Let's get back on dry land for a moment, shall we? After all, there are plenty of perfectly strange things on our surface world.
Owls always have an interesting place in human culture. In Japan, they are carved everywhere because the word for owl sounds like one of the words for happiness. The Romans thought owls were vampires ("striges;" thus the term "strigiformes" for the order of owls) even though they were also the sacred bird of Athena/Minerva. The overall impression is of a nocturnal, intelligent bird with a cute little beak, an oddly human face, and the ability to scare the crap out of people. In modern times, the association with witchcraft still stands.
You've got mail, Harry!
The Barn Owl (Tyto alba) is one of the most common owls in the whole world. It is so named because it often hangs out around barns to eat the tasty little mice therein. It has a pale, heart-shaped face, excellent hearing, and wings that are almost silent in the night.
The Barn Owl in particular also gives one perfectly good reason to believe that owls are true night spirits. They have a very humanlike face that gives them an 'uncanny valley' feel, which puts some people off. Some rural communities go so far as to call Barn Owls "Death Owls," "Demon Owls," and "Ghost Owls." A lot of the birds die because of the association with evil. There may well be a reason for this:
OK, y'know what? Somebody needs to use that screech in a movie. That is one of the single most intimidating displays you will ever see from an owl. It's even more intimidating when combined with bobbing, a motion used to make the owls look scarier, and beak-clicking. Well, their scare tactics are working:
...Yeeeeaaah, this thing should replace the black cat as the usual witch's familiar. Look forward to another owl on 7/15 - the release date of the last Harry Potter movie! MIDNIGHT SCREENING FTW!