We have 4 different ducks (9 months old) that will suddenly have trouble standing. First they lean back just a little, then fall forward and catch themselves with outstretched wings. Then they sit down and won't move. In about 4 or 5 minutes they will get up and walk away as if nothing was wrong. If I'm outside with them (we have 17 ducks in total) for an hour, I will usually see one of them have a sitting spell. The ones experiencing the problem seem to be eating fine, and they look and act healthy -- except for the few minutes when they fall down and stay down. They are eating Purina layer pellets and are free range grazing during the day. We keep them in a coop with pine shavings at night. We have a kiddie pool (in addition to a chicken waterer), so they can keep their nostrils cleaned out. The pool has gotten pretty nasty lately as the weather has been in the mid to upper 90s during the day. So, even though I was cleaning it out every other day, it did get to looking kinda strange. So, 4 days ago I started setting the pool up with fresh water once a day, emptying it later in the day and letting it dry. Any ideas???