Thayer's Gull (thayeri)

(last update: January 22, 2013)

Thayer's Gull adult basic January

Thayer's Gull adult, January 20 2008, Nile Creek, Vancouver Island, BC. Picture: Guy Monty.

Thayer’s Gull
The taxonomic history of Thayer’s Gull is an ongoing and somewhat contentious topic. The BOU treats Thayer’s as a subspecies of Iceland Gull (L. glaucoides), while the AOU assigned it species status in 1973. The AOU’s ranking seems a step up from its former grouping where Thayer’s was considered conspecific with Herring Gull.

For a concise historical summary on taxonomy, we recommend the article by Ron Pittiway.

This page does not imply Thayer’s is a resolved taxon, but the increase in claims of Thayer’s Gulls in Europe (mostly from Ireland, but also from Britain, Norway and Denmark), has made a database of descriptive photos more necessary.  
Many birders consider Thayer’s a recognizable form, as highlighted by Howell and Elliott in their paper dealing with adults. A thorough understanding of this paper is a requisite for anyone wishing to appreciate the questions surrounding Thayer’s Gull:

HOWELL/ELLIOTT PAPER

In North America, identification of this so-called “white-winger” is confounded by
(1) dark-end Kumlien’s Iceland Gulls (L.g. kumlieni),
(2) American Herring Gulls with “thayeri” wingtip patterns, and
(3) various Glaucous-winged hybrids (namely with Western Gull and American Herring Gull). The latter two are secondary identification concerns as they’ve been addressed in the literature and are relatively straightforward to work out.

The greatest identification concern is undoubtedly with kumlieni. It must be known that thayeri and kumlieni have the potential of overlapping in every respect, including size measurements, bare-part and eye color, wingtip melanism/pattern, and even upperpart coloration. The phenomenon of nearly complete overlap with kumlieni is thought to occur in a small minority of individuals (apparently most common on the western Great Lakes). Interbreeding has been attributed to these presumed intergrades, but the extent of hybridizing has yet to be adequately researched.
It is here European birders will need to take the most caution with the added caveat that Kumlien’s seems much more likely to appear in Europe than Thayer’s (this assumption is based on the fact that Thayer’s is rare on the East Coast of North America). But this is not to say that “classic” Thayer’s cannot, and have not, appeared in Europe.

As a resident of the Great Lakes region (southern Lake Michigan), Amar ayyash has the unique experience of seeing both Thayer’s, Kumlien’s and intermediates through the winter season. So, what are some near-constants that appear in the majority of Thayer’s and not in Kumlien’s (or vice versa)?