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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 (#litres_trial_promo)) 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 (#ulink_51bb2a47-98ce-54fd-859d-5dbfa66649a7)) 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 (#litres_trial_promo)).
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 (#litres_trial_promo)), 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.
There are two inland species of North American dark-headed gull, Franklin’s gull Larus pipixcan, and Bonaparte’s gull L. philadelphia, neither of which breeds near the coast.
From the Gulf of St. Lawrence, via Newfoundland, Labrador, Greenland and the Canadian Arctic Archipelago, we find a gradual disappearance of the temperate, sub-arctic and some low arctic species as we progress towards the shores where the sea is still near-freezing in July—the true High Arctic. In Newfoundland we reach the limit for breeding gannets, ring-billed gulls and common terns, and perhaps also Caspian terns. The Leach’s petrels breed as far as Newfoundland Labrador, but no farther, and it is doubtful whether the double-crested cormorant now breeds as far. South-west Greenland is less ‘arctic’ than opposite parts of the Canadian Arctic Archipelago at the same latitude; and it is not surprising that some species extend beyond Labrador to West Greenland, though not to Baffin Island and the other Canadian islands. Such species are the razorbill and common guillemot, the latter having only one small colony in West Greenland. The European cormorant extends to West Greenland and previously had a small outpost in Baffin Island, from which it has now disappeared, and it is also extinct in Newfoundland Labrador, after much human persecution. The puffin does not breed in the Canadian Arctic but goes far north in Greenland where it is of a distinctive, large arctic race.
Species which extend in breeding-range all the way from Newfoundland to Arctic Greenland and Canada are the herring-gull, great black-back, kittiwake, arctic tern and black guillemot. All these except the blackback reach the High Arctic, if we regard the Iceland gull Larus argentatus glaucoides, as a herring-gull, as we think we should.
The glaucous gull Larus hyperboreus, does not now breed in Newfoundland, but nests commonly from Newfoundland Labrador all the way to the High Arctic, as does the arctic skua Stercorarius parasiticus; two other skuas, the pomarine S. pomarinus, and the long-tailed skua S. longicaudus, do not breed in Labrador, but farther north in both Canadian and Greenland Arctic. On the west side of the Atlantic-Arctic the fulmar Fulmarus glacialis, breeds no farther south than Greenland and Baffin Island, although it nests south to about latitude 50° north on the east side of the Atlantic.
This leaves the three High Arctic sea-birds of the West Atlantic for consideration—the little auk Plautus alle, Sabine’s gull Xema sabini, and the ivory-gull Pagophila eburnea. All three breed in the more northerly parts of the Canadian Arctic Archipelago and Greenland, though the first may not have more than one colony west of Baffin’s Bay. Sabine’s gull is a rare bird that often nests in arctic tern colonies. The ivory-gull is the most northerly bird in the world in the sense that it breeds nowhere south of the Arctic Circle, but as far north as the land goes. The extraordinary, rare, Ross’s or rosy gull Rhodostethia rosea, which normally nests in the aldergroves of some north-flowing rivers of eastern Siberia, has once bred in Greenland.
The breeding sea-birds of the lands and islands north of the Arctic Circle belonging to the Atlantic or the Atlantic section of the Arctic Ocean.
With the exception of a few gulls, sea-birds entirely desert the arctic regions bordering Baffin’s Bay and Davis Strait in October and do not return until April. From no other part of the northern hemisphere is there so great a withdrawal of sea-birds to avoid a period of inhospitable climate.
The eastern arctic islands—Jan Mayen, Bear Island and Spitsbergen, Franz Josef Land and Novaya Zemlya, which lie across the Polar Basin where it abuts on the North Atlantic, have a very similar breeding sea-bird community to that of Greenland, though none has so many members. We can best make this comparison in the form of a table, adding columns for the Canadian Arctic, Arctic Russia-in-Europe and Arctic Norway. (see here (#litres_trial_promo))
We now come to the seabird community of Iceland, Faeroe, the British Isles, Scandinavia, the Baltic, and the North Sea and English Channel. This community is very homogeneous, considering the range of latitude over which it is spread, though there are some members which do not reach the south end of this range and a few which do not reach the north. Among the species which are found over almost the entire twenty degrees of latitude are the Manx shearwater, the storm-petrel Hydrobates pelagicus, the gannet, the shag Phalacrocorax aristotelis, the cormorant, the herring-gull, the lesser blackback Larus fuscus, the great blackback, the black-headed gull L. ridibundus, the kittiwake, the common and arctic terns, the razorbill, the guillemot, and the puffin. Species which occupy the more northerly parts of this temperate European stretch include the great skua Catharacta skua, and Leach’s petrel (Iceland, the Faeroes and Britain only), the fulmar, the arctic skua, and the black guillemot. The glaucous gull, little auk and Brünnich’s guillemot breed (in this part of the Atlantic) only in Iceland.
There is a central group of sea-birds which breeds neither as far north as Iceland nor as far south as Atlantic France; this is headed by the common gull Larus canus, and includes also the little gull L. minutus; its other members are terns, the whiskered tern Chlidonias hybrida (only casual, in Holland), the black tern C. nigra, the white-winged black tern C. leucoptera (casual only), the gull-billed tern and the Caspian tern. The populations of all these terns are low, and only two of them (black and gull-billed) have recently bred in Britain, and that casually; their headquarters lie between Holland and the South Baltic. The Baltic Sea, though it has as many breeding terns and gulls as any other part of this stretch of the east Atlantic, lacks tubenoses and has no gannets, shags, kittiwakes or puffins. The long-tailed skua has a somewhat specialised breeding distribution in Lapland, mostly inland. The remaining birds of this temperate stretch of the east Atlantic breed from Britain, the North Sea or the Baltic south beyond its limits; they are the roseate, little and Sandwich terns. Britain is the European headquarters of the roseate tern.
