Owls

Owls are mentioned several times in Tolkien's tales, but always as references or metaphors, and no actual owl makes a direct appearance at any point. All we can say for sure is that they were evidently well known to characters from across Middle-earth, which implies that they were as widespread during the Third Age as they are today.

The word 'owl' (or Old English ule) is a very old one indeed. Essentially the same word for the bird occurring in Proto-Indo-European, and it appears to have originated as an imitation of the owl's hooting call. Latin had several words for 'owl', among which was ulula, which shared its origins with modern 'owl'. From that Latin word we get modern English ululate, to howl or wail (and indeed the word 'howl' itself also seems to descend from the ancient form of the word, or from a close relation).

These would be difficult instructions to follow even for those more familiar with owls than Bilbo. Barn owls do not hoot, but screech, for which reason they're sometimes known as 'screech owls'. Indeed, this is the only kind of owl likely to be found in Middle-earth that are given this name (there are other 'screech owls' in the Americas, but they would not be found east of the Great Sea during the Third Age). Thus a 'barn-owl' and a 'screech-owl' are actually the same kind of owl, a kind that does not in fact hoot.

So, Thorin's instructions were ambiguous at best, to the point where Tolkien considered revising this section of the story. These revisions did not find their way into print, but they would have had Bilbo cry like a night-hawk, then hoot twice like an owl. Bilbo would have been exactly as confused by this new set of instructions as in the original version, but Thorin's intended meaning is at least easier to discern.