I have 4 female duck that are 4 mo. old. They are free range 100% of the time with access to an 8 acre lake. Fed 18% protein duck pellets. About 3 weeks ago one of the Welsh Harlequins started separating herself from the others. She'd follow behind slowly (almost labored), would not play in the water with the others. I've noticed that she's trembling slightly and her underbody feathers are unkempt. She is still eating. She seemed to fully recover about a week ago, but now the symptoms have reoccurred. Will brewer's yeast or niacin help? Any other suggestions would be wonderful.