iirc this was a major concern for face transplants. Person A gets horribly burned; person B dies; A gets B's face; B's mom sees A walking around with B's face and has an existential crisis.
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?