About half the members of this east and north Atlantic temperate sea-bird community are truly oceanic; that is, they may be found in mid-ocean, up to the greatest possible distance from land, wherever there are suitable feeding waters. Storm-petrels, Leach’s petrels and fulmars are the oceanic tubenoses of this community, and we now find that the Manx shearwater also has a right to be considered oceanic. Among the auks the dovekie and Brünnich’s guillemot from the north join the puffins, razorbills and guillemots in ocean wanderings. Here, too, are found all the four skuas of the northern hemisphere and one, but only one, gull—the highly specialised kittiwake. In the waters a hundred fathoms deep or less, that is, on the so-called continental shelf, we find all the birds previously mentioned, together with the gannet, the black guillemot, and gulls of the genus Larus—the great blackback, the lesser blackback and the herring-gull. Once we are within sight of shore quite a number of species are added to our list, and the tubenoses, except for the Manx shearwater and fulmar, drop out. Here are the terns, the black-headed and common gulls, and also the cormorant and shag, the one haunting mostly seas in sight of sandy shores, and other seas in sight of rocks.
By far the most impressive of the sea-bird haunts are the breeding cliffs, where the different species are zoned vertically as well as horizontally. Whether the rocks be volcanic or intrusive or extrusive or sedimentary, we are sure to find Larus gulls breeding on the more level ground a little way back from the tops of the cliffs—fulmars on the steeply sloping turf and among the broken rocks at the cliff edge, puffins with their burrows honeycombing the soil wherever this is exposed at the edge of a cliff or a cliff buttress, Manx shearwaters or Leach’s petrels in long burrows, storm-petrels in short burrows and rock-crevices, razorbills in cracks and crannies and on sheltered ledges, guillemots on the more open ledges where they can stand; perhaps gannets on broad flat ledges or on the flattish tops of inaccessible stacks, cormorants with their nests in orderly rows along broad continuous ledges, shags in shadowy pockets and small caves and hollowed-out ledges dotted about the cliff, kittiwakes on tiny steps or finger-holds improved and enlarged by the mud-construction of their nests, tysties or black guillemots in talus and boulders at the foot of the cliff. These wild, steep frontiers between sea and land are exciting and beautiful. They probably house larger numbers of vertebrate animals, apart from fish, in a small space, than any other comparable part of the temperate world.
Not many sea-birds of the east Atlantic do not breed on cliffs; but the skuas nest on moors, and the terns and black-headed gulls nest on sand and shingle. Many of the Larus gulls, and recently the fulmar, are catholic in their taste in nesting sites, and may be found on moors and even sand dunes. Quite a large number of sea-birds can be inland nesters, even including tubenoses. Fulmars now nest up to six miles inland in Britain, and many of the Larus gulls at much greater distances. The black-headed gull, in particular, is often a completely inland species, since some individuals nest in England as far as they can from the sea, e.g. in Northamptonshire, and may never visit it except in casual search for food.
As we go south along the Atlantic seaboard of the Old World we leave behind in the Channel Islands and Brittany the last elements of certain temperate cliff-breeding sea-bird species—the gannet, lesser blackback, great blackback, arctic tern (only a casual breeder so far south), razorbill and puffin. South of the Bay of Biscay we encounter a large sub-tropical and tropical community of about forty species (a few of which belong to sea-bird families but which have become river-birds or inland birds), which is distributed in four main geographical regions—the Lusitanian coast (the Atlantic coast of Spain and Portugal), the Mediterranean, the Atlantic coast of Africa north of the equator, and the Atlantic Islands. These last comprise the Azores, Madeira (to which pertain the Desertas and Salvages), the Canaries and—near the equator—the Cape Verde Islands. Many species breed, of course, in more than one of these regions, though only the herring-gull (rather doubtfully the little tern and cormorant) breeds in them all.
Of the species in the table, the crested pelican Pelecanus roseus, the pigmy cormorant Haliëtor pygmeus, the Mediterranean black-headed gull Larus melanocephalus, and the lesser crested tern Thalasseus bengalensis breed on no North Atlantic shore, and the rare slender-billed and Audouin’s gulls, Larus genëi and L. audouinii, are primarily Mediterranean species. It will be noted that three tubenoses have established themselves in the Mediterranean, but that no less than eight species breed in the Atlantic Islands, which have a greater variety of species of this order than any other part of the North Atlantic.
The distribution of breeding sea-birds on these coasts is best illustrated in tabular form :
Of the four main groups of these Atlantic islands, Madeira and the Cape Verdes have probably the largest sea-bird communities, with ten or a dozen species each. One tubenose, the North Atlantic great shearwater, Puffinus diomedea, nests on all of them as well as on the Berlengas of Portugal. Bulwer’s petrel, Bulweria bulwerii, and the little dusky shearwater, Puffinus assimilis, also nest on all four island groups. The Madeiran fork-tailed petrel, Oceanodroma castro, nests on all but the Canaries. The Manx shearwater nests on the Azores and Madeira, but not yet farther south. The little storm-petrel reaches south to the Canaries (although in small numbers and probably to these Atlantic islands only). The rather rare soft-plumaged petrel, Pterodroma mollis, is believed to nest on Madeira; it does so on the Cape Verdes. The beautiful frigate-petrel, Pelagodroma marina, breeds on the Salvages (which belong to Madeira but are nearer the Canaries), the Canaries and the Cape Verdes.
The red-billed tropic-bird, Phaëthon aethereus, the brown booby and the frigate-bird (man-o’-war bird) do not appear farther north than the Cape Verdes. Here the cormorant, which had dropped out in Morocco, reappears as a new race, primarily South African. The bird communities of these islands are only moderately well-known. Most of the sea-birds nest on rocks whose comparative inaccessibility has been both a temptation and a deterrent to the visiting ornithologist. As for the coast of West Africa and the islands lying close to it, no organised investigation of the sea-bird communities of this difficult region has yet been made. We know that one group of species breeds on the Atlantic African coast to Morocco, but no farther south—the shag, herring-gull, the whiskered tern, probably the gull-billed tern, possibly the slender-billed gull. Farther south both white and pink-backed pelicans, Pelecanus onocrotalus and P. rufescens, and the grey-headed gull, Larus cirrhocephalus, reach the tropical sea-coast in some places, and the brown booby nests on at least one island off the coast of French Guinea. The Caspian tern, whose world distribution is, to say the least, peculiar, may have breeding stations on this coast, and the little tern, which we had left behind in Morocco, reappears as a separate race on the coast and rivers of the Gulf of Guinea.
