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Larus Glaucoides Complex Robert Y.Mcgowan and Andrew C Taxonomic research update Historical and taxonomic review of the Iceland Gull Larus glaucoides complex Robert Y.McGowan and Andrew C. Kitchener he Iceland Gull Larus glaucoides is L. glaucescens, or as three separate species. now generally regarded as comprising The relationships among the three Iceland Tthree subspecies: L. g. glaucoides, Gull taxa have been debated by ornithologists which breeds in Greenland, L. g. kumlieni for 180 years,from Sabine (1819) to Chu (1998), (‘Kumlien’s Gull’), which breeds mostly on without a clear resolution, which gives some Baffin Island, and L. g. thayeri (‘Thayer’s idea of the problems presented by this group. Gull’), which breeds throughout the Cana- In an earlier paper, Weir et al. (1995) dian High Arctic (fig. 1). Iceland Gull taxa reported that the frequency of kumlieni have at various times, however, also been among wintering adult Iceland Gulls in the treated as subspecies of Herring Gull eastern Atlantic had increased substantially L. argentatus or Glaucous-winged Gull during the twentieth century.This change . (2000). et al Reproduced from Weir Weir from Reproduced Fig. 1. Breeding records for three Iceland Gull Larus glaucoides taxa, about 1950-95: glaucoides ; kumlieni ; thayeri . Complete breeding distributions of all three taxa are still uncertain (=?), and a few thayeri records are from up to 20 years before 1950. M indicates the main southward thrust of thick, multi-year accretions of pack ice; line of open circles shows postulated southern limit of High Arctic. © British Birds 94: 191-195, April 2001 191 McGowan & Kitchener: Historical and taxonomic review of the Iceland Gull prompted us to review historical distribu- Most records of immature Iceland Gulls tional data for all three Iceland Gull taxa.We had to be excluded from our study owing to suspected that any changes in their respec- the highly variable nature of the plumage of tive distributions that could not be explained such individuals and the lack of ringed birds by changing environmental factors might of known origin. We then assigned each his- help to elucidate the relationships among, torical specimen and each individual and the validity of, the three taxa. By carrying described in published records to one of our out a detailed historical and morphological three defined taxa, in order to determine review of Iceland Gull taxa over the past 200 changes in both the breeding and the winter years, we hoped to develop hypotheses on distributions of these taxa over the last two their relationships that could be tested by centuries. Historical data were relatively few molecular studies. compared with the availability of more Specimens were routinely collected by recent information (e.g. post-1950). Further members of Royal Navy and other explo- details, including sample sizes, are given in rations of the Northwest Passage, from 1818 Weir et al. (2000).We also examined changes to 1860, which covered almost the entire in the distributions of other large gulls in the range of the Iceland Gull. Although only a same region, to determine whether these few of the earliest bird specimens collected were due to a common set of environmental still exist, many full descriptions can be or ecological variables. found in published expedition reports and We found that, until about 1860, nominate zoological literature. It is clear from these glaucoides bred from Greenland to the studies that most early naturalists were able western High Arctic of Canada, but by about to distinguish readily between the nominate 1900 it was essentially confined to Green- race of Iceland Gull L. g. glaucoides and the land (fig. 2). Until 1860, thayeri was known larger Glaucous Gull L. hyperboreus, a fact only from the western Canadian High Arctic, which it is important to note when inter- but from 1900 to 1950 it was found preting descriptions from these early accounts. throughout the High Arctic of Canada and in The most reliable character for defining a small area of northwest Greenland (fig. 3). Iceland Gull taxa is the intensity and extent At high latitudes in Canada, therefore, of wingtip melanism in adults, although this thayeri had evidently replaced glaucoides, varies more or less continuously from none with which it was formerly sympatric in the in glaucoides, through the intermediate west and with which it probably interbred. kumlieni, to darkest and most extensive in The first known kumlieni were recorded thayeri. A secondary, but less reliable, char- from west Greenland by the 1850s, and by acter involves the iris colour of breeding 1900 the western and northern limits of adults. Details of the differences separating most of its breeding range in the eastern the three forms are given in table 1. Canadian Low/High Arctic were known. By Table 1. Definitions of Iceland Gull Larus glaucoides taxa, based on adults only. The melanism score of the primaries is derived from the extent and relative intensity of melanism on the upper surface of the wing. Feature thayeri kumlieni glaucoides Number of primaries with melanism (3)4-6 1-5 0 Melanism scores P10-P6 2.0-4.5 0.2-2.5 0 Iris colour of breeding dark intermediate clear greyish-yellow adults (purple/brown speckling) Sources: Dwight 1906, 1917, 1925; Salomonsen 1950; MacPherson 1961; Fjeldså & Jensen 1985; Godfrey 1986; Ingolfsson 1967, 1970; Snell 1989, 1991a, b;Weir et al. 2000. 