Jay Carlisle's little lesson to me at the Deer Flat CBC on the difference between Merlins and Sharpies caused me to look longer and harder at a little guy we had in the cottonwood tree near the Avimor entry this afternoon. I would probably has simply passed it off as Sharp-shinned Hawk normally, but I took the time to draw it so it would force me to look at the field marks more closely. Here is my field drawing and the relative proportions of the head and tail feel pretty good. The only problem is, I still can't determine if it is a Sharpie or a Merlin. I did not get any looks at the wings, back, or in flight and unfortunately it disappeared while I had my head down drawing it.
* smallish head
* lighter throat
* black weak moustache
* distinct eyebrow
* bill does not protrude much from head
* brown streaks on chest
* yellow feet
* black and white striped tail
* round white tip on tail
* kestrel/dove size
Looking in my field guides...the rounded tail looks good for a Taiga Merlin from Peterson's and the distinct eye brow and weak mustache support it. Because it was using a tall tree to hunt from also leads me away from Sharpie as I usually happen upon Sharpies in medium sized lower bushes. The broad white tail tip being rounded could also indicate Coopers, but the overall size was so small that I have a hard time with that.
Comment from Jay: My impression from your drawing is of a Merlin ... and the high perch, though not conclusive, also suggests that. Also, black & white tail bands (versus gray & black) is a Merlin trait. The biggest key would be relative length of the wing (how far the folded wings come down the tail ... or, how much the tail extends beyond the folded wing) but, in the absence of that, I'd guess Merlin as well. Also, I think the tail would appear longer in your drawing on an Accipiter...
UPDATE: On my way from my day job to my evening job I saw that the bird was back in the tall cottonwood and I had better views while it munched on what appeared to be a Junco. As it bent down to pluck meat and feathers I could see that the wings extended to about the same length as the tail or short of it and crossed in the back a bit. It was pretty uniformly dark on the wings and back. The light was better too and I was able to see the face with more clarity. All this combined with my afternoon study of Merlins vs. Sharpies and another call to Jay helped me to confirm that it is indeed a Merlin. Avimor's 100th bird recorded as seen!