I am unfamiliar with a study from British Columbia tracking Gray-cheeked Thrush (but I would be happy to be directed to any such studies). There was a study that compared migration routes of different Swainson's Thrush populations from BC, and that might be what is being referred to here? That article can be found online at:
Gray-cheeked Thrush has been known to occur relatively far south in western BC, as well as on the west side of the Rockies (see http://www.birdatlas.bc.ca/accounts/speciesaccount.jsp?sp=GCTH&lang=en) so it wouldn't at all surprise me if some of these birds migrate on the west side of "the divide". Regardless, it's cool to see new records of birds for regions from NFC detections.
From: <bounce-2412475-53236792...> <bounce-2412475-53236792...> on behalf of Christopher Heckscher <checkscher...>
Sent: Thursday, November 9, 2017 6:37 AM
Cc: Carrie Voss; Kate Stone
Subject: Re: [nfc-l] Gray-cheeked thrush?
I agree with Ken this is definitely a Gray-cheeked Thrush. Interestingly, GCTH tracked from breeding grounds in western British Columbia via geolocators and GPS units show routes eastward across Montana prior to moving south toward the Gulf of Mexico.