The African darter, Anhinga rufa, reed-cormorant, Haliëtor africanus, and the African skimmer, Rynchops flavirostris, haunt the rivers and in places reach the coast; but they are not sea-birds: and on islands in the Gulf of Guinea the noddy and the white-tailed tropic-bird, Phaëthon lepturus, breed. It is suspected that the frigate-bird may nest on this coast, but its breeding-place has not been found. Neither has that of the bridled tern, Sterna anaetheta, or the sooty tern, S. fuscata, although both species are seen in considerable numbers. There is at least one other riddle: a population of the royal tern, Thalasseus maximus, haunts almost the whole coast of West Africa from Morocco to some hundreds of miles south of the Equator. Systematists have separated it from the West Atlantic population as a subspecies (albidorsalis), on valid differences, and it does not appear to leave this coast, yet no ornithologist has yet seen its nest or even its eggs.
Only in the tropical parts of the Atlantic are there still these distributional queries. In the temperate and arctic zones the breeding places of the birds are well-known and described. And with this little mystery we conclude our tour of the Atlantic, for we are back on the equator and can strike west to the St. Paul Rocks, where we began.
FIG. 2a The breeding sea-birds of the North Atlantic, arranged by five geographical regions. No species breeds in more than four. Number of species; see opposite page for actual species
The sea-birds of the North Atlantic can be listed in the form of a table (Appendix, see here (#litres_trial_promo)), and plotted according to which parts of the ocean they breed in, in the form of a diagram (Fig. 2 (#litres_trial_promo)). For the purpose of completeness, the secondary sea-birds have been included, those belonging to families whose fundamental evolution has probably been non-marine (like anatids and waders) or which are only sea-birds in winter (divers and grebes). Only the more important of these are on the diagram, and they are not otherwise treated in this book. It is interesting that more than half of them are northern ducks which winter at sea, though usually within sight of shore.
It must also be pointed out that several species belonging to the families of primary sea-birds have secondarily taken to life inland, on rivers, or on estuaries, and may reach the sea only incidentally or not at all. Certain West African species, in particular, are river-birds (the pelicans Pelecanus onocrotalus and P. rufescens, the reed-cormorant Haliëtor africanus, the darter Anhinga rufa, the skimmer Rynchops flavirostris). The terns of the genus Chlidonias are primarily lake and marsh species throughout their range. In North America the gulls Larus pipixcan and L. philadelphia are purely inland species in the breeding season, and the tern Sterna forsteri and the pelican Pelecanus erythrorhynchos almost so. In South America the terns Phaëtusa simplex and Sterna superciliaris are purely river-species.
FIG. 2b Actual species. Arrows point to replacement species or to nearest ecological counterparts
One hundred and eleven species of primary and thirty-two of secondary sea-birds have been identified by competent observers at sea or on some shore in the North Atlantic since 1800: a total of one hundred and forty-one. Of these one primary sea-bird, Alca impennis the great auk, and one secondary sea-bird, Camptorhynchus labradorius the Labrador duck, are now extinct. Of the survivors, eighty-two primary and thirty secondary sea-birds actually nest, or have nested, on or near a North Atlantic or Mediterranean shore or a shore of that part of the Arctic (north of the Circle) that communicates directly with the North Atlantic (this brings in six arctic species: ivory-gull, Ross’s gull, little auk, white-billed northern diver, brent-goose and Steller’s eider). Two further species (Larus pipixcan and L. philadelphia, see table (#litres_trial_promo)) are purely inland breeders.
Most remarkably, the number breeding on the Old World and New World sides is almost exactly the same. We can derive the following summary of breeding-species from the Appendix (#litres_trial_promo); the totals include the two North American purely inland species, and the two extinct species. Doubtful (“?” in the Appendix (#litres_trial_promo)) and casual cases are deliberately included—most of them are from tropical West Africa north of the equator where the breeding of the species in question seems likely but, owing to the scanty exploration of the coast, is not formally proved.
We can see that if we add the six purely arctic breeders to those species which are common to both east and west sides of the North Atlantic, we have fifty-five, out of a total of 116, or about half. Of the remaining sixty-one species, 24 breed only on the west side of the North Atlantic, four on the west side and in the Arctic; and six purely on the east side, and seven on the east side and in the Arctic. Those on the east side include four ‘sea-birds’ which breed in the Mediterranean area but not in the North Atlantic (the crested pelican, pigmy cormorant, Mediterranean black-headed gull and the lesser crested tern).
The general conclusion is of considerable ecological interest, showing how exactly the sea-bird communities of both sides reflect one another. Although only about two-thirds of the members of the community on one side of the Atlantic are found in that of the other, the species comprising the remaining third ‘balance each other’ and occupy very much the same ‘niches’ or places in nature. Opposite species which pair off by occupying similar niches are grouped together in the list in the Appendix, see here (#litres_trial_promo).
A NOTE ON NON-BREEDERS AND CASUAL WANDERERS
Apart from these 116 breeders, the limbo of twenty-six primary sea-birds and one secondary sea-bird (the spectacled eider Somateriafischeri, which has been recorded twice in Norway, though it breeds on the other side of the Polar Basin) consists of casual wanderers, with three remarkable exceptions. These are all tubenoses (two shearwaters and a storm-petrel) which breed in the southern hemisphere but which cross the equator in large numbers to ‘winter.’ The most familiar of these in Britain is the Tristan great shearwater Puffinus gravis, which is rather similar, and certainly closely related to the heavier North Atlantic or Cory’s shearwater, P. diomedea. Incidentally we suggest confusion between the two would be reduced if P. diomedea were consistently called ‘North Atlantic shearwater’ and P. gravis ‘Tristan great shearwater’—not just ‘great shearwater.’
