Interestingly, there has been extensive conversation on the CarolinaBirds
list within the last week about separating these two.  I lifted the post
below for my own future study, but thought some here might find it of
interest.  I have no personal knowledge of either species, so I can't speak
to the validity of any statements made.  I snipped comments that did not
directly relate.  The post "Greenfield Lake 9-24-12" begins the thread on
the CarolinaBirds list.  http://birding.aba.org/maillist/NzC  It's all way
above my skill level, but I find I always learn something from these
in-depth discussions!
Mona Bearor
S Glens Falls, NY
----------------------
Quoted from CarolinaBirds  9/25/2012

Harry's comments regarding Gray-cheeked & Bicknell's are mostly appropriate
but if the bird lacks any rusty tones on the tail, I'd say a call of
Gray-cheeked is safe. Bicknell's has a warm cinnamon coloration to the tail
and wings - not as much as a Hermit Thrush, but noticeable. The reason that
a bird in this complex with warm cinnamon tones to the tail cannot be
conclusively called Bicknell's is that the minimus population which breeds
in Newfoundland can mimic this plumage. But, no Bicknell's population mimics
nominate Gray-cheeked. Thus, a bird which has those warm-colored tones is
not automatically Bicknell's, while a bird which lacks them is reasonably
Gray-cheeked. 

<SNIP>

However, over the past year, I've been looking at multiple articles and
photos of these birds - seeking a way to get a handle on this identification
quagmire. I believe it IS possible to identify a Bicknell's Thrush - without
hearing vocalizations (which I wouldn't trust unless the observer was
previously very familiar with vocalizations of both species). 

Bicknell's has the afore mentioned rusty tones on the back, wings and tail
which contrast with a grayer nape. The lower mandible has extensive yellow -
typically 2/3 of its length. Both marks are variable and by themselves not
definitive BUT to claim Bicknell's, these marks must be present. There's
more. Bicknell's is small - the smallest Catharus in our region. Conversely
Gray-cheeked is large - in fact the largest, averaging an inch longer
(that's a lot). Judging size in the field is tough - unless one is lucky
enough to have something to compare it to - but size is important. Finally,
there is the folded primary arrangement. Gray-cheeked is long-winged;
Bicknell's short. This should be evident (on extremely close birds) in two
ways: 1) Comparison of the folded primaries (extension) compared with the
tertials. Gray-cheeked: the primary extension is greater than the tertials.
Bicknell's: slightly less. 2) on Gray-cheeked, the longest primary (P-8) is
the second bottom-most primary in the folded wing. In Bicknell's, it's P-7
(third from the bottom). Really good photographs can reveal these
differences. I've seen such photos in Google Images.

To identify Bicknell's, an observer needs to build a case from multiple
marks. In the presence of a bird which clearly exhibits all these features
(photos a must), I would be comfortable accepting the bird as identified to
species. If not - better be safe (Gray-cheeked/Bicknell's). Conversely, if
the bird really doesn't have any of the attributes of Bicknell's -
especially a larger bird lacking any warm tones - I'd have no problem with a
call of Gray-cheeked regardless of where the bird was seen.
Mike Tove
Cary, NC

-----Original Message-----
From: Vermont Birds [mailto:[log in to unmask]] On Behalf Of Nikolas
Kotovich
Sent: Thursday, September 27, 2012 7:31 PM
To: [log in to unmask]
Subject: Re: [VTBIRD] Gray-cheeked in Montpelier

I had a Gray-cheeked/ Bicknell's Thrush this mourning as well at Delta Park.
Tail and back appeared to be uniform olive gray, olive gray flanks.
The bird lacked buff spectacles of Swainson's and had uniform gray face.


Unless anyone knows otherwise I don't think Gray-cheeked can be reliably
separated from Bicknell's Thrush in the field.

-Nik Kotovich