Gertie, as she was originally called may have suffered damage to her ovary, said vets at Cambridgeshire England, where the chicken roosts. But that's when a new hormone might have kicked in and started flooding her system. That hormone would have been testosterone - one normally associated with males. As a result, she may have grown male characteristics like a rooster's comb, stopped laying eggs, and even started acting and thinking like a rooster.
There have been several cases of animals changing sex in recorded scientific history, and the phenomenon is actually quite common in clown fish. Clown fish live in a social hierarchy where the males compete amongst themselves and the female is at the top of the social order. When these females die, however, the "alpha" male clownfish physically changes into a female. And in the film Jurassic Park, the protagonist Dr. Grant comments that some species of African frog are known to change their sex from female to male. What the film doesn't mention is that this trait is actually more common than we once believed. Flowers change sex in the form of orchids, and invertebrates change sex in the form of shrimp. Even humans have been known to go through "natural" sex change in rare circumstances without the assistance of surgery and often much against the wishes of the changed individual. In most cases the gender change is from an apparent female form at birth to a gradually more male appearance as life progresses. By the time these changes become apparent the subject could have lived their entire lives up until puberty as a female only later to learn that their body was destined to go a different way. The cause of the condition is commonly thought to be lack of 5-alpha-redictuse or beta-hydroxysteroid dehydrogenease. There have, however, been some recorded cases of male to female changes taking place as well.
So when Gertie became a male it wasn't the sort of thing that happens only once in Earth's history. In fact, it has been the foundation of the evolutionary path for more than one species. But that no doubt certainly doesn't make it any less strange when the crows of the chicken can be heard breaking out across the early morning hours on the farm. But what was the cause in this particular case? While veterinarians suspect a damaged ovary, it's still not entirely known and it has remained up until this point, unexplainable. But it does not appear to be, as some have suggested, an April Fool's prank.(link)
