Thayer's Gull adult May
In 2001 and 2003, two extensive papers appeared in the Finnish ornithological magazine "Alula", dealing with adult Thayer's Gull and adult Kumlien's Gull. Up to now, this is by far the most extensive research on these species, and anyone interested in this topic is encouraged to get a copy of th issues. Below, you will find the content of this paper, illustrated with many digital SLR-images from various photographers and different locations.
The full title reads: Identification and Variation of Winter Adult Thayer’s Gulls with Comments on Taxonomy, by Steve N.G. Howell & Martin T. Elliott, IN: Alula 4/2001.
"we" in the text below refers to the original authors. If any errors occur in this text, please let me know and mail to marsmuusseatgmaildotcom.
Identification and Variation of Winter Adult Thayer’s Gulls
with Comments on Taxonomy
PART I: INTRO & METHODS
PART II: EYE COLOUR & SEPARATION FROM OTHER TAXA
BELOW IS PART III: TAXONOMY & SUMMARY
Much has been written on this subject, but we would like to highlight an apparent ambiguity and inconsistency in the definition of the characters (and thus taxonomy) of Thayer’s Gull. There appear to be two different views operating. On the one hand, Godfrey (1986) noted “there is so much individual variation, even complete overlapping of characters [between thayeri and kumlieni], that some individuals of one race are almost certain to be mistaken for others." Godfrey (1986) also noted that some adult so-called Thayer’s Gulls “have wing tips fully as pale as Glaucous-winged, or even paler," and portrayed a virtually "white-winged” so-called Thayer’s on p. 264 (although no indication is given of how it was identified as Thayer’s). Perhaps these birds were identified as Thayer’s because they occurred in its presumed geographic range? On the other hand, if Godfrey’s assertions are correct, then many reports of interbreeding between Thayer’s and Kumlien’s would be spurious since the reported “Kumlien’s” could not be distinguished in the field from pale-winged "Thayer’s."
Measurements may have been employed to separate specimens of these taxa. Unfortunately, plumage types and measurements may not necessarily be linked, that is, a hybrid index based simply upon wing-tip pattern and melanism may be "a poor indicator of the hybridness of individual gulls” (lngolfsson 1987). That is, hybrids might show "pure" parental plumage but differ from parents in hybrid-related mensural characters that might not be evident in the field; conversely, an analysis of specimens (even assuming they are sexed correctly), that relied simply upon measurements to sort taxa might be similarly compromised. Wing-tip patterns correlated to measurements of correctly sexed specimens would be of interest, but Ingolfsson’s (1987) caveat should be borne in mind when interpreting mensural data.
Western Gulls and Glaucous-winged Gulls are considered separate species despite extensive and widespread inter-breeding over a large area from northern Washington to central Oregon (Hoffman et al. 1978, Bell 1996, 1997). Occasional individuals of apparently pure Glaucous-winged Gulls, as well as presumed hybrids, pair and nest with Western Gulls in central California, some 800 km farther south (Bell 1997; C. Corben, pers. comm.; SNGH, pers. obs.), If this situation were treated as analogous with one apparent interpretation of the Thayer’s Gull problem, then these extralimital breeding southern Glaucous-winged Gulls and hybrids might be considered "pure" Western Gulls, simply because they occur within the breeding range of Western. Large gull taxa are characteristically distinguished by patterns of wing-tip melanism, in combination with overall size and structure, and we believe this is the best approach to defining Thayer’s Gull (as discussed above).
Pittaway (1999) provided a useful summary of written material concerning Thayer’s Gull, including some Canadian literature that is not widely accessible. From his digest it appears that the northernmost populations of so-called Thayer’s Gull, on Ellesmere Island, include pale-winged birds ("impossible to tell from Kumlien’s") that are some 1400 km north of the known breeding range of Kumlien’s and nearer the range of Iceland. In addition, typical dark-winged Thayer’s reportedly breed locally in north-western Greenland (Salomonsen 1951). Reported interbreeding of Thayer’s and Kumlien’s comes from the southern edge of the range of Thayer’s, on Southampton Island and islands in northern Hudson Bay (Gaston and Decker 1985), and on southern Baffin Island (Snell 1989). Weir et al. (2000) mapped apparent distribution changes among thayeri, kumlieni and glaucoides over the past 200 years, but data was sparse for huge areas and long time periods, and we question their criteria of wing-tip melanism for defining thayeri (see above). Most importantly, the above authors who reported interbreeding of thayeri and kumlieni failed to define these taxa critically from the outset, so their results are difficult to evaluate (as also noted by Garner and Mactavish 2001).
