I had 2 runner x ducks go broody last year, both sisters. I forgot to pick in the eggs for a week or so and one went sitting, they need quite a large amount of eggs to make them go broody- mine both sat initially on 15 or so eggs. The first hatch had 12 ducklings and the 2nd had 15. The first one didn't go broody until June though and the 2nd started sitting in July.