The Tristan great shearwater nests only on Nightingale and Inaccessible Islands, in the Tristan da Cunha group; possibly a few may survive on Tristan itself. The population remains vast, though ‘farmed’ by the Tristan islanders, and an annual penetration of the North Atlantic by off-season birds has put the species on the list of regular and expected visitors to both West Atlantic and East Atlantic waters, as well as some arctic waters of Greenland. The northward movement reaches the North Atlantic in May, mostly on the west side at first, but odd birds appear in Irish and west British waters in June and have even been seen then in the Skagerak; one of us saw a few already at Rockall in mid-May (1949), and they were abundant there and in moult by late June (1948).
The Tristan great shearwater seldom comes close to land, and it is never common in British waters within sight of shore; but some distance to sea off west England, Ireland and the Hebrides it is always present in July and August; and some elements usually penetrate northabout into the North Sea, descending to the latitude of Yorkshire. The Tristan great shearwater is much more common than the Northern Atlantic shearwater in our seas; indeed, the Mediterranean race of the latter P. d. diomedea, and Cory’s race P. d. borealis, have each only once been taken ashore in Britain, although birds which may have been of Cory’s subspecies have several times been seen at the entrance of the Channel. The only Scottish record is of one, seen at sea close to Aberdeen on 10 September 1947, by R. N. Winnall. Normally as Wynne-Edwards and Rankin and Duffey have shown, Puffinus diomedea does not get much farther north in the Atlantic than 50°N., and that at about 30°W. It is much more common on the North American coast than on that of Britain, although this coast is much farther from its base; ‘they seem to arrive on our coasts early in August,’ writes Bent, ‘and spend the next three months with us, mainly between Cape Cod and Long Island Sound.’ The Tristan great shearwater also probably reaches its greatest abundance on the North American coast, particularly in the area of the Newfoundland Banks, where it is known as the ‘hagdon’; from here it extends every season along the coast of Labrador to Greenland;—it has been recorded near Iceland.
The other southern hemisphere shearwater that regularly visits North Atlantic waters is Puffinus griseus, the sooty shearwater. It is much rarer than the Tristan great shearwater, though it has been seen in British waters regularly enough to be classed as an autumn visitor. It breeds in New Zealand and its islands, in southern South America and its islands, and the Falkland Islands (in places many miles inland), and ranges the Pacific as well as the Atlantic; its Atlantic population is low compared with that of the other southern shearwater. Unlike the Tristan great shearwater, it probably makes its way into the North Sea by the Channel; and it is regular in small numbers in the Western approaches. At Rockall on 17 May 1949 J.F. saw none, but from 18 to 27 June 1948 R.M.L. found them always present there, singly and up to eight together, that is in the proportion of about one to a hundred hagdons. On the Newfoundland Banks, where it is in the same proportion, the fishermen called it the haglet. It reaches Greenland and Icelandic waters, and has been seen once as far north as Bear Island.
The storm-petrel from the south is Wilson’s petrel Oceanites oceanicus, which nests in vast numbers in the antarctic continent and on the southern islands of South Shetland, South Orkney, South Georgia, Falkland, Tierra del Fuego and Kerguelen. It disperses into, and across, the Equator in the Atlantic, Indian and Pacific Oceans. Wilson’s petrel has been the subject of an exhaustive monograph by Brian Roberts (1940), who mapped the dispersal in the Atlantic month by month (Fig. 29 (#litres_trial_promo)). Records north of the Equator are only irregular and sporadic between November and March, but in April the species is spread widely over the western half of the North Atlantic as far as Cape Cod. In May the petrels spread eastwards reaching from Cape Cod across the Atlantic towards Portugal and the Bay of Biscay, off which there is quite a concentration in June. By July there is a band of Wilson’s petrels across the whole North Atlantic with its northern border at about 40°N., but not reaching Britain. In August the eastern Atlantic petrels disappear, though on the west a concentration remains with its nucleus near Long Island Sound; and this persists in reduced population in September, by which time most Wilson’s petrels are making their way home. In September they reappear again off Portugal, and the homeward stream in October runs south along the north-west coast of Africa, continues its line across the Atlantic to the corner of Brazil, and carries on mainly down the east coast of South America; in November and December the concentration is at its greatest in the triangle Rio de Janeiro-South Georgia-Cape Horn.
There are only about ten records for this abundant and successful species, in Britain. It does not normally reach our islands, though elements cannot be within much more than a few hundred miles of Cornwall in June and July. Most of the British records are between October and December—suggesting young non-breeding birds, inexperienced in the ways of wind and wave.
Among the two dozen casual sea-bird visitors to the North Atlantic undoubtedly the most exciting are the kings of the tubenoses—the albatrosses, whose occurences in the North-Atlantic-Arctic are really monuments not so much to the fact that from time to time the best-adapted birds make mistakes and get right out of their range, as to the extraordinary powers of endurance and flight of the world’s greatest oceanic birds. Five albatrosses have strayed into the North Atlantic, four of the genus Diomedea, which includes the largest kinds, and one Phoebetria. All breed in the southern regions of the southern hemisphere.
The most frequent in occurrence has been the black-browed albatross D. melanophris, of which we can trace nine records. The first of these is astonishing; on 15 June 1878, north-west of Spitsbergen and north of latitude 80°N., the whaler-skipper David Gray shot one that is now in the Peterhead Museum; it was farther north than the species ever gets south, even though it nests to latitude 55°S. Another northerly record is from West Greenland, and others have been shot south-west of the Faeroes and in the Oslo Fjord, Norway; one is even alleged to have reached Oesel in the Baltic. In 1860 (Andersen 1894) a female black-browed albatross turned up among the gannets of Mýkinesholmur in Faeroe, and came to the cliff every season with them until 11 May 1894, when it was shot by P. F. Petersen. For many years the only British record was of one which was caught exhausted in a field near Linton, Cambridgeshire, on 9 July 1897; but on 14 May 1949 an immature albatross which was probably of this species was seen at the Fair Isle, between Orkney and Shetland. It was first noticed soaring off the south face of the Sheep Craig, the famous landmark on the east side of the island, and obligingly glided over George Waterston, G. Hughes-Onslow and W. P. Vicary, who got a fine view of it (Williamson 1950, 1950b). Further, in September 1952 one was picked up alive in Derbyshire (Edmunds, 1952; Serventy, Clancey and Elliott, 1953).
