I think the reason you don’t see zombie birds is that, while the zombie virus might be able to reanimate the dead in ways that blissfully mock the laws of thermodynamics, it’s too much to then expect the avian undead to also violate aerodynamic principles by flying about in an equivalent slow and halting manner as they search for brains to peck out.
It’s one thing to suspend disbelief regarding corpses rising from the grave and shuffling about in quest of cannibalistic munchies, but dead birds with the ability to sluggishly levitate would be something of a stretch. Especially if they were missing part or all of a wing or their tail due to decomposition.
On the other hand, it would be farcically amusing to see.
