I think in some hens, the sight of a nest full of eggs can trigger the hormones that make them brood.
I believe this could be true. We leave a few EE eggs in a nest in the freerange area so the EE won't hide her eggs, and when the Delaware hens find that nest, there is an incredibly loud, excited show about it. Usually the Del will sit on the nest for a while, too (then eventually leave - good girls!). Dels are not a broody breed, but I can see how that much excitement could provoke a broody hen's hormones.
I believe this could be true. We leave a few EE eggs in a nest in the freerange area so the EE won't hide her eggs, and when the Delaware hens find that nest, there is an incredibly loud, excited show about it. Usually the Del will sit on the nest for a while, too (then eventually leave - good girls!). Dels are not a broody breed, but I can see how that much excitement could provoke a broody hen's hormones.