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’.
Birds resembling mandtii have been reported previously from temperate regions of the eastern North Atlantic, though none of these records have been formally assessed and accepted by national records committees. They include an extraordinary inland record of a bird at the freshwater Talkin Tarn, Cumbria, on 20th–22nd December 2013 (Birding World 26: 493). Others include birds in Iceland during October 1961 (Petersen 1977); the Netherlands in February–March 2015 and June 2015 (Slaterus et al. 2015); and the Faroe Islands in March 2018 (Olofson 2018). In addition, there are a number of reports of this subspecies from the northeastern states of the USA (for examples see Macaulay Library 2020)
What defines ‘Mandt’s Black Guillemot’?
The Lincolnshire bird was in non-breeding plumage, appearing clean white on the head and body, including the scapulars and rump (Roberts 2021). It was an adult (second-calendar-year or older), with unmarked white upperwing-coverts and rump, lacking the dark spots/bars of juvenile plumage, which are retained until the summer of the second calendar year. This largely white appearance in non-breeding plumage is apparently shown only by mandtii and not seen in the other four, more southerly subspecies of Black Guillemot (Cramp 1985; Garner 2015), which are considerably darker in non-breeding plumage and, as a group, differ markedly from mandtii.
Martin Garner recently drew attention to the distinctive appearance of mandtii (Garner 2015) and that played a key part in raising awareness of the potential significance of the Lincolnshire bird. However, when the British Ornithologists’ Union Records Committee (BOURC) came to consider the Lincolnshire record, it became apparent that the precise characteristics by which mandtii is defined and can be reliably separated from the other four subspecies of C. grylle were not available. The original description by Lichtenstein (Mandt 1822) of a bird from Svalbard was unhelpful, since it offered no clear criteria by which mandtii was defined. More recent published material is somewhat vague, including Dement’ev et al. (1968), Cramp (1985), Godfrey (1986), Gaston & Jones (1998) and Sibley (2000). For example, Cramp (1985) stated only that ‘birds from high Arctic in both plumages [are] whiter on upperparts than birds from south’.
However, further research revealed a publication by Danish ornithologist Finn Salomonsen, which focused on the taxonomy of the Atlantic auks (Salomonsen 1944). This included an analysis of the characters of the five subspecies of Black Guillemot and set guidelines by which mandtii could be diagnosed. This comprehensive review of seasonal and geographical variation utilised a series of specimens from various Scandinavian collections. With regard to mandtii, Salomonsen wrote: ‘In winter-plumage C. g. mandtii differs very much from C. g. grylle and allies, being much whiter on upper-parts… The white feather-edges on the entire upper-parts are much broader, the head is almost white, the black spot on lore and ear-coverts entirely lacking; the rump is pure white, the feathers being white to base… the scapulars are either entirely white with or without dark shafts, or possess more or less dark colour along shaft or at base...’. Salomonsen quantified certain plumage features including the amount of white on the distal part of the greater upperwing-coverts, with mandtii showing 2–44 mm, and arcticus (atlantis) 18–36 mm (fig. 1). Salomonsen also recognised the extent of white on the inner web on the underside of the outermost primary as a useful feature to separate mandtii from other subspecies; in mandtii this appears as a large white patch that extends 18–29 mm beyond the end of the longest underwing-covert. Other subspecies show a smaller white patch of up to 17 mm (although see below).
An identification feature noted by Salomonsen for first-winter birds is the presence of ‘white tips to the secondaries, as this feature is never found in the adult birds, nor is it even approached by any of the subspecies of the gryllegroup.’ This diagnostic feature could therefore be extremely useful when identifying young birds.
The taxonomic arrangement Salomonsen used for the Black Guillemot was summarised and updated in Nettleship & Birkhead (1985). They referred to the ‘very distinctive mandtii’, stating that the ‘mandtii subspecies has much more white in the wing than any form… In mandtii, the greater wing coverts are constantly white almost or right to the base… The overall winter dress of mandtii is also much whiter than any other form.’
Analysis of museum specimens
To evaluate the features described by Salomonsen (1944), 55 specimens of mandtii in breeding and non-breeding plumage held at National Museums Scotland, Edinburgh, and the Natural History Museum, Tring, were examined by RYM. A degree of variability was shown in some features, for example the extent of white in the wing-patch and the broader white tips to black-based dorsal feathers. Greatest consistency was shown by two of Salomonsen’s quantified characters: the extent of white on the greater upperwing-coverts, and the extent of white in the remiges (see ‘A’ and ‘C’ in fig. 1).
One related issue was the subspecific taxa recognised and examined by Salomonsen (1944); he described a new subspecies, C. g. atlantis, for populations from North Atlantic breeding localities, but atlantis is synonymised with arcticus by IOC. Nevertheless, despite this slight complication, the study demonstrated a few clear qualitative and quantitative differences between mandtii and the other four subspecies of Black Guillemot. Central to Salomonsen’s review was his quantitative assessment of the extent of white in the wing, which he established by measuring four wing-feather parameters on a series of specimens of all subspecies (see fig. 1).
