Thayer's Gull (thayeri)
(last update: January 22, 2013)
Thayer's Gull adult September
Our findings. summarized in Table 1, suggest that adult Thayer's Gulls more frequently show pale irides than appears to have been described previously. Most of the adult Thayer's Gulls we observed (87%) showed either pale brown irides or yellow or green irides with varying degrees of brownish mottling (scores 1.0-2.5). A significant proportion (27%) had largely pale irides (scores 2.5 or 3.0) that superficially resembled those of an adult Herring Gull even at distances greater than 20 m. Five percent showed irides that could not be separated by color or pattern from those of a typical Herring Gull (score 3.0). Only 9% of our sample showed irides that were wholly dark or medium brown (scores 0 or 0.5). Our data indicate that a yellow-green base color of the iris. variably mottled or speckled with brownish, is actually fairly common in adult Thayer's Gulls: thus, the literature appears to overstate the reliability of iris color as a character distinguishing adult Thayer's and Herring gulls.
Although the data are rather limited, we found no clear evidence of a seasonal change in iris color. We hope that further study will reveal to what extent, if any, adults' iris color changes with reproductive condition and/or season, and the age at which Thayer's Gulls can start to show pale irides (rarely, a score of 2.5 can be reached by Thayer's Gulls in their second winter; King, pers. obs.).
The adult Thayer's Gull featured on the back cover was photographed by King at Marshall, Marin Co., California, on 20 December 1997. At long range (> 30m) in good light, this individual showed a "staring" yellowish iris, reminiscent of an adult Herring Gull. Closer inspection (down to 20 m) revealed very restricted fine brownish mottling on a yellowish iris (a score of 2.5). Other characters visible in the photograph supporting the identification of this bird as an adult Thayer's Gull are its relatively small body size, rounded head, medium gray upperparts, rather smudged dusky head and neck markings, rather small bill lacking a prominent gonydeal angle, dull yellow basal half of the bill, prominent purplish red orbital ring, and matt rather than jet black markings on the primary tips.
Adult Thayer's Gulls can also resemble the occasional adult Kumlien's [Iceland] Gulls [L. (glaucoides?) kumlieni] that show blackish primary markings. The iris of the adult Kumlien's Gull is generally described as variable, ranging from clear yellow to dark (Lehman 1980) or as "golden, amber, or even, rarely, brown" (Millington 1993). Therefore, while the iris of adult Thayer's Gull may average darker than that of Kumlien's Gull, we expect substantial overlap; a study of iris color in Kumlien's Gull similar to ours would help clarify the extent of this overlap.
We thank Stephen C. Rottenborn for his review. This is contribution 854 of the Point Reyes Bird Observatory.
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Grant, P. J. 1986. Gulls: A Guide to Identification, 2nd ed. Academic Press, San Diego.
Lehman, P. 1980. The Identification of Thayer's Gull in the field. Birding 12:198-210.
McGeehan, A., and Millington, R. 1998. The adult Thayer's Gull in Donegal. Birding World 11:102-108.
Millington, R. 1993. Identification and status of Kumlien's Gull. Birding World 6:101-106.
Zimmer, K. J. 1990. The Thayer's Gull complex, in Advanced Birding (K. Kaufmann, ed.), pp. 114-130. Houghton Mifflin, Boston.
Zimmer, K. J. 1991. Plumage variation in "Kumlien’s" Iceland Gull. Birding 23:254-269.