Iceland Gull and Thayer's Gull are medium-sized, white-headed gulls that nest on scattered cliff colonies throughout coastal regions of the Canadian Arctic Archi-pelago (Northwest Territories and Nunavut) and Greenland. These gulls are distinguished from other gulls by their size, soft-part coloration of adults, in particular a reddish-purplish orbital ring in breeding condition, and breeding habitat. Pairs nest primarily on small rocky ledges and outcroppings of vertical sea cliffs, often in association with Glaucous Gulls (Larus hyperboreus) and occasionally Common Ravens (Corvus corax). A large coastal cliff colony on south Baffin Island impressed Dewey Soper (1946: 235) to write, "The latter afforded a memorable sight as in a restless cloud they wheeled hysterically in dextrous evolutions against the bleak facade of the great promontory."
The Iceland Gull is separated into 2 subspecies based predominantly on markings and patterning of primary-feathers of adults. Thayer's Gull is monotypic. While the breeding ranges of these species approach closely or narrowly overlap, a precise definition being elusive, the chief wintering ranges for these species are rather widely disjunct. Birds breeding along west Greenland south of 70°N and on east Greenland make up Larus glaucoides glaucoides; most but not all adult nominate glaucoides lack patterning on their wings and have correspondingly light mantles. Birds breeding east, south, and southwest of Baffin Island, Digges Sound area, and west to eastern Southampton Island make up Kumlien's Gull (L. glaucoides kumlieni): adults have highly variable wing-tip patterns, sometimes patternless, others similar to Glaucous-winged Gull (L. glaucescens). Mantles of kumlieni are usually darker than those of the nominate glaucoides. Birds breeding in scattered colonies in northwest Greenland and in the Canadian Arctic Archipelago north and west of kumlieni make up Thayer's Gull (L. thayeri). Iris color and speckling varies greatly in adult thayeri, even in the western part of the breeding range. The great majority of adults in westernmost populations have more extensively patterned wing-tips and darker mantles than kumlieni .
The great majority of nominate glaucoides winter in the Arctic on coastal regions of southwest Greenland, in open leads in sea ice and in open-water polynyas. In all likelihood, many kumlieni also overwinter in the Arctic. Winter sight records of nominate glaucoides and kumlieni in the United States, southern Canada, Iceland, Faeroes, and Europe are mostly immature birds in coastal regions. In contrast, most thayeri appear to winter in the coastal Pacific Northwest, much less frequently to the Great Lakes region, and only very casually on the East Coast, the continental interior, and Gulf Coast. There are polynyas in the western Arctic in winter, but it is not known whether a portion of thayeri, in particular birds from western populations, overwinter in the north.
The taxonomy of Iceland and Thayer's gulls is unsettled, and whether they should be treated as 1 or 2 species is disputed. These birds share a breeding habitat distinct from Herring Gull (L. argentatus). Breeding chronologies are similar. Morphological and plumage characters overlap broadly. My studies, heretofore unpublished, are suggestive of continuous phenotypic variation from darkest to lightest extremes in plumage of adults across the breeding range. Where breeding ranges of kumlieni and thayeri overlap (e. Baffin I., e. Southampton I., Digges Sound), there is no evidence of assortative mating: gulls as dark or darker than the type of thayeri bred with others much lighter than the type of kumlieni, including birds lacking visible wing-tip melanism. Based on this, I believe only 1 species should be recognized with all taxa placed under Iceland Gull. Where practical, discussions in this account combine the 3 taxa in-volved (nominate glaucoides, kumlieni, and thayeri) as a whole. The names "Iceland Gull" and "Thayer's Gull" are used when referring to the taxa as separate species following current classification by the American Ornithologists' Union (1998).
Iceland and Thayer's gulls are among the least known of all North American gulls, and few studies are dedicated to their natural history. There are important summaries of known or presumed infor-mation on identification of nominate glaucoides, kumlieni, and thayeri (Gosselin and David 1975, Lehman 1980, Zimmer 1991, Howell and Elliott 2001), including caveats and cautions. Important information concerning arrival, breeding and departure times, habitat, and nest-site preferences is available from scattered localities throughout the Arctic (Salomonsen 1951, Macpherson 1961, Parmelee et al. 1967, Sutton and Parmelee 1978, Gaston et al. 1985, Goethe 1986). Timing of first Pre-basic molt is now quite well understood (Howell et al. 1999). Lack of basic knowledge relates to logistic difficulties of studying a high-arctic cliff-nesting species, inaccessibility of northern wintering areas, and, for Iceland Gull in particular, infrequent occur-rence in the south. Further studies are needed on almost all aspects of the biology of these intriguing and enigmatic gulls.
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