My drake mallard mates by ripping the feathers out under the wings of the females. He does not bother to grab the mallard hens anywhere else and that is the only place the feathers are missing. It might be your drake, they do get carried away at times.It would be a good idea if the little mallard had company as long as the KC doesn't pick on her and the Mallard is alert. Do your ducks have a shelter like a pen with a cover over top?Might be a good idea to keep them up for a few days and keep an eye out on the sky or around your area. Actually where the Mallard when this happened? Could she have been in an enclousure with chickens wire ?