In my experiences owls don't pluck the birds but take the heads. I have had birds killed by owls and the whole bodies are there except the heads. A fox may leave a good pile of feathers or very little. Here lately I have been seeing fox. I know there has been a female and not long ago a couple of her kits showed up on my cameras. Good luck...
https://www.backyardchickens.com/threads/hawk-attack-prevention-ideas.1495695/post-24962418
https://www.backyardchickens.com/threads/hawk-attack-prevention-ideas.1495695/post-24962418