For the second night in a row after putting my Muscovy ducks in their coop for the night, I observed one of the drakes acting like something was stuck to his butt. He stretches his head towards his vent area and snaps at open air then stretches his head on his other side and does the same thing. He will repeat this several times then go about his normal routine then about 5 or 10 minutes later he does this again. He is eating, drinking and pooping normally. Also, he seems to be the alpha duck, at least presently. There are 4 drakes and 4 ducks, 3 drakes will be going to different home soon before other issues arise . None of the other ducks do this at all and it only happens to this one drake in the coop at night. I keep their coop and pen clean and change out the bedding and straw regularly, their pool is dumped and refilled daily, as long as its above freezing. Any thoughts?