According to Sibley's illustrations, the olive-backed form of Swainson's Thrush bears perhaps more resemblance to Gray-cheeked Thrush than it does to the russet-backed form of the same species. However, his illustrations show the olive-backed form as having a much broader buffy collar wrapping around behind the auriculars than either Gray-cheeked or the russet-backed form. Is this depiction accurate? If so, it seems it might be a useful clue in separating Gray-cheeked from the olive-backed form of Swainson's, should either one ever happen to show up in Oregon. Not that I expect to see one of either, but chance does favor the prepared mind. In terms of detecting a bona-fide Gray-cheeked, I think the chance would be much greater if one happened to hear one among the tens of thousands of Swainson's, which pass over the western Coast Range every fall. The flight call is distinct enough from that of Swainson's, that I think one could be confident of picking it out, if he heard it well. It certainly is something those, who listen to night-migrating Swainson's should be aware of.