I don't have science to back it up but it is my theory that it behaves like an incomplete dominant gene. Meaning that if you cross broody to non broody you will get "aggravating broody", quits laying, sets but may or may not raise chicks., instead of reliable broodiness. I think there is a lot more to it than that, but that is kind of how it works. I consider reliable broodiness to mean multiple clutches per year with high hatch rates, no breaking eggs, carrying off bad eggs as needed, and absolutely no pooping in the nest. I think a lot of chickens that are considered broody are just the aggravating kind, and they will throw a percentage of hens just like them, and they will raise some chicks if everything goes just right. In working with true broody chickens, every pullet in the flock will become broody after the first few eggs are laid, not just a few hens in the flock going broody and maybe not until they are two or three years old.
