Thayer's Gull (thayeri)
(last update: January 22, 2013)
Thayer's Gull adult April
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
|Table 2. Iris colour variation in 283 adult Thayer's Gulls in central California (see Methods).|
|N =||2||7||19||27||21||23||5||(King and Howell 1999, N = 104)|
|N =||1||5||12||15||5||5||1||(This study, 44 presumed males)|
|N =||0||4||9||7||7||2||1||(This study, 30 presumed females)|
|N =||2||19||23||25||16||14||6||(This study, 105 sex not determined)|
|Aggreg sample %||2%||12%||23%||26%||17%||16%||5%||100 % (n = 283)|
|Totals||5||35||63||74||49||44||13||n = 283|
In central California, only about 14% of adult Thayer’s could be called truly "dark-eyed" (scores 0.0 to 0.5; Photos 3 and 6 in original article), while 20% are “pale-eyed" (scores 2.5 to 3.0; Photos 4-5 in article), although rarely the staring, clear pale lemon of a Herring Gull. Most birds (66%) are somewhere between, but their eyes do not appear obviously pale in the field. Other authors have commented on the variation in eye colour of Thayer’s Gulls, yet the dark-eyed stereotype persists. We note, though, that “dark"’ is a relative term, and that compared with most large Western Palearctic gulls, Thayer’s generally is relatively dark-eyed. Whether there is seasonal change in the eye colour remains to be documented, but pale-eyed birds are known from breeding colonies, and Sutton and Parmelee (1978) even suggested “dark-eyed birds may be absent from, or rare in, some parts of the species’ [breeding] range.” Pale-eyed birds may, therefore, reflect a geographic subset of the breeding population, perhaps the more northerly-breeding birds. Critical field observations in the Canadian Arctic could address this issue.
Thus, although popular field guides perpetuate the notion of dark eyes as a character of Thayer’s Gull, eye colour in adult Thayer’s Gulls is highly variable and, for any given individual, appears of no help in reliable separation from Kumlien’s Gull (contra Weir et al. 2000, McGowan and Kitchener 2001). It is also possible that eye colour may vary with age, for example, some four-year olds ("adults") could have darker eyes than older birds as occurs in other species of large, pa1e-eyed gulls. Good views are needed to determine eye colour, and eye scores of 2.0 and even 2.5 can appear "dark" in dull, overcast conditions or at medium to long range; conversely, such eyes appear strikingly pale (although rarely as much as a typical Herring Gull) when viewed head-on in bright sun.
In general, the separation of adult Thayer’s Gull and adult American Herring Gull L. a. smithsonianus is relatively straightforward if one has reasonable views of a bird. Nonetheless, we review here our experience with these two species in North America, which is where most observers will encounter Thayer’s. Separation of Thayer’s Gull from certain hybrid combinations is more problematic, particularly with respect to American Herring Gull x Glaucous-winged Gull hybrids, and the full range of variation in characters of this hybrid combination has yet to be established.
Thayer’s Gull is a "large white-headed gull," averaging slightly smaller overall than Herring Gull, but there is much overlap in size. In non-breeding (basic) plumage Thayer’s has variably extensive dusky clouding, mottling, and streaking on the head, neck, and, sometimes, sides of the chest (Photo). This variation may reflect, in part, the timing of a bird’s post-breeding (prebasic) moult (Howell 2001) in combination with age (heavier markings often being a sign of immaturity). The head markings range from relatively mottled to streaked and average denser and slightly more diffuse than on Herring, but probably do not differ enough to be of substantive use in field identification. When adult Thayer’s leave central California (mainly in March) they are still in non-breeding (basic) plumage (as are some smithsonianus in this region). Wing moult timing of Thayer’s is similar to smithsonianus wintering in California, with adults completing primary moult from November through January, and mostly by late December. In general then, overall size, extent and pattern of head streaking, and moult timing are not overly helpful for specific identification. More helpful are structural features which, although difficult to quantify, give Thayer’s a distinctive aspect, at least when an observer becomes familiar with the taxon. The head often appears relatively rounded, lacking the shallow, sloping forehead and flat-topped head often shown by Herring Gull, and the bill is relatively small and slender with a moderate gonydeal angle (but similar to many female Herring Gulls). Other observers have commented how these features lend Thayer’s a gentle aspect or dove-like head shape reminiscent of Common Gull L. canus, but this refers mainly to female Thayer’s. Some small Herring Gulls (presumably females) appear as small and "cute" as many Thayer’s, and some male Thayer’s appear as large-billed and flat-headed as many smithsonianus. Also, birds facing into a wind, and any swimming or flying birds, for example, while feeding, often have the forehead feathers sleeked down, which increases the apparent bill length, detracts from any small-billed appearance, and essentially negates the use of perceived structure as an identification character. The legs of Thayer’s often look shorter than on smithsonianus, which contributes to the smaller overall stature of Thayer’s.
