I have a hen that has 9 little ones she hatched, they are about 4 wks old. for the past 6-7 days the Mommy has singled out one duckling to pick on. I mean beat the crap out of. It cannot come out of hiding for the mommy will track it down and peck it. It has lost most of its feathers on its back. I removed it from the enclosure with the mom. What would make her do this to just one single duckling? When I fiest noticed her doing it I could tell which one was getting beat, so I thought it was just a teaching thing, but found today that she has beat it up pretty bad. I have had her for about 2 years and she has raised over 60 ducklings with no problems. Just wondered if maybe this little one could have something wrong with it for her to single it out. It eats fine, swims, keeps up with the the others, No visible issues at all. I am Stumped.