Interesting discussion. It is a question I have never considered actually, as I have never had or even contemplated this type of opportunity.
Shooting a roosted Bird before fly down, or shooting a bird seeking refuge from a predator in a tree would be out of the question for me.
However if I am calling it a bird, and he for some reason flies over and lands in a tree next to me I am not quite sure how I would respond to that. It would seem to me at that point that the hunter has done his job incalling the bird and fooling him, and there is little difference between shooting a bird in the tree and shooting one on the ground as far as difficulty in hitting the target goes.
I have had birds fly to me from some distance off in late and midmorning, but I have never had a bird fly and land in a tree near me. I very seriously doubt that this is an ethical question that I will have to toil with, as I would think it is unlikely to occur.