Found this bird at the Zoo. It looked a lot like a Townsend's Warbler but the face was wrong. I thought it might be a rare Black-throated Green Warbler. Got these fair ID photos and the consensus is that the bird is a Hermit Warbler. There was also a suggestion of a possible Townsend's X Hermit hybrid, but the lack yellow on the under-parts and at least a suggestion of streaking on the flanks ruled against that. Thanks to Guy McCaskie for helping out on the ID.