With a fairly pale, putative 1st-cycle Thayer’s Gull in Scotland at the moment, now is probably a good time to look into some of the variation shown by Kumlien’s Gull.
The following photos were all taken at St John’s, Newfoundland, in late January 2013. They illustrate some of the variation at the darker end of 1st-cycle Kumlien’s Gull. Thayer’s Gull is still a rarity in Newfoundland, and the large numbers of Kumlien’s Gulls, all showing bewildering variation, make looking for one a rather daunting task! However, given enough scrutiny, a typical Thayer’s Gull should still stand out, even when you have to sift through thousands of Kumlien’s Gulls.
This rather dark bird differs only subtly from paler examples of Thayer’s Gull: it has already moulted many scapulars, the secondaries are greyish rather than dark brown, and the pale fringes to the primaries are slightly longer (reaching the secondaries at rest).
Compared to typical Thayer’s Gulls, this bird shows less contrast between the outer and inner webs on outer primaries. Note that secondaries can appear darker than the greater coverts; this depends on the angle of the wing towards the photographer.
This bird has rather pale secondaries, of the same colour as the inner primaries.
This bird has moulted only a few scapulars. Note that the juvenile scapulars have quite extensive brown centers.
The same bird, now with its wings open. Secondaries rather brown. Compared to most Thayer’s Gulls, note the rather pale outer web to P6 (dark in Thayer’s).
The above two pictures show examples of birds with extensive brown juvenile scapulars. However, rather than thin primary fringes as in Thayer’s Gull, they show frosty whitish distal area on each primary.
This bird has rather dark juvenile scapulars.
This one has paler tertials than most 1st-cycle Thayer’s Gulls (and has moulted many scapulars).