Depends entirely on the situation, really. Like a large castle is going to be relatively incapable of keeping out various wildlife on a massive scale, so a bird or rat running around isn’t really an issue off hand. Maybe keep an eye on it to note any sus behavior, but otherwise animals exist. But if they’re in sensitive areas, then it becomes considerably more alarming. Sure, it could just be a rat, but it also could be a fuckin’ druid in which case killing the bird/rat/bat/whatever on sight might be a reasonable response. This also has the added benefit of allowing for a guard who might be a fan of animals and not want to hurt them, which gives the player an alternate path/chance to take things with.

I miss DMing