What am I? The above composite and the next pic are the same individual from a week or 2 ago.
Silent fall empids are their very own special can of worms. Michael O'Brien wrote one of the best Empidonax ID articles I've come across, focusing on fall birds here. When this bird popped up my immediate impression was Yellow-bellied; its eye ring is medium prominent and it seemd fairly greenish (the Willow/Alder complex should be browner and have less of an eyering and Least usually has a stronger eye ring and colder tones). It's shot at a pretty high ISO in deep shade so I think a lot of the color is lost. In the backlit shot below the throat (except the part blown out by the sun) blends inot the face which would be a good point for Yellow-bellied. The pics do seem to show a grayish collar/nape. O'Brien's article, though makes point that Yellow-bellied and Acadian (which I'm basically discounting based on location and timing) are greenish throughout the upperparts. If the photo's accurate then that might be a point for Least.
Yellow-bellied has a longer primary projection than the very short primaries of Least. The pics are equivocal, though I think the top composite suggests a length more consistent with Yellow-bellied. Of course if I'd remembered (or noted) the primary length and/or the collar detail instead of utilizing the usual spray-away-with-the-camera-shutter -ID-confirmation-method I'd maybe be more confident on the ID. Maybe next time.