Abstract
A Black Guillemot Cepphus grylle at Cut End on the River Witham in Lincolnshire on 7th–10th December 2017 attracted considerable local attention. The bird’s largely white appearance resembled that of the distinctive, high-Arctic ‘Mandt’s Black Guillemot’ C. g. mandtii, and it attracted wider interest as potentially the first British record of this subspecies. During the BOURC review, it was found that the characters that define mandtii and exclude other subspecies were not widely known. This short paper describes the review process, and in particular a diagnostic measurement of the extent of white in the underside of the primaries.Five subspecies of the Black Guillemot Cepphus grylle are recognised in IOC taxonomy (Gill & Donsker 2020). ‘Mandt’s Black Guillemot’ C. g. mandtii has a circumpolar, high-Arctic breeding distribution, which includes northern Alaska, northeast Canada, west and east Greenland, Svalbard and northern Siberia. During the winter months when the pack ice forms, some mandtii move south to high latitudes in the North Atlantic (Cramp 1985). This distinctive taxon, which is extensively white in non-breeding plumage, is a plausible vagrant to British waters. The remaining subspecies are largely sedentary in the cooler temperate North Atlantic and Baltic Sea. Nominate C. g. grylle is resident in the Baltic; C. g. arcticus occurs in the North Atlantic south of mandtii, including coastal eastern North America, Greenland, Britain & Ireland, Norway, southwest Sweden, Denmark and European Russia from Murmansk to the White Sea; C. g. islandicus is restricted to Iceland and C. g. faeroeensis to the Faroe Islands. Two previously described subspecies, C. g. atlantis and C. g. ultimus, are not recognised by IOC, being synonymised with arcticus and mandtii, respectively. In eastern North America and Greenland, birds resembling mandtii apparently intergrade with arcticus; Butler et al. (2020) suggested that further investigation is required to resolve the distinctions and relationships between the subspecies, owing to ‘complex variation and clinal trends’.