When in the presence of a Roostro they will lay eggs that produce Stony Chickadoos which will grow up into either a Stony Hen or a Roostro. This process is more efficient in a coop, especially when upgraded. After laying 6 eggs, it will become an Elder Hen.
Stony hens are thought to have evolved from hen hens who once trekked deep into the mountains of the Far, Far Range. Perhaps these birds were seeking a clarity of mind that one can truly obtain through meditation amongst these lofty, barren crags. But most likely, they were just stupid and got lost.
On the Ranch:
Stony hens in close proximity to roostros will periodically lay eggs that produce stony chickadoos. However, keeping too many hens or roostros in close proximity makes them anxious and egg production will come to a halt. Savvy ranchers with an understanding of the complex nature of chicken romance always keep their coops from exceeding 12 grown chickens.