Last summer, two of our ducks went broody (a Cayuga and a Buff) and our other Buff duck insisted on sitting with one of the broody ducks on eggs but wouldn't sit on her own eggs. She would sit on either nest and would stay on it even if the other duck got up. Every morning and evening, I would make the broody girls get up and eat, poop, bathe, etc, and they would do all that and very quickly go back to sit on their eggs, but "Helper Buff" would usually take her sweet time before going back to the nest(s), sometimes hours.
Here they are in their duck house, along with our other Cayuga girl who would occasionally make a social visit to the broody Cayuga's nest.
And another pic of the Buffs "telling me off." (Although they would accept Lettuce or worms readily enough, the little feathered piggies!"
Oh, and the bright blue thing in the middle nest is a plastic 3D-printed egg that a friend made me. The ducks didn't seem to think too highly of it, probably because of the "unnatural" color.
Also, our Welsh Harlequin and her babies were housed separately at night but were allowed supervised mingling with the rest of the flock during the day sometimes. Several times she plunked herself down with the broody Cayuga. My husband said that she, as an "experienced mama," was showing the Cayuga "how to get in the right frame of butt" for hatching.