The features ‘A’ and ‘D’ can be measured (or estimated) only from specimens or live birds in the hand, with ‘A’ being the amount of white in the distal part of the greater upperwing-coverts, and ‘D’ the distance between the feather shaft and the white patch on the outermost primary (fig. 1). Another feature, ‘B’, the distance between the tip of the primary and the white patch, shows a degree of variation across the subspecies, and so is less useful.
The fourth feature, ‘C’, is the measurement on the underwing from the tip of the corresponding primary underwing-covert to the distal limit of the white patch on the outermost primary (fig. 1). While this feature could potentially be determined on good-quality images of the underwing, an estimated measurement would depend on the image also showing a feature whose (known) measurement can be used for scaling. Fortunately, one image (plate 1d) shows the left underwing of the Lincolnshire bird fully extended.
Another plumage feature involves white markings on the upperwing primary coverts which project as a ‘spur’ or a ‘bud’ from the large white patch; this was described by Garner (2015) as present in mandtii and not in other subspecies of Black Guillemot. Examination of museum specimens revealed that some mandtii (2 of 15) do not have those white markings while some specimens of other subspecies (3 out 151) show a similar pattern. Hence, it appears that although this feature may be suggestive of mandtii, it is not diagnostic. It is also relevant that Salomonsen, who made a very thorough examination of many Black Guillemot specimens of various subspecies and noted other features useful for identifying mandtii, did not discuss this feature.
Assessment of the Lincolnshire bird
Photographs of the Lincolnshire bird (plate 1) revealed a number of characteristics described by Salomonsen which support the identification as mandtii. For birds in non-breeding plumage these are:
- Its overall appearance is very pale.
- The white head lacks a black loral spot and black ear-coverts.
- Extensive white on the rump and scapulars.
- The white upperwing-patch appears uniformly white, with limited or no dark speckling, suggesting that the greater upperwing-coverts are extensively white-tipped, with little black at the base (see fig. 1).
As described above, we also investigated Salomonsen’s measurement of white on the underside of the primaries. Images of the bird were sharpened (using Paint Shop Pro v19) to delineate the tips of primary underwing-coverts more clearly against the white on the underside of the primaries. An ‘image wing length’ (from the carpal joint to the wing-tip) and an ‘image white-on-primary’ (Salomonsen’s C-value) were measured directly from an enlarged image of plate 1d, displayed on a computer monitor and measured using Vernier calipers. Both authors undertook these measurements independently. The ratio of these measurements (we determined that C was 16.0% (CJM) or 16.1% (RYM) of the wing length, with the latter figure taken) was then used in conjunction with published wing lengths from Salomonsen (1944) and Butler et al. (2020) to calculate C for the Lincolnshire bird. The wing-length range for mandtiiis 159–175 mm (hence C for the Lincolnshire bird is 25.6–28.2 mm); for all subspecies the wing-length range is 151–182 mm (hence C for the Lincolnshire bird is 24.3–29.3 mm). Salomonsen’s C-values for subspecies other than mandtiilie in the range 0–17 mm, apart from one arcticus that he listed as 22 mm. His C-values for mandtii are in the range 18–29 mm. Hence, the Lincolnshire bird falls well within Salomonsen’s range for mandtii whatever its actual wing length might be.
Although this approach relies on extrapolation from a single photograph, we have confidence that our measurements of the length of white on the outermost primary were sufficiently accurate to allow reliable identification. Repeating the analysis with measurements from the original (unmodified) image gave a calculated range of the C-value of 26.5–29.1 mm based on the min/max wing length for mandtii (above). Furthermore, incorporating a small degree of shrinkage in wing length (1.6%, from Ewins 1985) for comparison with measurements from museum specimens would, in any case, slightly increase the C-values. The result is a critical, quantified feature in support of the bird’s identification as mandtii.
There are no provenance issues with the Lincolnshire bird, which is highly likely to have a wild origin. The species is not currently kept in captivity in Europe, the ‘Zootierliste’ inventory listing no ‘current holdings’ of Black Guillemot and just six ‘former holdings’, the most recent from 2015 (www.zootierliste.de). With the identification confirmed, the Cut End, River Witham, Lincolnshire record was accepted by BOURC, and the subspecies C. g. mandtii added to Category A of the British List (BOU 2020).
In the light of the analysis described above, it may be useful to examine Black Guillemot specimens in European museum collections; this might uncover more extralimital records of this high-Arctic subspecies. Indeed, it may be possible to identify other European records of live birds if sufficiently high-quality photographs of the relevant underwing features exist.
We thank Steve Gantlett for allowing us to use his images of the Lincolnshire bird for this analysis, and George Holliday for sharpening the images. Mark Adams and Hein van Grouw at NHMUK, Tring, kindly facilitated collection access. In addition, the following are thanked for help in tracing the Salomonsen reference: Alison Harding (NHMUK, Tring), Jan Bolding Kristensen (Natural History Museum of Denmark) and staff at the Scottish Ornithologists’ Club (SOC) Library, Aberlady. We acknowledge the reviewers for helpful comments that improved the text.
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Christopher J. McInerny, BOURC Secretary, School of Life Sciences, University of Glasgow, Glasgow G12 8QQ; e-mail [email protected]
Robert Y. McGowan, Department of Natural Sciences, National Museums Scotland, Chambers Street, Edinburgh EH1 1JF; e-mail [email protected]