A couple of times, I've found a dead mouse in the ducks' paddock. The other day it was pretty fresh, and I know that KK (the cat) hadn't been in there. It had to have been the ducks. I've googled it, and sure enough, ducks do catch (and eat!) mice. I didn't know we were getting more mousers when we got the ducks.
By the way, KK and the ducks get along quite well. I have no worries about leaving them out together. I think the ducks are too big and scary for KK to mess with them, and they actually are sometimes seen following her around. (Or, are they chasing her?)