HYBRIDIZATION OF GLAUCOUS GULLS LARUS HYPERBOREUS AND HERRING GULLS L. ARGENTATUS IN ICELAND

Until recently, the Herring Gull Larus argentatus and the Glaucous Gull L. hyperboreus were completely allopatric in western Europe, although having overlapping distributions in Canada and the Soviet Union. But beginning about 1925–1930 a large-scale invasion of argentatus from the Faroe Islands and/or Britain into Iceland occurred, resulting in extensive hybridization with hyperboreus, already present there. The hybrids are viable and apparently fully fertile. In the summers of 1964–1966 371 gulls were collected from breeding colonies in various parts of Iceland and the samples analysed by the use of hybrid indices based on the colour of the five outermost primaries. The populations of northwestern Iceland, where the greatest concentration of hyperboreus was found at the time of the argentatus invasion, were found still to consist largely of pure hyperboreus, with 18–23% of the birds showing signs of argentatus admixture. In eastern Iceland, where argentatus first appeared in numbers, and along the south coast, birds of hybrid origin predominate, 3–43% of the birds being pure argentatus while pure hyperboreus are absent except for one colony in southeastern Iceland where they made up 23% of the birds. Significant differences in composition are found among several of the colonies of eastern and southern Iceland, and possible reasons for this are discussed. Among 53 mated pairs no deviation from random matings with respect to hybrid indices was indicated. The high proportion of pure argentatus found in some colonies in spite of this indicates that pure argentatus are still immigrating from abroad. Two colonies in eastern Iceland sampled both in 1965 and 1966 showed a significant change in composition towards argentatus, further evidence for continued immigration of argentatus. A few other colonies have changed in the same direction over longer periods. Although there must be some differences between the two forms in ecological requirements as shown by their distributions, their ecologies appear similar in Iceland. Both forms take a large portion of their food from the intertidal zone of the sea-shore, and although a slight difference in the selection of foods from the shore was seen in winter in one locality, this probably did not have a genetic basis. However, argentatus may feed to a larger extent on refuse than hyperboreus. This may enable argentatus-like birds to occupy the relatively barren (with respect to intertidal foods) east coast of Iceland in numbers. An examination of several hundred museum skins of argentatus and hyperboreus from other areas shows that occasional hybridization occurred in western Europe long before the extensive contact was established in Iceland, but gene flow between the two was always limited as their ranges were widely separated. The two gulls are effectively reproductively isolated from each other where synipatric in Canada, while the situations in Alaska and in parts of the Soviet Union are unclear. Since the area of overlap without hybridization is much larger than the area of hybridization, the two forms are best regarded as separate species. Future studies may show in which direction the hybrid situation in Iceland will develop, but meanwhile it is suggested that the hybrid populations in which birds of hybrid origin amount to 50% or more be referred to as Larus argentatus/hyperboreus.