Polymorphism and behaviour in the Arctic Skua (Stercorarius Parasiticus (L.))

Abstract

Arctic Skuas are birds of the Arctic and sub-Arctic. The majority of them are light-coloured, but there is a Dark form which constitutes 70 to 80% of the population at the southern­ most limit of the breeding range of the species in northern Britain, decreasing further in frequency towards the north. Birds intermediate in colour phase can be recognized. The colour variation is largely due to two allelomorphs at a single locus. The heterozygotes are variable in expression and the Intermediate class contains representatives of both homozygotes. The Skua colony on Fair Isle, Shetland, was intensively studied from 1949 to 1963. During this time most birds (both chick and adult) were individually marked and the phases of nesting birds and their young noted. There were no differences between mating types in either breeding success or fecundity. In matings between phases, 60% involved pairs where the male was darker than the female. This arises because Dark males pair earlier than Pale ones. Since Arctic Skuas pair for life (although ‘divorce’ and ‘remarriage’ are not uncommon), this behaviour gives an advantage to males carrying the allelomorph for dark colour unless early hatching clutches are at some disadvantage. It is suggested that later hatching may be favoured in inland and northern parts of the species range, where the main food is small mammals (which usually become plentiful only later in the summer). The frequency of Pale birds is much higher in such areas. 'There is a fowl called the Scutiallan, of a black colour and as big as a Wild Duck, which doth live upon the vomit and Excrements of other Fowls whom they pursue and having apprehended them, they cause them to Vomit up what meat they have lately taken, not yet digested: The Lord’s Works both of Nature and Grace are wonderful, all speaking forth His Glorious Goodness, Wisdom and Power ' (Brand 1701).