A Great horned owl will take chickens, the best way to prevent attacks is to keep the secure at night. Lock the chickens up in the coop at night. If you have a run attached to your coop also make sure to block off the entrance to the run so if the owl breaks into the run it can't get into the coop. This also works for the majority of predators, most animals attack at night and if your birds are locked up inside the coop at night then you stop the majority of predator related deaths. During the day a owl will rarely attack, they usually spend the day hiding from crows or guarding their nest, if they have chicks/eggs. To prevent raptor attacks, owls & hawks, you can set out fake crows in the trees and around your coop and run. Also, you can weave string through the trees above the areas your birds free range, this will slow down an attack because it will surprise a hawk as he smashes into the string, this usually makes the hawk stop the attack and if it doesn't the chicken will usually be out of there by the time the hawk realized what happened. Hawks can see the string and some will avoid it but those who do not will avoid it in the future. And one last thing you can do is stand out by the chickens while they free range, a hawk will not attack if you are standing there, but the hawk will watch and wait for you to leave, if you go inside the coop for a second or run inside to get something quick the hawk will take its chance and attack your chickens.