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), and plotted according to which parts of the ocean they breed in, in the form of a diagram (Fig. 2). 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) 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; the totals include the two North American purely inland species, and the two extinct species. Doubtful (“?” in the Appendix) 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.
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
