This was a very small Empidonax flycatcher. 

 Here's the description of that I entered into eBird on the day of the sighting:

Empidonax was seen by Ryan Votta & me about 8:15 this morning. I ruled out Hammond's Flycatcher
rather quickly because the primary projection was fairly short - too short for a Hammond's. However, even with the short primary projection, the tail length appeared short. (Tail didn't look long enough for a Dusky Flycatcher tail.) The eye-ring was bold. This eye-ring was round, appearing fairly even in width rather than the slightly tear-shaped eye-ring of a Dusky or Hammond's. The bill was petite. Upper mandible was dark while the underside of the lower mandible was yellowish for almost the entire length. (I think there may have been a slight smudge at the tip of the bill, but so slight as to be difficult to detect.) The upperparts were grayish/olive with the head appearing slightly more grayish. The throat and conspicuous wing bars were whitish. The chest had a slight shading that formed a bit of a band. Otherwise, the underparts were washed with yellow. The bird flicked it's tail slightly. The bird held its wings fairly still - lacked the twitching motion of wings typical of a Hammond's Flycatcher. The only vocalizations we heard were occasional calls. The call was a perky "whit" - not quite as dry as a Dusky Flycatcher "whit" and not the high "peep" of a Hammond's call note. [I should add that I had described my interpretation of the call note as a "whit" with a slight similarity to the call note of Audubon's Warbler.]
The bird stayed on the north side of the inflow ditch the remainder of the morning. It foraged low in the understory of the cottonwood grove. Vegetation it frequented included: cottonwood saplings, roses, hawthorn, Russian olive, and the pear tree that is in full bloom. The bird spent time very low in the vegetation almost exclusively (eye level or lower).

As in the description (the previous field), I eliminated Hammond's Flycatcher and Dusky Flycatcher rather readily.  See the previous field for details.     

 I was confident of the ID before consulting any field guides.  However, I always have the National Geographic Field Guide to the Birds of North America and The Sibley Guide to Birds in my car.  I'm sure I consulted those field guides sometime after the intial sighting of the bird.