Thank you so much for sharing this. It makes perfect sense! I know the methods I used before of guessing by tail feathers was totally wrong. Fortunately, in my case, the guessing is really just for fun, as I'm allowed to keep roosters where I am.
Given that I've found all the information on those type methods of supposed feather sexing on private blogs, I figured it was more anecdotal, or they have gotten lucky with their guessing. Not so much actual genetics or science. I've had the best luck waiting on combs to give me a clue at the 5/6 week mark. Earlier with my d'Uccles, who seem to pop combs by 2 weeks if they're cockerels.