They're medium-large owls. At first I thought they were short-eared owls, but the photos on Google images don't match:
|Short-eared owl from Google Images|
Nor do they appear to be a Northern Hawk Owl.
|Northern Hawk Owl from Google Images|
Typically these mystery owls are audible (in pairs) in late evening, all through the night, and into early morning. Notice the yellow eyes and black beak. They have a harsh screeching cry, unlike the melodious Great Horned Owl's call. From my days as a field biologist, I'm very familiar with Great Grey owls, Barred owls, and Spotted owls, and know without question this isn't any of those species.
Yesterday morning I caught some shots of the mystery owl. I was on maximum zoom and frequently taking photos through a screened window in very early morning light (meaning, dim), so the photos aren't always clear or focused.
Below are the clearest shots I took. Any thoughts about the species?
UPDATE: You guys nailed it! It certainly seems to match with a juvenile Great Horned owl. In fact, here's a YouTube clip with the very sounds we've been hearing: begging cries. Since we have NOT been hearing any adult Great Horned owls nearby, I can only conclude these two youngsters (we always hear them in pairs) have just been booted out of the nest and are trying to figure out how to be adults.