I think it would be hard for a weasel to kill a free ranging standard sized chicken. Think about the size difference. I don't know that a weasel could get a full-grown chicken pinned down or to stay still long enough to hurt it. A lot of standard sized chickens will go after mice, rats and other small animals. A weasel would probably become prey instead of a predator if it tried to kill a chicken during the day. It's a much easier meal for a weasel at night when chickens are helpless.