Muscovies can be pretty lazy ducks. I know that my male ducklings are Much less active than my females, the boys like to plop down a lot, even if they're just eating or drinking real quick, they will still plop down then jump right up a second later to waddle back out to the yard. I wouldn't worry about them unless you see some obvious signs of weakness in their legs, like shaking/wobbly knees or if they act like they're painful.
Quote: