I found a dead rat my Pekins obviously caught in their house one morning. My female Pekin caught a frog last summer but it was too big for her to eat so I made her let it go. The other day I found a single pinky mouse in some grass seed, so I offered it to the first duck to come and get it. My tiny Pekin drake grabbed it up and mushed it about his bill before dropping it. The female came along and said "Let me show you how it's done" and gobbled it right up.
I wouldn't count on them for pest control though. I just know now not to use poisons anywhere. I set out snap traps just outside their pens at night, beyone reach of their bills. I've had some luck with that.