From an article in the NYT's Scientist At Work series:
In owls with a facial disc, the ears are hidden behind it, and are asymmetrical — one is higher than the other. That allows the birds to locate prey both horizontally and vertically for more accurate detection.
I wonder if that's why dogs and cats sometimes cock their heads.
The article, which concentrates on owl researcher Denver Holt's work observing snowy owls in Alaska for what they can tell us about climate change, is worth a read for a variety of reasons.
Isn't the aerodynamic sculpting just marvelous? Looks like a seal with wings.
Photo credit for both pictures: Daniel J. Cox. They are part of a slideshow that accompanies the article.