It could be a female, you'll know if she lays an egg! For the person before me, talking about fat males and mating females, you do know that male birds so not implant the eggs in the female birds, right? A lone female bird can and will lay eggs with no male in sight. Happens all the time! The female will sit at the bottom of the cage, lay them, sit on them. And eventually give up because it's not fertilized BUT she will lay them all by herself.

its a transsexual

Your bird is fat. But it's always a little difficult to tell with birds. Your bird may have always been female, or it really is just a fat male. Unless you know of your bird mating any time recently with a male bird, I'd say he's just chubby.