Like the teenage dream of the imaginary perfect boyfriend or girlfriend I sometimes wonder if the search image some folk have a vagrant juvenile Thayer’s Gull in Britain, is a bird which doesn’t actually exist!
So to spice up the discussion here are 2 juvenile Thayer’s Gulls. The primaries are still growing, so I think its fair to assume that where they were collected very near to where they hatched (might even be from same brood?). They are from Igloolik, in the northwest corner of Foxe Basin, Nunavut, Canada. This is core range Thayer’s zone according to the maps. Both birds indeed look basically very similar, both like juvenile Thayer’s Gulls (as I have seen them in Pacific North west).
Critically, one has obviously paler primaires than the other, paler tertials and when I looked carefully at the secondaries they are paler and don’t seem to contrast at all with the rest of the wing. I think a dark secondary bar would not be apparent in the field.
That, as I have argued many times, is the normal variation which can be ascribed to juv. Thayer’s Gulls. The bird I saw in Derbyshire, considered ‘acceptable’ or not- looked just like…a juvenile Thayer’s Gull. The parsimonious explanation is that it was one, as are most of the other ones which have occurred in recent years in western Europe, even though they vary somewhat in appearance- (just like the juveniles of all large gulls)!
Can of worms or closer to the truth?! Great article by Peter Adraiens b.t.w. in latest Birdwatch mag. Well worth a read. Not what some might expect!
juvenile Thayer’s Gulls, copyright NHM