How can I tell which kind of owl is living in my backyard by sound alone?

Ok, so for the last few weeks we have heard a screechy repetitive noise in our backyard. The sound can only be heard after dusk and is rarely heard in the morning. Eventually we were clued in to the fact that it was an owl, or owls, by the owl pellets in our backyard. Its interesting, but also kind of annoying. THe noise it makes is a kriiiil (then is stops for like 10 seconds) kriiillll (stops for 10 seconds). Repeat that x 1000. How do i know what kind of owl it is?