My mother has a Pekin duck that is a little over a year old and has recently begun to have foot problems. She says that periodically it looks like both feet sort of crumple up, and it can't walk for a while. Then, all of a sudden, they straighten up and it is keeping up with its buddies again. Could this be a hereditary issue, since her other ducks are fine, or is it some sort of vitamin deficiantcy?