I mean, animals (birds too) def have grieving rituals ("funerals") and emotional suffering caused by loss of a close one.

Seeing a reanimated corpse of that same individual would fuck beyond uncanny-valley anyone.

And even without knowing the individual prior to assimilation, there is no way birds don't recognise it as something different.
It just isn't physically harming them & won't go away - what are they supposed to do?