I have 6 khaki campbells, 7 mos. old. A week or so ago, one of them began acting weak and having to lay down and rest. She has to flap her wings to keep up with the others, and has to keep lying down and resting. Some days, she doesn't do this at all and has normal energy. She seems to be eating, drinking, foraging, and laying normally. Tonight, one of the other ducks was acting this way too! Anyone know or guess as to what might be going on? Thanks!