When in the presence of a Roostro they will lay eggs that produce Briar Chickadoos which will grow up into either a Briar 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.
Briar hens are a chicken variant born from the wild thickets of the Far, Far Range. They were first discovered long ago by an explorer who noted that they seemed to have a complex tribal structure, and a fascinating series of chicken customs. He even may have joined the briar tribe as an honorary chicken himself! His records end there however, and we know nothing else about him or his findings. Oh, but his name was Carl. We know that.
On the Ranch:
Briar hens in close proximity to roostros will periodically lay eggs that produce briar chickadoos. However, keeping too many hens or roostros in close proximity makes them anxious and egg production will come to halt. Savvy ranchers with an understanding of the complex nature of chicken romance always keep their coops from exceeding 12 grown chickens.