Well, mice carry lots of germs and nasty stuff, but I'd say as long as you aren't infested and have a giant pile of mice doo in from the floor to the ceiling in your duck pen, it's fine XD! Ducks can hold their own pretty well from what I've seen. My kittens were terrified of my big girl! She'd waddle up and give them the eye and they'd run for the hills. So unless it's a predator or a large rat, don't worry too much about it. If you're concerned souly about mice, I'd suggest getting a couple of cats with claws. In a month my cats have kept birds, mice, and other small boogers from grabbing my duck's food. Now they are free to eat when they please without theives or poo in their dish. So I'd say that over all, mice are more annoying than they are dangerous, and a couple of kitties will fix that right up!