Herring Gull (Larus argentatus) - HBW 3, p. 609

Taxonomy: Larus Argentatus Pontoppidan, 1763, Denmark.
Systematics of present species and its close relatives represent one of the most complex challenges in ornithology, and typify the discord between evolution, biogeography, reproductive isolation, and taxonomy. The closely related taxa were often treated as a ring of subtle races, until the two extremes (L. argentatus, L. fuscus) met in W Europe and did not freely interbreed. Many forms are now treated as separate species, since it is uncertain how to apply the biological species concept to the complex pattern of variation and interbreeding. Present species is reported to hybridize extensively with L. hyperboreus in Iceland (but this disputed), with L. glaucescens in Alaska, but rarely with L. fuscus in Europe. Confusion prevails over the species boundaries of L. argentatus, L. cachinnans and L. fuscus, with many of the identifiable races assigned at one time or another to 2 or even 3 of these species. The most extreme lumping has included L. glaucoides, L. fuscus and even L. californicus within L. argentatus; a more traditional treatment includes L. cachinnans and L. armenicus. Owing to sympatry without hybridization in W France, L. cachinnans is now seen as a distinct species, further supported by molecular evidence. Much controversy surrounds the form “omissus” of the N Baltic region, with pink-legged individuals in a (formerly) predominantly yellow-legged population: it has been treated as a race either of L. argentatus or of L. cachinnans, but it is probably simply a morph of L. argentatus (based mainly on voice and primary pattern). Bluish-legged birds of Russian Arctic islands included in vegae, but sometimes separated as birulae. See also L. cachinnans, L. armenicus, L. fuscus. Four subspecies recognized.