I was actually quite amazed as to just how variable juvenile thayeri could be, with the trip really changing my opinion on these birds in a European context. In fact, ever since coming back from California, the bird I found (but identified and left alone as a probable dark Kumlien's) in County Fermanagh back in January 2012 has repeatedly come back to haunt me - it's pretty clear it was easily in range of Thayer's Gull. Anyway, I digress... here's a few shots, with notes on each featured bird following the images.
Juvenile Thayer's Gull, Venice Beach, Half Moon Bay, CA, February 2013
A heavy and thick-set individual, the bill is quite hefty and the bird possesses an almost Herring Gull-like structure. The upperparts are quite heavily worn and bleached. The tertials and primaries are the darkest parts of the gull, with the former lacking any internal patterning. Note the solid dark tail and heavily barred undertail and uppertail coverts.
Juvenile Thayer's Gull, Jenner, CA, February 2013
A relatively dark individual with dark brown, solidly marked juvenile tertials and dark brown primaries. The scapulars, though heavily worn and abraded, are dark-centred, paling towards the fringes. The bill is beginning to acquire a pinkish tinge to all but the very tip.
Thayer's Gull, Jenner, CA, February 2013
This is a particularly bleached individual, with rather concolorous pale upperparts. The tertials, though quite pale and extensively pale-tipped, are still some of the darkest brown feathers on the bird, along with the tail and primaries - the latter of which seem remarkably unaffected by the sun when compared to the rest of the plumage. The bill is still extensively dark.
An open wing shot of the same bird shows the obvious pale window on the inner primaries, pale inner webs of outer primaries and obvious dark secondary bar as well as an all dark tail and heavily barred rump.
Thayer's Gull, Jenner, CA, February 2013
Another heavily bleached individual; birds like this are not uncommon after spending a winter in the Californian sun! The tertials are still very much uniform, though quite pale- the primaries (what is left of them!) are also a mid-brown.
Thayer's Gull, Jenner, CA, February 2013
A pale and slender bird with distinctly two-toned bill and heavily bleached upperparts - would this be identified as a Thayer's in Western Europe?
Thayer's Gull, Petaluma, CA, February 2013
A bird that probably fits the general Western European concept of a 'classic' juvenile Thayer's Gull: dark bill (darkest at tip), uniform brown tertials, dark brown primaries with pale fringing, heavily barred undertail coverts as well as a uniform brown wash to the underparts.
Thayer's Gull, Petaluma, CA, February 2013
Dark bill and dark brown 'shawl', but otherwise with rather pale, heavily bleached upperparts - note how some of the inner greater coverts are almost entirely off-white! The tail, tertials and primaries are also particularly heavily affected on this bird.
Thayer's Gull, Petaluma, CA, February 2013
To me, this bird has more of a European Herring Gull feel to it (especially facially), and could get overlooked as such in Europe with the greyish-brown wash to the body and relatively sparsely barred undertail coverts. The tertials show some patterning at the tips, and the primaries - a dark brown - possess obvious pale fringes.
Thayer's Gull, Petaluma, CA, February 2013
A much more distinctive individual to that above, with uniform brown underparts, dark centred juvenile scapulars, dark centred tertials (with patterning towards the tips) and dark brown primaries. The flight shot illustrates the heavily barred (and predominately chocolate brown) rump and uppertail coverts, the all dark tail and uniform chocolate axillaries. One of the nicest juveniles we saw!
Presumed Thayer's Gull, Petaluma, CA, February 2013
Another heavily bleached bird that is unlikely to be confidently identified in a Western European context - can Kumlien's Gull be ruled out, even in California? The scapulars are extensively pale with pale 'horseshoe' markings to the tips, and the coverts are also largely pale. However, the tertials are quite uniform (despite being pale) and the primaries are uniformly dark with pale fringing. The tail is all primarily dark, but too is heavily bleached.
Thayer's Gull, Petaluma, CA, February 2013
Here's another one that has an almost Herring Gull-like feel to it with quite pale (and blotchy) underparts. The undertail coverts aren't particularly heavily barred either.
Thayer's Gull, Half Moon Bay, CA, February 2013
Again quite bleached, but a really nice bird - classic in all respects. You can just about make out the silvery quality to the undersides of the bird's primaries.
Thayer's Gull, Golden Gate Park, San Francisco, CA
Yet another heavily bleached bird - the primaries, tertials and tail have all suffered quite considerably. That said, plumage is otherwise typical Thayer's Gull.
Plenty food for thought here Josh. No doubt bleaching has an effect, but I wonder if some of the paler birds aren't just pale in the first place? Tremendous series of shots - wish we could have obtained images like these of the Gower birds - we may then have got closer to the truth! Still time I suppose?
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