No other albatross has been certainly seen in Britain: a record of the yellow-nosed albatross from the Lincolnshire-Nottinghamshire boundary on 25 November 1836 is not admitted to the British list. This species, D. chlororhynchos, has been certainly obtained, however, in south Iceland, at the mouth of the St. Lawrence river, in the Bay of Fundy (New Brunswick), and in Oxford County, Maine. A record of D. chrysostoma from Bayonne in France
(#litres_trial_promo) may possibly refer to this species, for D. chlororhynchos and D. chrysostoma are extremely similar, and almost impossible to distinguish in the field. D. chrysostoma, the grey-headed albatross, has, however, certainly been recorded once in the North Atlantic—from South Norway in 1837 (or 1834). The light-mantled sooty albatross Phoebetria palpebrata, a relatively small species which breeds on sub-antarctic islands, has been recorded from Dunkirk, France
(#litres_trial_promo). The greatest of all the albatrosses, the wandering albatross Diomedea exulans, has been taken, in France (Dieppe), Belgium (Antwerp) and on the Atlantic coast of Morocco; this magnificent animal has a wingspread up to 11
⁄
feet and may weigh seventeen pounds or more; we can imagine the excitement of those humans who encountered these South Atlantic wanderers on their North Atlantic wanderings!
Sometimes these wanderings may end in queer places; for instance, F. J. Stubbs (1913) found an albatross that he judged to be D. exulans hanging among the turkeys of Christmas 1909 in a game-dealer’s shop in Leadenhall Market. When he saw it ‘the bird appeared quite fresh, and bright red blood was dripping from its beak.’ There was no indication whence it had been obtained.
Unidentified albatrosses have been seen at sea west of Spitsbergen on 2 May 1885, by the Captain David Gray who shot the 1878 black-browed albatross; off the mouth of Loch Linnhe, West Highlands of Scotland, in the autumn of 1884 by W. Rothschild; and twenty miles north-west of Orkney on 18 July 1894, by J. A. Harvie-Brown (1895).
Apart from the three regular non-breeding summer visitors and the albatrosses, at least six other tubenoses have wandered into the North Atlantic from the South, or from the Pacific. The Cape pigeon Daption capensis, has been recorded from France
(#litres_trial_promo), Holland and Maine, but the three British records have been rejected from the official list on the grounds that sailors have been known to liberate captured specimens in the Channel. Quite probably they are valid. Peale’s or the scaled petrel Pterodroma inexpectata, has once been taken in New York State. Pterodroma neglecta, the Kermadec petrel, has been once found dead in Britain (on 1 April 1908 near Tarporley in Cheshire). One Trinidad petrel Pterodroma arminjoniana, was driven to New York by the hurricane of August 1933, and possibly this close Atlantic relative of the Kermadec petrel may cross the equator fairly often, as it breeds on South Trinidad Island (only), which is fourteen hundred miles south of the equator, surely no very great distance for a petrel. One collared petrel Pterodroma leucoptera, a Pacific species, was shot between Borth and Aberystwyth in Cardiganshire, Wales, at the end of November or the beginning of December 1889. The last wandering tubenose is the black-bellied storm-petrel Fregetta tropica, a sub-antarctic species which was first collected off the coast of Sierra Leone and has also been taken in Florida. It seems likely that this last species may cross the equator fairly regularly, at least as far as the Tropic of Cancer.
One Pelecaniform wanderer has crossed the equator into the North Atlantic from South Africa—the Cape gannet Sula capensis, which may reach north to the Canaries.
From the western United States the California gull Larus californicus (which may be a race of the herring-gull, see here (#ulink_9687426d-38ae-5553-8f1f-060f4c9131d0)) winters fairly regularly to Texas, and thus (in our definition) to the North Atlantic region. Another gull which enters the North Atlantic, from more distant breeding-grounds, is the great black-headed gull, Larus ichthyaëtus of the Black Sea and farther east, which has reached Madeira and Belgium and has been seen in Britain about eight times.
Four exotic terns have wandered into the North Atlantic. The South American Trudeau’s tern Sterna trudeaui, has once reached New Jersey. On the east side Sterna balaenarum, the Damara tern of South Africa, has migrated across the equator as far as Lagos in Nigeria. Thalasseus bergii, the swift tern, breeds on the west coast of South Africa north to Walvis Bay, whence occasional individuals may sometimes pass north across the equator. The elegant tern Thalasseus elegans, of the Gulf of California, has accidentally reached Texas. And finally Gygis alba, the tropical, white, almost ‘transparent’ fairy tern breeds north in the Atlantic to Fernando Noronha, and therefore probably occasionally operates across the two hundred miles that would bring it to the North Atlantic, though there is so far no formal record of this. It has a wide distribution in all tropical seas, but is very much attached to, and does not often fly far from, its breeding-grounds; nevertheless R. C. Murphy (1936) ponders: ‘Since there are seasons when powerful southeast trade winds blow from Fernando Noronha across the equator almost as far as the mouth of the River Orinoco, speculation offers me no clue as to why Gygis has not succeeded in jumping the next gap and establishing itself in the West Indies.’
The remaining wanderers are from the North Pacific—auks from that cradle of the sub-order of auks. Aethia pusilla, the least auklet, has not actually reached the Atlantic, but one was found ‘halfway’ from the Pacific to the Atlantic, in the Mackenzie delta in May 1927. The ancient murrelet Synthliboramphus antiquus has been found three times in the Great Lakes area, but no farther east. Aethia psittacula, the paroquet auklet,
(#litres_trial_promo) has actually reached the Atlantic by turning up in, of all places, Sweden: in December 1860 one was captured in Lake Vattern! If the least auklet has not reached the Atlantic, its congener Aethia cristatella, the crested auklet, has, for even if we reject (as most do) the alleged Massachusetts record, we must accept that of 15 August 1912 when one was shot north-east of Iceland. Finally Lunda cirrhata, the tufted puffin, was obtained by the great naturalist Audubon in Maine: other records from the Bay of Fundy and Greenland are erroneous.
