Shocked. I would not have thought this, but it seems hennypenny, you could be right.... http://www.animallaw.info/statutes/stusok29_5_410.htm
This is very strange-- the laws protecting owls are federal, and state laws are not supposed to be able to trump them. Perhaps this is with permit in hand? I would tread very, very carefully here.
But, that aside, even if you do legally dispose of the owl, you still have a chicken and guinea buffet in your trees and yard every night! It is only a matter of time before something else harasses or takes your birds. In my thoughts, it is much wiser to make sure your birds roost securely inside a coop at night to keep them safe, and enjoy the free pest control and beauty that the owl(s) in your area offer.