I was in Cave Creek Research Station in Arizona (April 18, 2010) photographing Hummers, when this little Flycatcher came in for a drink.
The thin eye ring, light wing bars, long tail, habitat...make me think Dusky or Gray but that would be a guess.
I have sent this to lots of birder, photography friend's and they have told me what they think.
Hmmmm. I'm certainly no expert on sight ID of empidonax but I'm leaning towards one of the "Western" Flycatchers for this one. Here's my thinking, FWIW:
I believe that Hammond's, Dusky, and Gray all normally have black tips to the underside of the bill. Your bird's lower mandible looks yellowish all the way out to the end. I may be overdependent on this mark, though, so don't take that as gospel.
The bird is rather green and isn't particularly brown, so this leans away from Alder and Willow. That nice eyering is also bad for Willow in particular.
I think the primary extension is too long for Least. The last two primary tips (P9 and P10) seem to have a big gap from the rest.
The eyering seems to be a little pointed at the back which is good for "Western".
I think a "Western" in April in AZ is likely Cordilleron but that migrant Pacific Slope is also possible. Of course the splitting of the Western Flycatcher is somewhat controversial.
The combination of large-looking head, shortish tail, long primary projection, and the whitish eyering with little post-ocular teardrop suggest Hammond's to me. The coloration of the lower mandible in Hammond's Flycatchers is more variable than the field guides would have you believe and I find that they can be fairly plastic when it comes to how long and wide the bill looks. Generally they seem to have narrow short bills that are all-dark, while other individuals pale lower mandibles and bills more suggestive of a Dusky Flycatcher. The head color seems to gray and the back is too olive (not green enough) for a "Western" type.