FIG. 2c Bathymetrical sketch-chart of the Atlantic Ocean
CHAPTER 2 (#ulink_6267e577-835c-5b43-ad61-a30b1abff6c9) EVOLUTION AND THE NORTH ATLANTIC SEA-BIRDS
GEOLOGISTS DIFFER in their opinions of the origin of the Atlantic Ocean. The followers of the geomorphologist Alfred Wegener believe that it is a real crack in the earth’s crust whose lips have drifted away from each other, and this opinion is lent verisimilitude by the neat way in which the east coast of the Americas can be applied to, and will fit with extraordinary exactitude, the west coast of Europe and Africa. It must be stated that, while the present opinion of most geographers is that the resemblance of the Atlantic to a drifted crack is purely coincidental, this is not shared by all students of animal distribution and evolution, some of whom, find the Wegener theory the most economical hypothesis to account for the present situation.
Whatever the truth is, there is no doubt that the boundaries of the Atlantic, and their interconnections, have varied considerably; thus halfway through the Cretaceous Period, about ninety million years ago (during this long period nearly all the principal orders of birds evolved), there were bridges between Europe, Greenland and Eastern North America cutting the Arctic Ocean from the North Atlantic completely; and from then until the late Pliocene—perhaps only two million years ago—there was no continuous Central American land bridge, but a series of islands.
Our present knowledge of the tree of bird evolution owes much to Alexander Wetmore and his school, who have so notably added to our knowledge of fossil birds during the last twenty years, especially in North America. Birds do not appear very frequently in the sedimentary rocks—their fossil population does not generally reflect their true population in the same way as that of mammals is reflected. However, if land-birds are rare in the beds, water-birds are relatively common, and the periods and epochs in which all our sea-bird orders, and many of our sea-bird families and genera, originated are quite well known. A recent paper by Hildegarde Howard (1950), of the school of Wetmore, enables us to show a diagrammatic family tree of birds (Fig. 3 (#litres_trial_promo)), with special reference to sea-birds, and to collate its branching with the approximate time scale of the epochs, so cleverly established by geomorphologists in recent years from studies of sedimentation-rate and the radioactivity of rocks. It will be seen that the primary radiation of birds and the great advances into very different habitats consequent upon the first success of the new animal invention—feathered flight—took place in the Cretaceous period, the first birdlike feathered animals having been found as fossils in Jurassic deposits of the previous period, over a hundred and twenty million years old. In the Cretaceous period—the period of reptiles—ostriches were already foreshadowed, as were grebes and divers, and the pelican-like birds, and the ducks.
In the Cenozoic period—the period of mammals—the radiation of birds into all nature’s possible niches continued rapidly, especially in the first two of its epochs—Eocene and Oligocene—from sixty to thirty million years ago. In these epochs grebes can be distinguished from divers, and a bird of the same apparent genus (Podiceps, or, as the North Americans have it, Colymbus) as modern grebes has been found. Gannet-boobies of the modern genus Sula have been found in the Oligocene, as have cormorants of the modern genus Phalacrocorax. The only penguin fossils known are later—of Miocene age—but it seems probable that they share a common stem with the tubenoses, which would mean that their ancestors branched off in the Eocene. The tubenoses diversified in the Oligocene—from this epoch we have a shearwater of the modern genus Puffinus; and from the Miocene Fulmarus and albatrosses. The ducks started their main evolution in the Cretaceous, and by the Oligocene we find modern genera such as Anas (mallard-like) and Aythya (pochard-like); in the Pliocene we have Bucephala (Charitonetta)—one of the tribe of sea-ducks.
For the Lari-Limicolae, the order which includes waders, gulls and auks, the fossil record is rather indefinite, mainly owing to the difficulty of distinguishing the present families by bones alone. However, we know that the auk family was early—an Eocene offshoot; that the waders and gulls diverged in the Oligocene; and that the gulls, terns and skuas probably diverged in the Miocene—which means that an important part of the adaptive radiation of this order was comparatively late. One of the early auks, the Pliocene Mancalla of California, out-penguined the great auk, Alca (Pinguinus) impennis, for it had progressed far beyond it in the development of a swimming wing.
FIG. 3
Diagrammatic family tree of sea-birds, mainly after Hildegarde Howard (1950)
According to Howard (1950) a few living species of birds have been recorded from the Upper Pliocene, but large numbers of modern forms occurred in the Pleistocene. Of course in the Pleistocene the oceans approximated very closely to what they are today, with the Central American land-bridge closed, the Norwegian Sea wide open between Arctic and Atlantic Oceans, the Mediterranean a blind diverticulum of the North Atlantic. We need this picture as a background to a consideration of the North Atlantic’s present sea-bird fauna, for we shall find that it has few sea-bird species of its own, and only two genera; for the primary sea-bird species which now breed in the Atlantic (and Mediterranean) and in the neighbouring parts of the Arctic, and nowhere else in the world, are no more than twelve: the Manx shearwater Puffinus puffinus
(#litres_trial_promo); the very rare diablotin and cahow of the West Indies and Bermuda (Pterodroma hasitata and P. cahow); the storm-petrel Hydrobates pelagicus; the North Atlantic gannet Sula bassana; the shag Phalacrocorax aristotelis; the lesser black-back Larus fuscus; the great blackback L. marinus; the Mediterranean gulls L. melanocephalus and L. audouinii; the Sandwich tern Thalasseus sandvicensis; the razorbill Alca torda, the puffin Fratercula arctica; besides the extinct Alca impennis, the great auk. The two present genera peculiar to the North-Atlantic-Arctic are Hydrobates and Alca.