Top: Thayer's Gulls. Bottom: Herring Gulls.
The upperparts of Thayer’s Gull average slightly darker than smithsonianus. In overcast conditions, slight differences in the grey tones of the upperparts can be noticeable (assuming the angle of light is comparable), but often, such as in direct sunlight, Thayer’s and smithsonianus do not appear appreciably different in upperpart colour. The dorsal wing-tip surfaces of resting Thayer’s typically are slaty blackish (not jet black as typical of Herring), and the ventral wing-tip surfaces are extensively silvery with reduced blackish markings. Details of Thayer’s wing-tip pattern are discussed above. Smithsonianus typically shows more extensive black on the dorsal wing-tip surfaces and the ventral wing-tip surfaces are blackish, although with some apparent geographic variation in pattern (Jonsson and Mactavish 2001). Some smithsonianus show a very thayeri-like wing-tip pattern (Plate 3, II); such birds are commonest in north-eastern North America (Macpherson 1961), rare (at best) in central California in winter (SNGH pers. obs.), and are best separated from Thayer’s by size, structure, bare-part colours, and upperpart tone.
Orbital ring in Herring Gull (3x left): note orange to yellowish tone.
Orbital ring in Thayer's Gull (3x right): note purple to pinkish tone.
The legs of Thayer’s tend to be brighter and deeper pink in winter than on smithsonianus, but this is only an average difference: some Thayer’s show paler flesh-pink legs, while a few smithsonianus have bright pink legs, similar to typical Thayer’s. The eye colour of Thayer’s is variable (see above) but is rarely as cleanly pale as Herring Gull. The orbital ring of Thayer’s ranges from purplish pink to pinkish red (Plate 1, Photo 4), rather than the chrome yellow to orangeish of smithsonianus. The bills of mid-winter adult Thayer’s typically are pale greenish or greenish yellow with a brighter yellow “saddle spot" on the culmen at the point of decurvature, that is, above the gonys spot, which is orange-red. Fewer than 10% have bills that are overall yellow to rich yellow (mainly from late January or February onwards) or show some pinkish tones basally, and only up to 10% of apparently full adults have some trace of a dark subterminal mark, or bar, anterior to the red gonys spot. By contrast the bills of mid-winter adult smithsonianus in California tend to be less greenish overall (although many are similar to Thayer’s) and up to 80% or more of adults show dark subterminal marks, or bars. Note that bill colour and pattern both vary seasonally: bills in mid winter are paler, or more greenish, and have more distinct dark marks. In late winter bills become brighter and lose dark subterminal marks.
These two taxa and their hybrids all interbreed extensively in western North America and the resultant offspring can apparently show any combination of parental characters (Bell 1996, 1997; pers. obs,). While plumage, especially wing-tip pattern (e.g., Plate 3, V), of this intergrade combination can resemble Thayer’s Gull closely, the intergrades typically differ in their larger and bulkier structure, although small females probably can overlap large male Thayer’s in overall size. In particular, the bills of Glaucous-winged x Western hybrids are larger and stouter (with a more swollen gonys) than the relatively slender bill of Thayer’s Gull, a difference that is obvious, or at least noticeable, in direct comparison (e.g., Plate 1). Other features include upperpart colour (typically darker than Thayer’s, e.g., mostly Kodak 6.0-7.0 and darker), orbital ring colour (often including some yellowish), and bill colour (relatively bright yellow on some, a Western Gull trait; or flesh-coloured basally with a dark subterminal band, a Glaucous-winged trait).