In conclusion, Pittaway (1999) advocated lumping Thayer’s with Iceland, although in some ways the situation is not dissimilar to that of the better-studied Western and Glaucous-winged gulls, which are considered separate species. That is, adult Kumlien’s show a variable admixture of Thayer’s and Iceland characters (e.g., Zimmer 1991), in the same way that Western x Glaucous-winged intergrades span the complete range of combinations of parental characters. However, if Thayer’s and Kumlien’s do interbreed extensively and are often indistinguishable, then why are Thayer’s Gulls so rare in the Atlantic and Kumlien’s so rare in the Pacific? It is also noteworthy that dark-winged presumed Kumlien’s generally appear to be separable from presumed Thayer’s, at least based upon wintering birds in Newfoundland and California, with relatively few potential intergrades (eg., only 1.7% of our California sample). This suggests that, unlike Western and Glaucous-winged gulls, the present-day contact zone and degree of interbreeding between thayeri and kumlieni may be quite limited in terms of their overall population sizes. Still, in evolutionary terms, Kumlien’s Gull may have been derived from interbreeding between Thayer’s and Iceland gulls, as suggested by several authors (e.g., Dwight 1925, Salomonsen 1951, Weir et al. 2000, McGowan and Kitchener 2001). Unlike the last authors, however, we believe that Thayer’s Gull and Iceland Gull are best treated as separate species given the information available, but what of kumlieni?
The variation exhibited by adult “kumlieni” (e.g., Zimmer 1991) appears unprecedented in any other single taxon of large white-headed gull. Indeed, whether this variable population even deserves a trinomial is a taxonomic problem, from which follows the question: of what would Kumlien’s be a subspecies - Thayer’s or Iceland? There also may be different types of "Kumlien’s Gulls." For example, is there
1) a fairly consistent hybrid swarm, of older origin, that interbreeds with Thayer’s at one edge of its range and with Iceland at the other (and which might even be an incipient species, as suggested by Dwight [1925:197] and AOU );
and 2) a contemporary form of "Kumlien’s Gull" produced from direct interbreeding of pure Iceland and Thayer’s?
As everyone interested in the Thayer’s problem has commented, more data is needed. Logistics in the Canadian Arctic are not trivial, however, and answers to some questions may be a long way off. In the meantime, quantification of the variation exhibited by adult presumed Kumlien’s Gulls (eg., wintering birds in Newfoundland) would be of interest, as would similar studies of Thayer’s Gulls from elsewhere in their winter range.
We quantify and summarise variation in wing-tip pattern and eye-colour of presumed adult Thayer’s Gulls wintering in central California. Almost all birds (98.3%) fitted into the concept of what adult Thayer’s Gull should look like, but wing-tip pattern was somewhat variable, and eye colour was highly variable. Only about 14% of adult Thayer’s Gulls we saw were "dark-eyed” in winter, and up to 5% as pale-eyed as typical Herring Gulls. Wing-tip pattern features that we suggest indicate a pure Thayer’s Gull (as opposed to Kumlien’s) are a black subterminal band on P10 and a complete subterminal black (or dark) band on P5, but lack of these characters does not automatically point to Kumlien’s Gull. In fact, most presumed Thayer’s we saw lacked a dark subterminal band on P10 (this character may be more typical of sub-adults than full adults) and 25% of adult Thayer’s lacked any dark on P5. Perhaps more useful is the pattern on P9 that is, whether or not there is a blackish medial band and whether or not the white mirror bisects the dark along the outer web, but this is much harder to see clearly in the field. From the point of view of an observer trying to establish the identify of an adult gull suspected to be a vagrant Thayer’s Gull, we suggest birds should not be claimed as that species if:
a) Their upperparts are not slightly darker grey than an American smithsonianus or British argenteus Herring Gull or noticeably darker than a Ring-billed Gull.
