Some of my 3yr old runners are having difficulty walking. They are kept in a large pen, and allowed to roam the yard for 30min-1hr mornings and evenings while I'm doing feeding/cleaning chores. For the past week or so I've noticed ducks tipping back on their tails and struggle to maintain balance during or after their roaming. They then lay down and are unable to walk. They seem to recover in about 30 minutes. They are eating and drinking normally and otherwise healthy. It started with one duck and now half of the flock are showing symptoms. Any comments or suggestions would be appreciated.