Sea-Birds. James Fisher

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      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, СКАЧАТЬ