That's a very attractive bird Allen (beautiful shade of blue), and an excellent capture of it as well. A good pose with just the right amount of space around the bird, and great sharpness. The colors and shapes in the background add nicely to the composition. Also, the lichens give a very natural look to the perch. All good, I like it.
Your gear is close to my wish-list setup - I have three of the four components to date, still saving up for the 400 DO II (sadly I'll need 9,300 Australian dollars to own one - such is the penalty of a smaller market and a weaker currency). At least the optical quality is excellent as you've proven with this shot!
Nice capture with a good pose and perch. Have you fed him on a diet of mealworms? I might have preferred him just a tiny bit smaller in the frame but that's a personal preference that I find I have been changing as time goes by. And perhaps a bit brighter in the mid-tones? But just fine as posted.
The perch the bird and the background all work really well for me. I like a large in the frame shot like this. I agree with Bill that the mid tones are too dark. Eastern Bluebirds are not this deeply blue, nor is the chest so deeply orange. I think you need to lift those a little and would consider toning down the blues a little as well. Easy fixes for an otherwise beautiful frame.
Beautiful bird, perch, BG. In addition to the advice above, I'd consider trimming a little bit below - just enough to hide the gap showing at bottom edge. That would make for a more solidly anchored finish down there.