Concerning zombified birds and other flying creatures which have become zombies: I think that, no matter how much they might be rotten, they can still fly. I had over 155 Giant Mosquito Corpses on my screen at the same time, and not one of them was without flight or movement. Similarly, I've had entire flocks of undead ravens, undead Peregrine falcon folk, and all other flying undead creatures under the sun on my map at some point in large enough numbers.
While those worlds are long-deleted, I did check the status on a few of those creatures, and found that they were all rotten beyond hope. Some less so than others, but mostly they were all rotten to the point where anything they had in terms of wings should have been unusable. This was not the case.
Of course, I'm sure you didn't need me to tell you that, but I thought my input might serve to confirm the fact a little bit. And, while I am here, I will also impart a few other tidbits of information, which might be of some importance considering the subject matter:
1 - Zombified birds (and other reanimated creatures of flight) can be grounded.
2 - Bolts are largely ineffective against undead.
So, if you were thinking of shooting them out of the air and hacking them up, that won't work.