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Original title in French: Les goélands du complexe [Larus argentatus-cachinnans-fuscus] : où en est la systématique?
A comprehensive review of the systematics of the [Larus argentatus-cachinnans-fuscus] complex.
Summary in English:
This paper is an updated translation of an original publication in Dutch Birding (Yésou 2002). The updates are given as footnotes.
The large white-headed gulls form a group of closely related taxa sharing distinctive phenotypic features (immaculate white head in breeding plumage, red spot at the gonydeal angle of the bill). The intensity of the grey mantle and the details of the wing-tip pattern, parameters which vary between taxa and are easily studied in museum collections, have classically been considered the most useful characters to sort out most taxa. The leg colour, flesh-coloured, pink or yellow of variable intensity, is also a character widely used in their classification.
The large white-headed gulls consist of up to 33 taxa (the number of accepted species and subspecies varies with time and author) which have a mostly northern Holarctic distribution: the only representatives in the Southern Hemisphere are five taxa considered as subspecies of Kelp Gull [L. dominicanus]. The northern large white-headed gulls consist of seven unchallenged species (the Great Black-backed [L. marinus], Glaucous [L. hyperboreus], California [L. californicus], Slaty-Backed [L. schistisagus], Yellow-footed [L. livens], Western [L. occidentalis] and Glaucous-winged Gulls [L. glaucescens]) and two groups of taxa of which the number of valid (sub)species is widely discussed: the ‘Iceland Gull Group’ (the most recent research suggests that the Iceland [L. glaucoides] and Thayer’s Gulls [L. thayeri] are different species, while ‘Kumlien’s Gull’ represents a hybrid population between them) and the ‘Herring Gull Group’.
Although this article reviews all taxa of white-headed gulls, it mostly deals with the Herring Gull Group, also known as the Larus argentatus-cachinnans-fuscus complex. This complex has long been considered as resulting from a process of ring speciation, for which the most commonly held scenario is as follows. The ancestor of the Herring Gull Group supposedly lived in central Asia. During some interglacial period, it expanded northwards. This resulted in a population, [fuscus] or an ancestor of [fuscus], which went into an eastward expansion around the northern Holarctic. The divergence led to populations which, among other points, became paler and paler (present-day [heuglini], [vegae] and [smithonianus]). The ring was closed when birds crossing the Atlantic Ocean diverged into argentatus which overlapped with fuscus without interbreeding, behaving as good species. Meanwhile, the southern taxa had evolved from the ancestor population which had already given rise to the northern ring. This theoretical approach was however based on an uncomplete coverage of the group, as until recently very little was known on the physical appearance, phenotypical variation and biology of many, mostly Asian, taxa.
A deep interest in the Herring Gull Group has developped in both amateur and professional circles since the 1970s. This interest has benefited from new opportunities. The development of birding tours has improved the knowledge of various taxa, mostly from their wintering grounds. Following the end of the Cold War, some western ornithologists were given the possibility to study Asian taxa both on their breeding grounds and in the rich collections of Russian museums. The development of colour-ringing and, particularly, genetic studies has also helped to better our understanding of the appearance, biology and relationships of various taxa. This increasing interest has resulted in numerous publications, greatly improving the knowledge of many taxa and allowing a review of the systematics of the Herring Gull Group.
In the present state of knowledge, it seems more appropriate to consider the Herring Gull Group as consisting in at least eight species which are well differentiated at the phenotypic and genetic level, and possibly up to ten species. The polytypic Herring Gull [L. argentatus] includes subspecies [argenteus]. Other polytypic species are the Lesser Black-backed [L. fuscus] (subspecies [graellsii] and [intermedius]), Yellow-legged [L. michahellis] (subspecies atlantis restricted to the Azores) and East-Siberian Gulls [L. vegae] (subspecies [mongolicus]). It is argued that the subspecic treatment adopted for the three European species, in accordance with widely recognized rules for subspecies designation, must help in distribution study. The treatment of [mongolicus] as a subspecies of [L. vegae] follows the rules for species ranking proposed by Helbig et al. (2002), pending further research (species rank has previously been proposed for [L. mongolicus]). The American [L. smithsonianus], Heuglin’s [L. heuglini], Caspian [L. cachinnans], Armenian [L. armenicus], and Steppe Gulls [L. barabensis] are monotypic (the ranking of [barabensis] however remains disputed, some authors consider it as a subspecies of [L. heuglini]). Various Asian taxa (particularly nominate [vegae] and [mongolicus]) and nominate [argentatus] exhibit strong individual variation in leg colour, from flesh to yellow, but this occurs in a way which does not allow subspecific separation : thus ‘omissus’ must be considered a synonym of nominate [argentatus], and ‘birulai’ a synonym of [vegae] (note the inverted commas indicating that these names have no taxonomic value). Regarding ‘taimyrensis’, recent study of the type series and fieldwork in its type area strongly suggest that it refers to a (past) limited hybridization zone between [L. heuglini] and [L. vegae].
Further research is however needed before we fully understand the variations within each taxa of the Herring Gull Group, and their relationships. The phenotypic variation within [michahellis] remains partly unknown. The ongoing debate on the validity of the subspecies of [argentatus] and [fuscus] calls for improved description of phenotypic variation in both species. The variation within [cachinnans] remains understudied (is ‘ponticus’ a good subspecies?) and the high variation of the bare-part coloration in some taxa (particularly in [mongolicus] and [vegae] but also in 'omissus') is unexplained. And we have to learn more about the differences between [argentatus] and [smithsonianus]. Further research are also required in order to better understand the relationships between some taxon pairs, particularly [barabensis] versus [heuglini] (phenotypically well differentiated, not intergrading, but showing genetic similarities), [heuglini] versus [vegae] (phenotypically well differentiated but possibly intergrading to some degree), [vegae] versus [mongolicus] (biologically isolated and phenotypically differentiated but sharing marked similarities). Also, it is worth studying the relationships between the latter two taxa and [smithsonianus].
The information already available is however sufficient to show that the ring-species theory of a recent common origin to all taxa of the Herring Gull Group cannot explain the phylogeny of the whole group. Probably, radiation played a larger role than ring speciation. Some of the radiations which have occurred may date back to c 170 000 to 1 M years ago. Moreover, the speciation process within the Herring Gull Group probably began before the Pleistocene usually considered under the ring species theory, as fossil material referred to this group has been found in c. 4.5 M years old layers in North Carolina, USA.
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