The Two Phases of Stercorarius parasitieus (Linnæus)

Summary.(i). The distribution of polymorph ratios in wild populations, and changes or evolution in such distribution, are peculiarly suitable to quantitative treatment. Cases of polymorphism, whether in equilibrium, due to a balance of selective factors, or transient, due to the favouring and spread of one or more of the phases, are known among birds, and the following instances are discussed:the Guillemot ( Uria aalge ), the Screech‐Owl ( Otus asio ), the Reef‐Heron ( Demigretta sacra ) and the Fulmar Petrel ( Fulmarus glacialis ). (ii). Melanic polymorphs are well known in birds, and replacement has been demonstrated within historic times for Coereba saccharina , and Rhipidura flabellifera.(iii). In the Skuas S. longicaudus has very rare melanics, S. pomarinus about one in ten or twenty, S. parasiticus from 0–95 per cent. Considerations are advanced for and against the hypothesis that the melanic form is the mutant. (iv). Data on distribution of S. parasiticus , which is circumpolar, are collated and discussed. Greatest population concentrations are in the areas of the north Pacific and north Atlantic. Further data suggest that the control of the two phases may be unifactorial, and that the young may be identifiable the same season in which they are hatched. (v). References in the literature to proportions of the phases throughout the range are given in Table 1, and the distribution pattern shows that dark birds are much commoner in the oceanic and southern areas, while in the high north and the continental areas of the range pale birds predominate, sometimes to the exclusion of the dark. Pale birds are commoner in the Old World than in the New World areas, and it is possible that the melanic has spread from the two oceanic areas northward and along the continental edges—more rapidly in the Pacific, where the northernmost populations still have a fair number of dark birds. Isolines for 25, 50 and 75 pale birds per cent, are given in text‐fig. 5. (vi). The dimorph‐ratio cline thus revealed is probably of the “stepped ramp” type, though only in a few places can an intra‐group cline be distinguished ( e.g. Iceland). Reversals and anomalies occur in areas of discontinuous and small populations ( e. g. the British Isles). Remarkable rates of change in dimorph ratio are also recorded in such places. No reliable indication of long‐term change is to be found, though there are some suggestive instances. (vii). Temperature does not appear to be correlated with the cline. Humidity shows broad relations, but on close scrutiny the coincidence breaks down. (viii). Heterogeneity, due to selective mating, occurred in one sample examined, but not in three others.