What skill is used for imitating a voice or mimicking a sound like a bird call ora a dog's bark? My guess would be that it is either Disguise or Bluff, but I think the case could also be made for Perform or Wilderness Lore.
I'd go for perform myself as it's about the closest your going to get. The idea of mimicking a sound of an animal could be likened to someone throwing their voice or impersonating anothers voice which are "entertainment" type activities.
