Pretty sure things like ravens, bats e.t.c. are alright. any form of Abberation, Fey, monsterous beast (Dire wolf or badger for example) would be considered a threat Im sure.
The guards arnt going to be dancing about saying: "Ahhhh, a killer raven!"
Common sense as ZeroVega said :p
Edit: 'Would be interesting to see if they can see the difference between a normal common~or~garden raven and an epic raven though
'