These two taxa hybridise extensively in north-western North America and, again, the resultant offspring can apparently show any combination of parental characters (Patten 1980; pers. obs.). Both plumage and structure (at least of small females) of this hybrid combination can resemble Thayer’s Gull very closely. Many hybrids, however, differ in their larger size and bigger and heavier bills, but some appear very similar to Thayer’s and on rare occasions we have been unable to distinguish between these two options when confronted with a problem bird (mainly, though, in the case of first-year birds; Howell and Corben 2000). The most thayeri-like hybrids superficially resemble Herring Gulls with relatively extensive white in the wing tip (e.g., a white mirror on P9 and white tongues on P6-P8), “dusky" eyes, and pinkish orbital rings. Possible differences from Thayer’s in wing-tip pattern include the more extensive and more diffusely dark areas on hybrids, especially on P9-P10 (Plate 3, III-IV), but more critical examination of large samples is needed and, under field conditions, some birds appear to show “perfect" thayeri wing-tip patterns. In addition to size and structure, the bills of winter adults often show more extensive dark subterminal marks than typical of Thayer’s, and the orbital ring should be checked for an admixture of yellowish pigment. Most adults of this hybrid combination appear to be identifiable in the field; given observer experience and the presence of other gulls for comparison, but some resemble Thayer’s so closely that they can be passed off as that taxon. For example, the photo of the "fourth calendar-year-type Thayer’s" on page 30 of`Garner and Mactavish (2001) looks to us more like a hybrid than any Thayer’s we have seen - but we acknowledge that photos can be misleading.
Despite a purported dramatic easterly spread of thayeri in the past 150 years (Weir et al. 2000), the only possible results evident in Western Europe have been increased numbers of kumlieni, which Weir et al. (2000) considered to be derived from interbreeding of thayeri and glaucoides. Furthermore, if thayeri and kumlieni do interbreed freely, then the occurrence of the resultant intergrades in Europe seems as likely, or perhaps more so, than of pure thayeri. The occurrence of thayeri is always a possibility to consider, however, and a bird at Killybegs, Ireland, in February-March 1998 (McGeehan and Millington 1998) constituted the first well-documented claim of an adult Thayer’s Gull in Europe. Evaluation of this record awaits resolution of the range of variation in Thayer’s Gull and Kumlien’s Gull (Anon 1999).
Given adequate views, and noting the suite of characters discussed above under separation from American Herring Gull, the separation of Thayer’s Gull from most Western Palearctic large white-headed gulls should be straightforward. Only British Herring Gull L a. argenteus, Scandinavian Herring Gull L. a. argentatus and, of course, Kumlien’s Gull, need serious consideration.
Top: Herring Gull argenteus breeding in the Netherlands showing thayeri pattern on P10 and P9.
Bottom: unringed Herring Gulls in winter in Europe showing thayeri patterns.
Because we have not observed Thayer’s Gull alongside Western Palearctic Herring Gulls, some of the comparisons we make are subjective. As with American Herring Gulls (discussed above), problems are most likely between large male Thayer’s and small female Herring, which can appear very similar in overall structure, including bill size and shape. Structural differences of female Herring Gulls that may be apparent relative to Thayer’s include the longer body, neck, and tarsus of argentatus (only extremely small individuals of which might recall Thayer’s), and the longer curve from culmen to bill tip and, in flight, the slightly blunter wing-tip of argenteus. Unlike American Herring Gull, adult argenteus typically completes primary moult earlier than Thayer’s, that is, by late October or November, whereas nominate argentatus averages later, some completing in December (MTE, pers. obs.). The pre-breeding (prealternate) moults of argenteus and argentatus also average earlier than Thayer’s Gull; such that adult Thayer’s often retain dusky head and neck markings later into the winter. However, vagrant gulls can exhibit atypical moult schedules, so moult should be used with caution as an identification feature (Howell 2001).
Plumage differences should rule out most argenteus (this taxon only exceptionally - 1% of breeding birds in the Netherlands- shows a thayeri-like pattern on P9-P10), which usually has extensive black (Kodak 18-19) in the wing-tips on both inner and outer webs, and has paler (more silvery and less bluish) grey upperparts. In many aspects of primary pattern, argentatus is closer to thayeri and, despite averaging darker above, it overlaps with thayeri in the grey tone of its upperparts. A thayeri-like primary pattern is often shown on P9 and P10 of argentatus (e.g., Plate 4, d-g). Possible, albeit subtle, differences from Thayer’s Gull in wing-tip pattern may include: more extensive and more diffuse dark pigmentation on the inner webs, coupled with relatively limited black on the outer webs, of P6/P7-P8; and perhaps narrower white tongues on P5-P8.
Bare parts overlap in colouration between Thayer’s Gull and Western Palearctic Herring Gulls except, apparently, for the orbital ring. Herring Gulls can show deep pink legs and a greenish bill base in winter (both commoner in argentatus), and a dark-flecked iris (particularly in younger adults, but also rarely in individuals age 12 or older; MTE, pers. obs.). The orbital rings of British and Scandinavian Herring Gulls are mostly chrome yellow to orange, rarely scarlet on argenteus and red on argentatus (MTE pers. obs). Although Elliott (1981) and Grant (1986) reported, respectively, "purple" and "pink" orbital rings for European Herring Gulls, we are unaware of any evidence that either argenteus or argentatus shows a purplish-pink orbital ring typical of Thayer’s Gull. Herring Gull orbital rings can, however, fade briefly to flesh-pink during autumn primary moult, when the bill base also often becomes pinkish (MTE pers. obs,). Non-breeding adult Herrings can also appear to show a narrow dark orbital ring that might suggest Thayer’s, but close-range views reveal this is, in fact, the dark outer rim of the iris, exposed by shrunken orbital skin (M. Ahmad and MTE, pers. obs.).
