There's an interesting article on Feathersite that explains how sometimes a sex change will occur in poultry. Briefly, even though hens have 2 ovaries, only one normally functions. If that one stops functioning, the other takes over but produces more testosterone. Not only will a bird start crowing but other physical changes can occur. There are photos accompanying the article if anyone wants to read it:
http://www.feathersite.com/Poultry/BRKChange.html
I've read about this before, though I think my bird is a bit too young (5 months). I do think she may be mimicking the actual rooster that is crowing all day long. She really looks nothing like a boy in my opinion, and throughout the day she clucks like the other girls.