FIG. 9b The distribution of bridled guillemots in the East Atlantic breeding-populations: O: no bridled birds observed. A: under 1 per cent bridled. B: under 2 per cent bridled. C: under 5 per cent bridled. D: under 10 per cent bridled. E: under 20 per cent bridled. F: under 50 per cent bridled. G: over 50 per cent bridled.
All the four skuas appear to vary in plumage; the bonxie (great skua) particularly in the amount of rufous colour, especially among some of its southern forms; the three smaller skuas have a ‘normal’ pale phase of plumage with light breast and underparts, and yellowish or buff on the sides of their necks; and a ‘dark’ phase which is almost uniformly, or uniformly, dusky; and intermediates. The dark phase of the long-tailed skua is so rare that it has hardly ever been seen. Among the population of pomarine skuas, wherever they may breed, from five to twenty per cent. are dark; the distribution of dark birds is even, in the sense that there is no detectable gradient. Southern’s detailed analysis (1944) shows that no geographical area contains significantly more dark pomarine skuas than any other. Among the arctic skuas (Southern, 1943), however, the situation is quite different. In the southern parts of this bird’s breeding-range about three-quarters of the birds are dark; in the middle parts about half, in the Low Arctic less than half, and in the High Arctic a quarter or less. In north-east Greenland, indeed, the dark form is unknown. There are a few, rare, birds intermediate in colour between the pale and dark forms. This looks like a quivering balance between two ‘stable’ types. The proportion of the colour-forms in the British colonies is (Southern points out) subject to rather special considerations, since the colonies are generally small and scattered, and thus liable to random fluctuations—in fact between the limits of 50 and 86 per cent. dark. The mean probably lies at about 75 per cent.
Southern has attempted to correlate the distribution of the dark arctic skuas (Fig. 10) with temperature, relative humidity and various ecological factors. His material carries darkness with humidity over a considerable part of the bird’s total range; but the correlation breaks down in Norwegian Lapland—also, good meteorological figures are not available for all the arctic regions.
FIG. 10
Distribution of colour-phases of the arctic skua, Stercorarius parasiticus, from H. N. Southern (1943), showing isolines for percentage of the pale phase in the breeding population.
We found the same difficulty in correlating the distribution of the colour phases of the fulmar, Fulmarus glacialis, with climate and other environmental factors. In the Atlantic (though not the Pacific) part of the fulmar’s range the situation is in many ways the opposite of that among the arctic skuas; where the surface of the sea is above freezing (i.e. in the Low Arctic and rest of the range) the fulmars are nearly all light. The gradient runs from 0 per cent. dark in Britain to a hundred per cent. (probably) in the High Arctic of Spitsbergen and Franz Josef Land; in West Greenland (Low Arctic) the fulmars are very nearly all light. Finn Salomonsen suggested to Fisher (1952) a correlation between this distribution (Fig. 11) and surface water-temperature. Dark fulmars are only found in the areas where the water is nearly freezing, or freezing, in July, in which month the adults collect food for the chick fulmar. It is interesting to note that approximately the fifty-fifty situation in the distribution of colour-phases of the fulmar (as also of the bridling in the guillemot) is found at Bear Island, whose position is between Low and High Arctic. However, the Pacific fulmar appears to reverse the situation found in the Atlantic; the dark fulmars are found in the warmer parts of the Pacific fulmar’s range, and the light fulmars progressively towards the colder parts, though nowhere does this race of the fulmar breed in truly High Arctic waters.
FIG. 11
Breeding distribution of the fulmar, showing the approximate preponderance of dark birds in the populations, indicated by the dark parts of the circles (from Fisher, 1952)
The existence of these polymorphic forms of some birds constitutes a problem of the greatest interest, which travellers and amateur naturalists might well help to solve by collecting simple counts of the relative proportions of easily recognisable forms.
CHAPTER 3 SEA-BIRD NUMBERS AND MAN
EVERY BIRD has a history, which is a tale of adventure and fluctuating fortunes, of success, or of failure; for every bird, like every other animal, suffers change. In any study of the life of birds, and the place of birds in nature, an understanding of their numbers is fundamental.
Since most sea-birds are social animals, and nest in colonies in wild and beautiful places, their numbers can often be studied very closely, and with a great deal of enjoyment. So enthusiastic is the average amateur bird-watcher about visiting sea-bird stations, and ‘collecting’ islands, that it is safe to say that every important seabird colony on both coasts of the United States (not Alaska), and on those of the Faeroes, Britain, France, Belgium, Holland, Denmark, Germany, Finland, and Sweden is known to somebody who can distinguish its birds from each other; and most of those in Norway, Spain, Portugal, Iceland and St. Lawrence-Canada are known. The sea-bird stations of Greenland, thanks to a tradition of accurate observers from Giesecke to Bertelsen and Salomonsen, are better known than those of the Canadian Arctic, Newfoundland, the U.S.S.R., China, and perhaps even Japan. Those of the Antarctic and Subantarctic, and South America, are perhaps better known than those of the tropical Pacific. Probably a very adequate list of the sea-bird stations of the United States (excluding Alaska) or of north-west Europe could be compiled by some bibliophilic ornithologist with access to all the local as well as national bird and natural history journals of those countries. Such lists would be useful documents; they would have to be carefully dated, because of what history tells us of the fortunes of animals, and of change. Fisher has recently compiled a dated list of all the fulmar colonies of the world, and we have both, at different times, compiled lists of the world’s gannetries. It is surprising how certain it is possible to be of being complete, within reasonable limits. Thus after the publication of his Report on the 1938 survey of black-headed gull colonies P.A.D. Hollom (1940) had no colony known (or not known) to be occupied in 1938 to add or subtract from his list of 342 such colonies. When Fisher and Vevers (1943–44) organised a census of the North Atlantic gannet in 1939, only two small colonies of the twenty-three which then existed were overlooked in that year. When Fisher and Waterston (1941) reported on the fulmar colonies known to them in Britain in 1939 they believed that there were 208 separate stations at which the fulmars were breeding. Ten years later, after carrying on research and correspondence with the same intensity to discover the situation in 1944 and 1949 (during which ‘back information’ was also collected), Fisher discovered that he had overlooked only nineteen, all small (and some in very remote parts), and unimportant as far as the fulmar’s population, or the actual extent of its range, were concerned.
A census of the sea-birds of the North Atlantic is no longer a wild dream. A start has been made with certain obvious species, with limited distribution or small populations. The organisational problems are not insuperable; we have an ever-increasing body of highly competent bird-watchers available for, and keen on, the counting of nests: for a sea-bird census depends on the assessment of the number of occupied nests. Such a census has already been performed for several species on the coasts of Germany (Schulz, 1947), and, judging from the descriptions of the distribution of sea-birds in Sweden (Lundevall, unpublished), Denmark (Jespersen, 1946, and Løppenthin, 1946), the Netherlands (van Ijzendoorn, 1950) and Belgium (Verheyen, 1951), it need not be long before a census of the southern North Sea and Baltic could be complete. In Britain good surveys, if not censuses, exist for the sea-bird colonies