The separation of Thayer’s and Kumlien’s gulls remains problematic, with its crux summarised by the question: "At what point do Thayer’s become reliably separable from Kumlien’s?" (and see under Taxonomy? below). It is widely accepted that Thayer’s averages larger and longer-billed than Kumlien’s, showing on average more extensive and darker markings in the wing-tip, and, perhaps, averages darker-eyed. But are there any absolutes or simply overlapping averages? Are we trying to find characters that separate a parent from its hybrid offspring? With so many questions unanswered what follows does not, and cannot, claim to be the last word on this tangled subject. In particular, our comparisons of wing-tip pattern should be viewed as tentative and are presented here to stimulate fieldwork that may refine identification criteria. It must also be acknowledged that Thayer’s and Kumlien’s reportedly interbreed (cf. under Taxonomy? below) and consequently some birds should be intermediate in terms of structure, plumage colour, and wing-tip pattern. Such birds cannot be assigned satisfactorily to Thayer’s or Kumlien’s but, unfortunately, we still have little idea how common intergrades really are.
Below: Kumlien's Gulls in winter in Newfoundland.
In general, Kumlien’s Gull is smaller and smaller-billed than Thayer’s, and female Kumlien’s is much like Iceland Gull in these respects, although male Kumlien’s may be more like Thayer’s than Iceland (B. Mactavish, pers. comm.). It may be worth reiterating MTE’s impression on seeing Thayer’s for the first time: in most respects Thayer’s looks as much or more like Herring Gull than it does Iceland and, contra most authors, the gonydeal angle of Thayer’s can be quite pronounced. One of the main (in-hand) differences between Thayer’s and Kumlien’s of known sex may be bill size (but male Kumlien’s overlaps extensively with female Thayer’s). However, few mensural data appear to be published, particularly for Kumlien’s Gull. From Brewster (1883; n = 3 kumlieni, including type specimen), Brooks (1915; n = 1, type specimen of thayeri), Brooks (1937; n = 4 male, 4 female thayeri), Dwight (1925; n = 12 male, 12 female thayeri), and Godfrey (1986; thayeri and kumlieni, no sample
sizes given), it appears that Thayer’s and Kumlien’s overlap substantially, sex for sex, in almost all measurements (with thayeri averaging larger), except perhaps for culmen length: male Thayer’s (n = 23+) = 47-57 mm, male Kumlien’s (n = 5+) = 43-48 mm. However, how specimens such as those measured by Godfrey (1986) were identified as Thayer’s Gulls is unclear (see below, under Taxonomy?), and by other definitions they might include some Kumlien’s Gulls.
Other potentially useful features that have been mentioned to distinguish Thayer’s and Kumlien’s (e.g., Godfrey 1986, Zimmer 1991, Garner and Mactavish 2001) are moult timing, upperpart tone, and wing-tip pattern. Bare-part colours overlap completely (pers. obs., B. Mactavish pers. comm.) and appear to be of no use in identification.
Garner and Mactavish (2001) noted that adult Kumlien’s complete their moult earlier on average than do adult Thayer’s, reflecting the longer-distance migration of the latter. This sounds quite reasonable, but moult timing in gulls (and most species) is flexible, not fixed: thus, a Thayer’s Gull wintering in Newfoundland could moult on the same schedule as the Kumlien’s Gulls there, while a Kumlien’s that crossed the Atlantic to Europe could moult later (see Howell 2001).
The upperparts of Thayer’s (Kodak 4.5-5.5) should be slightly darker than most Kumlien’s (Kodak 3.0-4.0), but some presumed Kumlien’s (or intergrades with Thayer’s?) could appear as dark as Kodak 5.0 (eg., Plate 4, II). Thus, the upperparts of most Kumlien’s appear slightly paler than or similar to a British Herring Gull, while Thayer’s should appear slightly darker. Intergrades presumably could also be any intermediate shade.
