Ren - can you detect a slight skunk smell when you see these owls? If so, then they are most likely GHO. GHO's prefer skunk

Also GHO's will have the 'scary movie hoot' like someone else mentioned. Males have a deeper voice than the females, and they will often times talk back and forth. Owl's like to sit in trees and then swoop down on their prey. Unlike hawks who will soar above an area and then dive onto their food. So, if you see owls in your trees, there is a very good chance they are 'stalking' your birds. I have also heard of Owls landing in the run and then walking up the ramp into the coop and dragging a bird out. GHOs are big enough to carry off a 6 lb bird.