Chickens can undergo natural sex changes
All it takes is one dysfunctional ovary to turn a hen into a rooster.
Two sex organs are present when a female chicken is an embryo. But once its sex genes kick in, only the left one develops into an ovary. The right, sexless gonad typically remains dormant.
Most hens make do with just the one ovary, and live out their lives as mothers and egg-layers. However, medical malfunctions such as an ovarian cyst or tumor can cause a hen's left ovary to regress. In its absence, the dormant right sex organ may begin to develop. If it becomes a testical, or a combination ovary-testical, it will emit androgen — a male sex hormone. This induces a chicken sex change.
Recently, a hen named Gertie belonging to Jim and Jeanette Howard of Huntingdon, England, spontaneously transitioned into a rooster. It developed a wattle and cockscomb, put on weight, and began strutting and crowing in a notably masculine manner. The Howards renamed Gertie Bertie.