b) They show any signs of immaturity (complete dark bill band, dark marks in tail, primary coverts, alula, tertials, etc.) in combination with relatively reduced (for a Thayer’s) dark in the wing-tip; this could indicate a sub-adult Kumlien’s which might show more extensive dark primary markings than a full adult.
c) They have only a narrow (complete or incomplete) or no subterminal dark band on P6.
d) They have outer primary markings that do not appear slaty black (Kodak 14 or higher) on perched birds or that show more than one dark tone (e.g., an admixture of paler grey and blackish) from P6 outwards.
In this regard, beware of backlighting on birds in flight and note that underwing markings look paler than those on the upperwing; also beware of the potential problems with photos concerning exposure, development, and different films.
e) They show no indication of at least a partial blackish medial band on P9, that is, dark extending on to the inner web.
f) They have only narrow subterminal blackish bands on the inner webs of P6-P8. This can produce a banded or "saw-tooth" appearance to the closed primaries. Thayer’s tends to show a solidly dark upper edge to the closed primaries from P7 outwards; beware how partially open wings could affect perception of this feature.
If a problematic bird satisfies all of these criteria, its separation from Kumlien’s (or an intergrade), may rest largely upon perceptions of structure (especially bill size, but sexual variation confounds the in-field usefulness of this feature), wing-tip melanism, and, at some level, the philosophy of an individual observer or of a records committee. One must also accept that names cannot be applied confidently to every bird seen (eg., Plate 3, m-o; Plate 4, I; Photo 7). We also discuss identification criteria for Thayer’s Gull versus potentially similar North American and Western Palearctic taxa, and comment on the taxonomic status of Thayer’s Gull. We note that different criteria may be being applied to the same data: some authors defining Thayer’s Gulls on plumage characters (primarily wing-tip melanism and pattern), others identifying Thayer’s Gulls based upon presumed breeding ranges, and perhaps others employing mensural characters. We believe, based on the admittedly limited evidence available, that Thayer’s and Iceland gulls are best considered separate species and that “Kumlien’s Gull” is of uncertain taxonomic status and, at some point in time, may have been derived from interbreeding of Thayer’s and Iceland. In conclusion, we recognise that this paper is simply a small piece added to a very large and incomplete puzzle. We hope that our preliminary analysis stimulates comparable studies in other areas, so that more can be learned of this complex problem.
We thank Chris Corben for countless hours shared observing Thayer’s Gulls, and for many helpful comments on this topic. Matt Heindel contributed motivation to attempt some quantification of characters that might help define and distinguish Thayer’s and Kumlien’s gulls; Peter Rock enabled MTE to study a ringed population of British Herring and Lesser Black-backed gulls; staff at the British Museum and California Academy of Sciences kindly allowed MTE and SNGH, respectively, access to specimens in their care; Jeremiah Trimble at the Museum of Comparative Zoology (MCZ), Harvard University, Cambridge, Massachussetts, provided data and photos of the type specimen of Thayer’s Gull; Peter de Knijff, Mashuq Ahmad, James Lomax, and Bruce Mactavish contributed photos for study; Paul Doherty (Bird Images) provided invaluable additional video material; further logistical help came from Simon Perkins and Diana Stralberg. Versions of the manuscript benefited from comments by Mashuq Ahmad, Jon L. Dunn, Matt Heindel, Ted Hoogendoorn, Paul E. Lehman, Bruce Mactavish, Killian Mullarney, David A. Sibley, and Will Russell.
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Steve N. G. Howell, Point Reyes Bird Observatory, 4990 Shoreline Highway, Stinson Beach, CA 94970, USA.
Martin T. Elliott, 36 Queen Street, St. Just, Penzance, Cornwall TRI9 7JW, United Kingdom.
|Thayer's adult, May 31 2009, Churchill, Manitoba, Canada. Picture: Tom Johnson.