This is a bird that was milling around with several Swainson's Thrushes on September 11th and I initially called as a Gray-cheeked Thrush in the field, based on an overall colder tone and the severe lack of any buffy eye ring or spectacles in the face. There was no rufous in the primaries or the tail, and the lower mandible was more than half black so Bicknell's was ruled out.
The bird was relatively small for a Gray-cheeked, and on the olive end of the gray-olive spectrum, so I popped a few extra pictures of it. Then I got a surprise when processing the above picture - the lower mandible is pale pink, not yellow. The yellow color comes from the gape. The bird is a first fall immature as indicated by the buffy tips to the coverts and the pointed tail feathers.
The pink bill had me wondering about western subspecies of Swainson's Thrush and other possibilities, and then subsequently I pulled out Peter Pyle's book "Identification Guide to North American Birds Part I" which indicated that the paler section of the lower mandible ranges from "pale flesh" to "yellowish flesh". I then pulled up my other GCTH photos and found that this was not the first time I'd seen this, just not usually in a bird that was more colorful and smaller than most. Bicknell's has a yellowish lower mandible with the yellow color covering >50% of the length. I'd assumed that Gray-cheeked had yellow bills as well, but apparently I'm wrong.
Based on size this bird may well be the subspecies Catharus minimus minimus, which is on the small end of the GCTH. If anything this bird was a little smaller than the Swainson's whereas GCTH are usually a little bigger. This is also a cautionary tale for using structure to ID Bicknell's, since this Gray-cheeked is very Bicknell's in overall structure.
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