The empids ( Empidonax ) flycatchers are one group of birds where it really is necessary to learn their songs. You can get them off the Cornell site. With variations in plumage, molts and dust and dirt, they can be next to impossible to Id by sight. Experienced birders can often ID them by " Gestalt," but that takes a lot of field study. I have heard it said the the Willow and Alder can't even identify each other by sight. That is why they sing so much. The least has a very quick che-bec, and the Will has a slower distinct fitz - bew. The Alder's song is similiar to the Willows, but distict enough to tell the difference. This is a very difficult group of birds to ID by sight. And by photograph, probably impossible. You said you heard it singing - record the songs from the cornell site and that will solve your problem.