The sea-birds which qualify by birth and residence to be members of the North Atlantic fauna (excluding purely Arctic and Mediterranean species) include thirteen tubenoses, seventeen cormorant-pelicans, fourteen gulls, nineteen terns, two skimmers, four skuas and five auks (besides various secondary sea-birds, notably about eighteen ducks, three divers and two phalaropes). If we are to understand how these have got into the North Atlantic we should analyse the present distribution of the sea-bird orders and groups as between the different oceans.
The most primitive group of sea-birds, yet the most specialized, is that of the penguins. The Sphenisci have fifteen species in all, of which eight breed in the South Pacific, seven in the Antarctic Ocean, five in the South Atlantic and two in the Indian Ocean. One (and one only) reaches the Equator, and thus the North Pacific, at the Galapagos Islands. No live wild penguin has ever been seen in the North Atlantic.
(#litres_trial_promo) It seems certain that the evolution of this order of birds has taken place in Antarctica and in the neighbouring sectors of the South Pacific.
The great order of Tubinares the albatrosses, petrels and shearwaters, probably originated in what is now the South Pacific. Nobody knows exactly how many species belong to this order, as there is a good deal of disorder in the published systematics of this very difficult group; but the number is certainly eighty-six, and may be over ninety. Of these fifty-four breed in the South Pacific, twenty-seven in the Antarctic, twenty-five in the North Pacific, twenty-four in the South Atlantic, seventeen in the Indian Ocean, thirteen in the North Atlantic, three in the Mediterranean, and only one, the fulmar, in the Arctic Ocean.
The Steganopodes are an order which is particularly well represented in the South Pacific and Indian Oceans. The pelicans, gannets, cormorants, darter, tropic– and frigate-birds number fifty-four species in all. Thirty-one breed in the South Pacific. Twenty-eight breed in the Indian Ocean. The North Pacific has twenty-three, the South Atlantic twenty, the North Atlantic sixteen, the Mediterranean six, the Antarctic three, and the Arctic two. The present distribution suggests that the order radiated from what is now the East Indian region—from south-east Asia or Australasia.
In the order Laro-Limicolae the family Chionididae, two curious pigeon-like sheathbills, Chionis, are found in Antarctica; and one also breeds in the South Atlantic and South Pacific.
In the family Laridae the gulls (subfamily Larinae) number forty-two. In the North Pacific sixteen of these breed, in the North Atlantic fourteen, in the Arctic eleven, in the South Pacific nine, in the Indian Ocean six, in the South Atlantic five, in the Mediterranean five, in the Antarctic two. Besides these two breed inland only in North America, one inland only in South America, and three inland only in the Palearctic Region. This appears to be the only group of sea-birds whose evolutionary radiation may have taken place from the north; the Arctic and neighbouring parts of the North Pacific and Atlantic appears to be the origin of the gulls. The terns (subfamily Sterninae) number thirty-nine, of which twenty-three breed in the North and twenty-two in the South Pacific, nineteen in the Indian Ocean, nineteen in the North Atlantic, fifteen in the South Atlantic, ten in the Mediterranean, two in the Antarctic, two in the Arctic and one inland only in South America. The radiation of terns appears to be pretty general over the world’s seas, and they may have originated in the tropics, perhaps in the Indian Region. The skuas (subfamily Stercorariinae) have only four species, one of which (Catharacta skua, the great skua) has its breeding-headquarters in the Antarctic; it also breeds in the South Pacific, South and North Atlantic. The other skuas have an arctic breeding-distribution which extends into the North Pacific and North Atlantic. The three skimmers Rynchops belong to a separate family, Rynchopidae; North Atlantic, South Atlantic and South Pacific each have two; the Indian Ocean has one. Some workers regard them as all of one species.
The family Alcidae (the auks) take the place in the north of the penguins of the south. Undoubtedly their origin has been in or not far from the Bering Sea. Of the twenty-two species, sixteen belong to the northern part of the North Pacific, twelve to the Arctic Ocean north of the Circle, and six to the northern part of the North Atlantic.
This concludes the list of sea-birds belonging to groups of super-family or higher status whose evolution has been marine. There are several further (secondarily marine) groups which contain sea-birds, or part-time sea-birds; thus all four members of the order Gaviae the divers, breed in the Arctic, and North Atlantic and Pacific regions, and winter at sea on the coasts of the oceans. Many of the twenty species of grebes, order Podicipedes, are marine outside the breeding-season, and six of them visit the coasts of the North Atlantic at that time. Among the geese and ducks many (see Appendix, see here (#litres_trial_promo)) are partly marine, and some (e.g. eiders and scoters) are largely marine in the breeding—as well as in the off-season: two eiders and three scoters breed in the North Atlantic-Arctic. Among the waders (Charadriidae) the subfamily Phalaropinae contains only three members, all of which breed in the Arctic, North Atlantic and North Pacific, and two of which winter in the open sea.
If we ignore these secondary sea-birds, and consider the 267 species of the primary marine groups, we find that the hierarchy is this: South Pacific 128 (51 per cent.); North Pacific 107 (40 per cent.); North Atlantic 74 (28 per cent.); South Atlantic 73 (27 per cent.); Indian Ocean 73 (27 per cent.); Antarctic 44 (16½ per cent.); Arctic 31 (11½ per cent.); Mediterranean 24 (9 per cent.); and purely inland only 7 (2½ per cent.).
It can be seen that the North Atlantic, with its seventy-four species, is much lower than either half of the Pacific than would appear warranted by its area. There is not the faintest hint, from the radiation of any of the sea-bird groups, that either North or South Atlantic has been the arena of any great evolutionary changes. The Atlantic has been colonised from without; by penguins from the Antarctic; by petrels from the South Pacific; by pelecaniform birds and terns probably from the Indian Ocean; by gulls and auks from the Arctic. The North Atlantic and the immediately neighbouring parts of the Arctic have but two present sea-bird genera and only thirteen species of their own. We need not be surprised at this indication that the Atlantic’s bird fauna is derived from that of other oceans if we accept Wegener’s theory of the origin of the Atlantic; but whether the Wegener theory is true or not it is quite clear that the North Atlantic has not been the home in which any important group of sea-birds has evolved. This is not to say that there has been no sea-bird evolution in the North Atlantic; but it has not usually gone beyond the differentiation of species. Of this it has, indeed, much to show. Some of the classic examples which E. Mayr (1942) has discussed are North Atlantic species. Mayr’s thesis is that one species can only become two after it has been differentiated geographically. He opposes the notion which has found favour in some quarters that speciation may occur by ecological differentiation or by the differentiation of behaviour.
