Sea-Birds. James Fisher

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      FIG. 8

      Breeding distribution of two closely-related guillemots or murres: Uria aalge, the common guillemot; and U. lomvia, the arctic guillemot (Brünnich’s guillemot^* in the Atlantic, Pallas’s murre in the Pacific). Areas of overlap shaded.

      Increases noted in the 1939–1949 enquiries were several, but only one, at Foula in Shetland, was significant and by checked observers (from 24 per cent. in 1938 to 29.4 per cent. in 1948–49). Increases on the margin of significance were recorded from St. Bee’s Head in Cumberland, Marwick Head in Orkney, and the Fair Isle. Apart from these small increases in the last decade, there was a significant increase of the percentage on Noss in Shetland from 15.5 in 1890 to 26.5 in 1938, which seems great enough to embrace a possible slight observer-error.

      Unfortunately, too few of the early bridled guillemot counts are reliable, though some from Berneray and Mingulay (‘Barra Head’) in the Outer Hebrides may be so. This had 20.2 per cent. in 1871; 12 in 1939; 9.8 in 1949; 12.6 in 1950. The decrease between 1871 and 1939 is significant, though the other apparent changes are not so. Elsewhere we have followed Southern in discarding such vague records as ‘about one in every nine or ten.’

      Nothing is yet known about the percentage of bridled guillemots along the coast of Norway, except that it has remained slightly over 50 per cent., at Bear Island from 1932 to 1948. At the Karlov Islands off the Murmansk coast the percentage was 42 in 1938. It seems likely, from the rather scanty figures from Novaya Zemlya, which Southern slightly misdates and misplaces, that the percentage may be about the same on islets in Pukhovy and Bezymiannaya Bays off that island (36.4 and 50).

      These changes are curious and it is clear that much remains to be solved about this interesting problem in distribution and evolution. Nor is much known about the distribution of the bridled form in the New World, save the following: H. F. Lewis found 128 bridled out of a sample of 724 (17.7 per cent.) in the colonies along Quebec Labrador in 1929. One of us found 51 bridled out of a sample of 295 (17.3 per cent.) at Cape St. Mary, on the south-west corner of the Avalon Peninsula of Newfoundland, in 1953. In June, 1940 at Funk Island and other parts of the east coast of Newfoundland within forty miles of it W. Templeman (1945) collected twelve common guillemots (? at random) of which six (50 per cent.) were bridled. When Hørring and Salomonsen (1941) compiled a list of all the common guillemots that had been then collected on the west coast of Greenland they recorded six out of thirty-two (18.7 per cent.) as bridled, but knew of no breeding-colony. Soon afterwards Salomonsen (1944) became aware of the colony in the Sukkertoppen district; but no count has apparently yet been made there.

      FIG 9a The principal breeding-colonies of the common guillemot in Britain. The percentages of bridled forms in the breeding-populations, as determined chiefly by H. N. Southern and his colleagues, are shown. Minus and plus signs in brackets indicate changes in the decade c. 1939–c. 1949 which are significant, or on the borderline of significance. Crossed circles mark sites of former colonies.

      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. СКАЧАТЬ