I've got a Great Horned Owl who lives in the tree just outside my orchard where my poultry are located. The owl comes up and sits in a tree just outside my back door and watches me during the day. He has never taken one of my birds.
He can't take any of my birds because he can not get inside their secure night pen.
OP, as long as you continue to let the owl into your barn, he will take chickens. He's found a nice easy food source and has no reason to give it up. So you can either continue to feed the owl, or you can keep the chickens where he can't reach them. The barn is not secure when the door is left open, so if the barn door will be left open in the future, then I suggest that the chickens have a pen, with a cover, inside the barn. Or they can be moved to an outside run that is covered. Or, they can have a corner of the barn that is secure and opens into an outdoor run that is covered against aerial attack and sturdy enough to defeat raccoons and stray dogs.
He can't take any of my birds because he can not get inside their secure night pen.
OP, as long as you continue to let the owl into your barn, he will take chickens. He's found a nice easy food source and has no reason to give it up. So you can either continue to feed the owl, or you can keep the chickens where he can't reach them. The barn is not secure when the door is left open, so if the barn door will be left open in the future, then I suggest that the chickens have a pen, with a cover, inside the barn. Or they can be moved to an outside run that is covered. Or, they can have a corner of the barn that is secure and opens into an outdoor run that is covered against aerial attack and sturdy enough to defeat raccoons and stray dogs.