help!

I have been going in circles trying to figure out what this is. I have seen it perched in a willow, elm, and this mulberry just about every morning for a month; from mid-January to mid-February. On the day I took this picture, I believe that I saw two of the same species but I haven't seen either one since then. Raptors are so difficult to identify...right now I'm thinking it is a prairie merlin.

Nebraska Game & Parks Wildlife indicates that Merlins are an uncommon winter visitor. I thought it might be a Prairie Falcon, but when I read more, then I think again...

The Broad-winged Hawk was another consideration, but based on its status/range, that would be really rare. I've also considered immature (because of the yellow iris) Sharp-shinned and Cooper's hawks, although I'm pretty certain that it is not a Cooper's Hawk because it was not very big. Besides, the tail is short. This is driving me (and my husband) crazy. Let me know if you know what this is or if you know someone who can help me figure it out!