I found some cooperative Tree Swallows last week. I took this picture as he was stretching his wing. Let me know what you think.
Canon EOS 7D Canon EF600mm f/4 IS L USM with Canon 1.4x II teleconverter
ISO 400
f/8 @ 1/640sec
Impressive wing for such a little guy. Nicely exposed.
I wish the back wing was a bit sharper, since it is such an important component of the image. I know you were already at f/8, but if time allowed, it might have been interesting to push it up a bit. You might have been able to push up the ISO to get a bit more flexibility here.
I usually like to frame fairly loose, but might consider a very small, 20% or so crop in from the left.
I like the crop as presented. I do agree with Randy regarding the "softish" back wing. I think f/11 or f/16 might have worked. Love that background and perch. Super pose of the bird, and well done on the exposure.