In terms of wing-tip pattern, it may be helpful to compare our results for presumed Thayer’s Gulls with a sample of 27 dark-winged adult presumed Kumlien’s Gulls. These dark birds were evaluated by MTE from video footage contributed by Paul Doherty and photos taken by Peter de Knijff, all from Newfoundland, in eastern Canada, in February and late January, respectively. All birds were judged to be at least in their 4th winter (i.e., 4th basic plumage) and, to qualify as "dark," individuals had to show melanism on at least 5 primaries, dark enough (estimated Kodak l2 or higher) and extensive enough (extending medially on to the inner web of P10 and sub-terminally on to the inner webs of P6-P9) to be comparable to (and potentially confusable with) the least-marked individuals in our sample of thayeri from California. Our sample of dark Kumlien’s was limited, but we suspect that no more than 5-10% of "adults" have dark marks on P5, and, as Zimmer (1991) noted, they may never show a complete dark sub-terminal band on P5. B. Mactavish (pers. comm.) notes that the ratio of adult Kumlien’s with dark on P5 may be closer to 5% and that birds do not need to have blackish wing tips to show a mark on P5. It seems reasonable to conclude that most Kumlien’s-like birds with distinct dark marks on P5 probably are sub-adults and/or intergrades with Thayer’s (e.g., Plate 4, I; Photo 7 in original article).
As can be seen from Plate 3, there is considerable variation in the wing-tip pattern of Thayer’s, the only constants being the presence of melanism on at least 5 primaries (contra Weir et al. 2000, McGowan and Kitchener 2001), and the consistently dark markings (estimated Kodak 14-17) for P6-P10 (with the exception of Plate 3, m-o, which may represent intergrades). Neither of these constants seems to completely eliminate the possibility of Kumlien’s, a few individuals of which may exhibit both criteria (although whether such birds represent intergrades is unknown). The problem, then, is how to separate this minority from presumed pure thayeri.
Characters that we suggest indicate a pure Thayer’s Gull (as opposed to Kumlien’s) are a blackish sub-terminal band on P10 and a complete dark sub-terminal band on P5. However, lack of these characters does not necessarily mean the bird is a Kumlien’s Gull: most presumed Thayer’s we saw lacked a dark subterminal band on P10 (which may be more typical of sub-adults than full adults), and 25% of adults had P5 unmarked (Photos 2 and 7). The presence of dark medial bands on P9-P10 is much commoner on thayeri than kumlieni, with markings on both webs similarly dark on thayeri (or with the inner web only slightly paler), but with inner web markings often distinctly paler on kumlieni (e.g., Photo 9). On adult kumlieni, the dark basal to the white mirror on P10 typically does not extend on to the inner web (Brewster 1883; and see figure 20 of Zimmer 1991). Those with dark medial bands may be younger adults (4-5 years old), as the only examples seen have these markings diffusely defined and relatively extensive towards the feather base (e.g., Plate 4, I, III); others presumably could be intergrades with thayeri (e.g., Photo 8). Also, while a dark medial band on P9 appears very rare on presumed kumlieni (again, perhaps mainly on younger adults and intergrades), on most thayeri (e.g., 96% of n = 48, the 2 exceptions perhaps being intergrades with kumlieni/glaucoides), dark extends on to the inner web of P9 as a partial to complete medial band (e.g., photo 7). Unfortunately, this feature can be virtually impossible to see under field conditions (and is difficult to record even using freeze-frame video). B. Mactavish (pers. comm.) notes that a complete medial band on P9 would be "probably next to non-existent” on Kumlien’s.
Below left 3x: Shape of P9 mirror in Kumlien's Gulls in winter in Newfoundland.
Below right 3x: Shape of P9 mirror in Thayer's Gulls in winter on west coast.
Another feature worthy of critical study is the pattern on the outer web of P9: for 90 thayeri we observed, black on the outer web of P9 was not interrupted by the white mirror on at least 82% (Plate 3, a-g, I). Of 23 dark-winged kumlieni where P9 could be seen clearly, 4% (only l bird, and showing signs of immaturity - a dark bill band and extensively dark outer webs to P7-P8), had an unbroken dark outer web to P9, whereas 96 % (22 birds) had the dark outer web interrupted by the white mirror (e.g., Photo 9). B. Mactavish (pers. comm.) notes, however, that an "unbroken dark leading web to P9 that breaks the mirror is uncommon but far from rare" on Kumlien’s so more quantification of this feature is needed.
Additional characters to note are: an isolated mirror on P9 (Plate 3, b, g) is possibly never shown by kumlieni (but is also rare in thayeri), and kumlieni typically has narrower dark marks on the outer webs of the outer primaries, often not extending to the feather shaft as is typical of thayeri.
END OF PART II