192British Birds 94: 191-195, April 2001 McGowan & Kitchener: Historical and taxonomic review of the Iceland Gull . (2000). et al Reproduced from Weir Weir from Reproduced Fig. 2. Known breeding distribution of three Iceland Gull Larus glaucoides taxa, in about 1900: glaucoides ; kumlieni ; thayeri ; glaucoides or kumlieni (taxon not verified)=?.The first northerly records of Herring Gull L. argentatus smithsonianus are shown as open circles. (2000). et al Reproduced from Weir Weir from Reproduced Fig. 3. Breeding records for three Iceland Gull Larus glaucoides taxa, about 1950: glaucoides ; kumlieni ; thayeri ; glaucoides or kumlieni (taxon not verified) shown as filled circle; uncertain or disputed records = ?. British Birds 94: 191-195, April 2001 193 McGowan & Kitchener: Historical and taxonomic review of the Iceland Gull 1964, kumlieni was recorded breeding in diate is known to occur with one parent Greenland (not shown in fig.1). The current (i.e. kumlieni × thayeri), and is strongly breeding range of kumlieni is restricted, suspected to occur with the other. lying between those of thayeri and glau- 4. The form kumlieni is highly variable in coides and overlapping both; kumlieni freely key morphological characters, being inter- interbreeds with thayeri (Gaston & Decker mediate between the two parent taxa in 1985; Snell 1989) and probably with glau- wingtip melanism and iris colour, and this coides. In contrast to their changing variation tends to be geographical with breeding distributions, the winter ranges of respect to the parent taxa. glaucoides and thayeri were found to have 5. The present range of the intermediate is changed little since they were first deter- limited, lying between and overlapping in mined, by 1860 and by the 1920s respec- part with those of the parent taxa; in addi- tively. Winter adult kumlieni,however,was tion, the hybrid has not spread into the unrecorded in the north Atlantic between High Arctic, where thayeri has directly Greenland and the British Isles until about replaced glaucoides. 1900; it remained rarer than glaucoides until This may not, however, have been the first 1915 (when samples were 75-100% glau- time that thayeri and kumlieni have met. coides), but became progressively more Their populations may have advanced and common after 1950 (when samples were 33- retreated several times in the Arctic in 80% kumlieni). response to climate change during the There is clear evidence that glaucoides present and past interglacials. They may not was replaced by thayeri on the breeding have hybridised at every meeting and, when grounds, involving an eastward shift of 40° they did, gene flow may have been in either longitude, whereas, in contrast, the distribu- direction. It appears that the brief systematic tions of other large gulls of the region shifted ornithology of the Arctic happened to coin- mainly northwards. Therefore, whatever cide with the most recent hybridisation factors had caused the eastward movement between thayeri and glaucoides, and the of thayeri were different from those which eastward shift by thayeri of about 40° of affected the distributions of other large gulls. longitude, followed by further introgression We also noted, however, that thayeri, as it by the intermediate into the remaining pop- spread eastwards, hybridised with glau- ulation of the Atlantic parent. coides, giving rise to the variable form Hybrid zones are common ornithological known as kumlieni. phenomena. In northern regions, most are Hybrid populations are often distin- explained by secondary contact during the guished from valid taxa by their inherent present interglacial, though most species do instability, both through time and in their not move far (Hewitt 1989). Hybrids are pre- geographical distribution. Otte & Endler sumed to be adaptively less fit, and they tend (1989) set out fairly simple criteria by which to occur in density troughs between the unstable, hybrid populations can be recog- parental peaks (Barton 1989). In the case of nised. Five of these criteria demonstrate that the Iceland Gull, the distributions of both kumlieni is not a valid taxon, but a variable parent taxa and the hybrid form shifted intermediate form resulting from introgres- rapidly, but kumlieni is now probably more sive hybridisation between thayeri and glau- numerous than thayeri, and glaucoides may be coides lineages: ten times more abundant than either of them. 1. Specimen data confirm that the postu- Other workers (e.g. Dwight 1906; Snell lated parent taxa (glaucoides and thayeri) 1989) have suggested that kumlieni is an formerly bred sympatrically and may have intermediate taxon between glaucoides and hybridised.
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