The following Phylloscopus-type warbler was photographed at Gambell, Alaska on October 1, 2011. Those who have examined the rather poor quality photos are divided between calling the bird a dull Willow Warbler (Phylloscopus trochilus) and a Common Chiffchaff (Phylloscopus collybita ), presumably of the Siberian (tristis) race. The most frequent refrain is that the photos look good for Chiffchaff but do not convincingly exclude Willow Warbler.
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The following comments are typical:
"I am not sure if they are diagnostic, but I'd be really surprised if this bird is not a Chiffchaff, and Siberian Chiffchaff at that. It appears to have a really dull grayish-olive tone above and no real hint of yellowish in the supercilium or underparts. I do think the tips of the primaries are visible in a couple images including PA010614 and they seem very short. Between color, the primary extension, and the late date (Chiffchaffs should be on the move later than Willows), I think he's got a Chiffchaff there.
I wouldn't be the one to make a conclusive case from these images though and I'm not sure anyone could. Better photos and audio (even a smart phone with no signal can get acceptable audio these days) might be needed."
"As for the warbler, they're not great shots and I'm always wary of making judgements based on soft photos where details are open to interpretation but I can see nothing to suggest it's anything other than a Chiffchaff. The general demeanour looks spot on as does the primary projection. Molting adult Willow Warblers can show a disconcertingly short pri-proj but by now they would have finished their post breeding molt and have a long projection. The all dark bill also supports Chiffchaff."
"Of course, what I see is a Phylloscopus Warbler with features of P. collybita / P. (collybita) tristis.
The greyish nape and mantle in combination with greenish-edged wings and dark legs are features of "Siberian Chiffchaff" tristis ("splitted" from collybita by some authors).
But the whitish-yellowish over-eye-stripe and the whitish breast aren't typical for tristis (...it should be buffer and more brownish in colour)...these features are more typical for individuals of the collybita-complex.
In result this bird shows features of P. collybita & P. tristis.
But due to the low quality I cannot exclude other eastern palearctic Warblers, like species from the trochiloides- group."