Feather sexing is a sex-linked trait. There is a great thread that explains all the sex-linked genetics, but basically only certain carefully done hybrids can be reliably feather sexed, just as the silver/red (called red sexlinks) and the barred gene (black sexlinks and the autosexing breeds). Some people claim to be able to reliably feather sex some pure breeds. I wouldn't rule out the results of careful observation, but I wouldn't rely on it without a lot of experimentation.
I LOVE the whole topic of genetics, so many projects I'd like to try. If anyone wants to collaborate, I'd be happy to do the genetics research and help you decide on what to cross. Anyone up for some sex-linked, super blue egg layers? Or maybe rose-comb auto-sexing breeds (to get around their propensity for frostbite) ? I think I know how to do both of those, just don't have the space and time to do it all.
and frostbite was bad for out leghorn this year, lost more than half his waddles
Poor thing. Glad it didn't get infected.