Poor girls....
Remember, my peeps, that this is one of those things developed by factory farm workers to make life easier on themselves, not the birds.
I believe that the best thing we can do is lead by example at every opportunity, rather than try to flog the unfloggable...
Now, back to the subject...do they grow back? It depends. A burned off (cauterized) beak will not grow back. A bird who's been debeaked but not sealed (cauterized) can grow back.
Also, a bird can be debeaked accidentally rather than done with a more odious intention...
How do I know this?
This is my darling Benzie...my favorite of all my roos, and the only pure spitzhauben roo I have left. He caught his beak in the wire while trying valiantly to flog my dog on the other side...
Don't cry for Benzie...this imp of the perverse got super-shake suppliments twice a day in the comfort of my lap while watching a DVD. He started following me around the pen with this "is it dinner time?" look on his face till he adjusted to eating differently, and now is showing signs of the beak growing back...