Oh I see, that lens+extender combo really fills the frame! In my humble opinion, the second image doesn’t need cropped at all. It’s got some space for the bird to “look” into, and I wouldn’t take much, if any out. The branch in the foreground does seem a bit distracting, but that’s one of those things that can’t be helped; the only thing you can do is try again! I love using lenses to capture things up close, whether it’s a zoom lens for something farther away, or a macro lens, for something up really close!
As to your question about catch-lights… I have never tried using flash for wild animal photography. (I have used it for pet portraits though). I imagine that bright flashes and camouflage don’t go real well together if you’re trying not to let the subject know that you’re there. However, if they don’t mind it, I’m sure it would improve your shots if you could actually make it work. It would have the added bonus of a little fill light, you could theoretically underexpose to get the background properly exposed and use flash to fill in the bird. I’ve never tried it, so that’s just speculation on my part.
As to 55-250 and 70-300 lenses, you’re right, you can occasionally get a picture that surprises you. But on the whole, those lenses just don’t measure up, and they really can’t compete, especially with a “fast” 2.8 L lens.
Attached are a few examples of what I mean, the second one is a “typical” shot, that’s what I normally got. The first one would be what I considered “sharp”. Taken with a Rebel T3 and Sigma 70-300 @ 1/640 sec. and f/8-f/9
Of course I should add that auto-focus could have been more the culprit here, I have no way of testing my theory any more, but either way, that’s what I was getting, whether it was the lens’s fault, or the autofocus…