral reefs and an extensive continental shelf, especially off Yucatan. Breeding sea-birds are scarce, except terns and the ubiquitous bigua cormorant, which is as much a fresh-water as a salt-water bird. The Sandwichtern, which is known as Cabot’s tern in North America, breeds in several parts of the Gulf coast of Mexico, which is more suited for terns than for any other sea-birds. On the grassy islands among the lagoons and marshes of the Texas coast, the gull-billed tern and Forster’s tern Sterna forsteri, are found. The beautiful Caspian tern Hydroprogne caspia, also nests in a few places on sandy islands, and there is an interesting outpost breeding-station of the white pelican Pelecanus erythrorhynchos, on the Laguna de la Madre, south of Corpus Christi, near the Border. The rest of the population of this fine bird is found in western North America.

Along the Louisiana coast, where there are many protected reservations, there are very big colonies of the laughing gull Larus atricilla, especially in the marshy islands of the Mississippi delta, which are overgrown with grass and low mangroves. One of the reservations is in the Breton Islands, 114 miles off the main Louisiana coast. Here are great colonies of terns on low flat sandy spits, including Caspian, Cabot’s and royal Thalasseus maximus (Bent 1921). Forster’s and common terns Sterna hirundo, also nest in the Breton Islands, as do numbers of the extraordinary black skimmer, an aberrant tern whose lower mandible is prolonged and with which it scoops food from the surface of the sea. The peninsula of Florida has to its west an immense continental shelf, along the lower end of which is a famous chain of Keys. Beyond Key West, at the terminus of the Key railway, many miles to sea, lie the dry Tortugas, flat islands of coral, their surface, largely of coral sand, clothed in parts with dense cactus as well as with bay cedar, with many bare and grassy spaces between. On the cedars and the cactus immense numbers of noddy terns nest: often over the nests of the sooty terns on the ground below.

The Florida coast has one of the best stations in the U.S.A. for the roseate tern. The darter, which most North Americans allude to as the water-turkey (it is a fresh water lover), and the double-crested cormorant of Florida, commonly nest in trees in many swampy places along the coast. Brown pelicans nest by lagoons and in mangrove-keys on both sides of the peninsula.

Naturalists accustomed to British coast conditions can have little notion of the interminability of the low-lying eastern coast of North America. Indeed, on the entire stretch of mainland coast from Southern Mexico to Maine, about four thousand miles, there is not a single cliff, nor indeed a mountain coming down to the sea. All through Florida, Georgia, the Carolinas, Virginia and Maryland to the New England States, runs a complex of lowland and shallow shores, broken in places by inlets such as those of Chesapeake and Delaware Bays and Long Island Sound. This is a tern coast. In the northern parts the effect of the Labrador current is felt and there is a fairly steep decline in temperature, which is why such tropical forms as the brown pelican and Florida double-crested cormorant drop out of the community in South Carolina. One tropical species which is distributed all along this coast, however, is the laughing gull; and the gull-billed tern reaches north to Virginia. Rather oddly, two terns, the common tern and Forster’s tern, appear to avoid the mainland coast from Florida to South Carolina, though they breed to the west and north of it.

The distribution of tern populations on this Atlantic coast has had a chequered history, and is dealt with in some detail in the chapter on Sea-Bird Populations (Chapter 3, see here).

In the New England States and Maine we encounter the first truly northern elements in the Atlantic sea-bird fauna, and a community of sea-birds which is intensively watched and studied, as is the very similar community on the eastern side of the Atlantic, ten degrees farther north. We now meet not only some of the terns but some of the gulls that breed in the British Isles. In Maine and New Brunswick, where little cliffs begin and the wooded coast closely resembles the skerry-guard of Stockholm, and other parts of the Baltic archipelago, we find the southernmost auks—black guillemots Cepphus grylle, puffins Fratercula arctica, and perhaps still a pair or two of razorbills Alca torda. We even find tubenoses breeding in Maine, birds which we had last encountered in the Caribbean Antilles. (Apart from Audubon’s shearwater and the rare diablotin (see here), which nest in various of the Antilles, no breeding petrel is found in the western North Atlantic south of Maine, save on Bermuda.)

The rocks and coral reefs of Bermuda, which is 580 miles from Cape Hatteras, the nearest point on the United States mainland, support an interesting little community of sea-birds, which consists of the northernmost outposts of the breeding population of an otherwise completely tropical species, the white-tailed tropic bird Phaëthon lepturus, besides the common tern, the roseate tern, possibly the Manx shearwater Puffinus puffinus, Audubon’s shearwater, and the cahow Pterodroma cahow, thought to be extinct for many years.

It is in the Bay of Fundy, then, on the borders of the U.S. and Canada (Maine and New Brunswick) that the northern birds really begin. Here in burrows in the island rocks nest the southern elements of the rather small Atlantic population of Leach’s petrel Oceanodroma leucorhoa. Here, too, are the representatives of the northern race of double-crested cormorant, which are separated by a gap of some hundreds of miles from the geographical race of the same species belonging to Florida and the Carolinas.

Other birds which come on the scene between Cape Cod and the Bay of Fundy are the great black-backed and herring-gulls, Larus marinus and L. argentatus, which are now quickly spreading south down the coast, and the arctic tern Sterna paradisaea, which still nests as far south as Cape Cod. If we move north to the Gulf of St. Lawrence, we can also bring in an outpost population of the European cormorant Phalacrocorax carbo, the ring-billed gull Larus delawarensis, which is very closely related to our common gull, the common guillemot Uria aalge, and, rather surprisingly, an arctic species, Brünnich’s guillemot Uria lomvia, whose breeding distribution extends from the Magdalen Islands via Newfoundland and Labrador to the High Arctic There is a curious relict population of the Caspian tern also here. In many ways the Gulf of St. Lawrence has arctic properties and there is, as we have seen, a very steep gradient in water temperature at its mouth, at the convergence of the west wind drift and the Labrador current. Here we find the southern outposts of the largest temperate North Atlantic sea-bird, the gannet Sula bassana—though the majority of its breeding-population is found on the other side of the ocean; and we meet our first kittiwakes Rissa tridactyla.

In structure the coasts of the Atlantic right round from Maine via Nova Scotia, the Gulf of St. Lawrence, Newfoundland, Labrador, Greenland and Iceland to Britain, have a good deal of similarity. They have a fairly even supply of estuaries, inlets, beaches, sands, cliffs, skerries, stacks and islands, and it is probable that the distribution of no sea-bird is seriously limited by lack of suitable nesting sites.