Spontaneous sex changes are rare in the animal world, but they do happen. Clownfish, for example (yes, Nemo) change from male to female as part of the standard mating process. But the hen situation is different: Her sex change is not beneficial to the species [source: Melina].
It starts with hen anatomy. A female chicken has one ovary and one inactive gonad, a remnant of early chicken development when sex genes haven't yet activated. A gonad can become an ovary, a testicle, or a combination of the two (ovotestis), but once those genes go the female route, that other gonad just sits there [source: Melina].
Now enter a cyst or tumor that damages the hen's ovary, and the gonad steps up to fill the sex-anatomy void. A gonad that develops into a testicle or an ovotestis instead of an ovary will start the release of androgens, a male sex hormone, and a sex change can result [source: Melina].
The typical hen-to-rooster transition begins with an end to egg-laying and progresses to behaviors and physical traits. A hen will start strutting and crowing, gain weight and grow the quintessential rooster wattles, dark feathers and cockscomb atop its head [source: Melina].
The result is essentially a sterile rooster. As far as the experts know, this change only happens to females; a rooster-to-hen conversion has never been documented [source: Melina].