I think I like the vertical only because it makes the bird a bit larger. You might crop it down a bit lower, leaving a little more space at the bottom and a little less at the top without actually centering it? Seeing the whole branch might be good. Just my two cents.
I like that bird. Unfortunate you couldn't have used DOF to separate it better from the background. You might consider retouching to remove the twig that seems to be part of a tripod with it's legs.
Portrait orientation ... a bit much negative space on top, given the little amount of space on the bottom. A haircut (little off the top please ) to rebalance, along with the corresponding sides to retain orientation.
) to rebalance, along with the corresponding sides to retain orientation.