So far the available evidence appears to uphold Mayr’s view—at all events, for birds. During the present century much systematic work in the description and measurement of birds has been conducted in American and European museums, and much practical and theoretical work on evolution has also been done. But it needed the persuasions of Mayr and Julian Huxley (1942), amongst a few others, to collate the work of the systematists and the evolutionary zoologists. Sea-birds lend themselves to evolutionary study because they are so largely confined to coasts for breeding purposes. This makes their distribution often linear rather than of the ordinarily spatial two-dimensional type; and this linear distribution makes it easy to apply Huxley’s concept that the characteristics of animals tend to grade from one part of their range to another in an orderly way. Some of these gradations had been recognised long before Huxley thought of the word “cline” because they are adaptations to the environment. For instance Bergmann’s Rule states that from the warmer parts of an animal’s distribution-area to the colder parts there tends to be an increase in its size. Thus the puffins, black guillemots and eider-ducks of the Arctic are considerably bigger than those of Britain. The main adaptive reason for this is that larger animals have less surface in proportion to their weight, and consequently heat is not lost from them (if warm-blooded) so rapidly as it is from small animals. Another rule, Allen’s Rule, states that warm-blooded animals of cold climates tend to have their heat-radiating surfaces decreased by a reduction in size of their extremities and limbs such as ears, tails, necks, legs and noses. There is also a general tendency (Gloger’s Rule) for animals to become darker as humidity increases.
If we examine those sea-birds which are widely distributed, we find clines in various characteristics, notably in size, i.e. total size, and also size of limbs and extremities, beak-length, wing-length, etc., and in colour. There are also clines in shape; for instance the fulmars of the north-east Atlantic have very thick bills, those of Baffin Island rather more slender bills, those of the North Pacific more slender bills still, and those of the Antarctic very slender bills indeed. No sea-bird is arranged quite evenly in its geographical distribution. Just as the distribution in space is never even, so are the gradations in character never even. From one part of the geographical distribution of a species to the other, change often occurs more as a series of steps rather than as continuous ramp.
Most working ornithologists today will agree that there are more subspecific names about than a true understanding of bird evolution requires. It is the species which has reality and significance. In this book we have tried to be sparing in the use of subspecies, and have rejected some that appear in many current text-books. Nevertheless, a study of the geographical races of the species of the North Atlantic sea-birds will lead us to examine here some of the more fascinating examples of geographic differentiation. The classic example among the sea birds is the chain of the Larus argentatus and fuscus group, the herring-gulls and lesser blackbacks, which may include some birds which are regarded as separate species, e.g. the California gull L.californicus, and the so-called ‘Iceland’ gull, or better the Greenland herring-gull, L. glaucoides or leucopterus. The relationships of this superspecies (Fig. 4a (#litres_trial_promo)) were first worked out by B. Stegmann (1934): we have included the results of subsequent systematic work in this map (Fig. 4b (#litres_trial_promo)) and in the discussion which follows.
It will be seen that the subspecies is composed of three chains of subspecies which unite in Central Siberia, where the resident breeding subspecies is Birula’s herring-gull Larus argentatus birulai. The two northerly chains link round the Polar Basin, the two end links of one overlapping with the two end links of the other. Where they overlap, the two races of one chain-end are ‘herring-gulls,’ of the other ‘lesser blackbacks.’ These behave as different species. It can be found convenient to make the ‘species’ separation in the chain, between the two races birulai and heuglini, thus calling the latter Larus fuscus heuglini (it is the first really dark-mantled gull in the chain). This is more practical than splitting the chain into argentatus and fuscus in the Bering Strait area, though this is probably the place of origin of the ancestral gull that gave rise to the whole chain; for if all the palearctic group were fuscus some confusion would surround the light-mantled Mediterranean forms.
Special comments can be made on various members of the chain. In the zone of overlap in Western Europe the herring-gulls are distinguished from the lesser blackbacks not only by form but by many habits. The lesser blackbacks breed often inland on moors, and when coastal tend to colonise flattish ground set back from the cliff-tops beloved of the herring-gulls. While the herring-gulls are dispersive in winter, the lesser blackbacks are almost entirely migratory, wintering south of all but their most southerly breeding-places, though some of the dark L. f. fuscus of Scandinavia winter in Britain, and recently a minority of the British race L. f. graellsii has ‘revived’ an old habit of wintering in England, especially in Cheshire and Lancashire. Both species are also extending their breeding-range north; L. a. argentatus has colonised east and north-east Iceland since 1909, and a herring-gull of this or the Scandinavian race omissus was breeding on Bear Island in 1932, though not 1948. The graellsii lesser blackback has established itself in south Iceland since about 1925, and a group intermediate between graellsii and fuscus in Denmark since 1922.
The North American situation is of great interest. As the herring-gulls range north-east they become generally paler in colour. The much-discussed Kumlien’s herring-gull L. a. kumlieni was for a long time held to be a hybrid between the ‘Iceland’ gull of Greenland and L. a. thayeri, Thayer’s gull of the Canadian Arctic and Thule corner of north-west Greenland. But there seems no doubt that it is a valid race (Taverner, 1933) with its own discrete breeding-distribution in southern Baffin Island, though on the western marches of its distribution there are apparently some forms intermediate between it and thayeri (Hørring, 1937) and colonies off south-west Baffin Island have been described as mixed (Soper, 1928).
FIG. 4
Breeding distribution and relationships of all subspecies of Larus fuscus, L. argentatus and related forms. a Diagram of forms, with leg and mantle-colour
