Yes I agree with hinkjc. Not all breeds are able to be sexed by wing sexing-fast feathering I don't know the ones that are that's a good place for some of the genitic studiers here on BYC to answer. And I might add sometimes they just amaze me with their facts and information.
Now my little story from experiencing my first go at buying wing sexed chicks.
This spring after easter I decided to buy some new babies to introduce some new blood and some different color in my mix, you know kinda break up the sea of mostly red in my flock. Just had a crazy notion to get them you know what its like walking in the feed store and there they so tempting and making it hard to walk away from them so I gave in and bought some. I don't know why I'd just hatched out around 300 red babies for selling mostly. I got 5 Gold sex-links because I surely didn't need any more roos that's for sure. Thought I was done I had satisfied my addiction, Oh No I wasn't done yet. The next week there I go to the feed store again for guess what more feed only and Lo and Behold there they are again more temptation. This time they were Barred Rocks a cage full, must have been 2 of 3 hundred of them little black and white fur balls. So I decided to get 5 more chickens remember I'm an addict, and I asked did she know the sex of these, were they marked someway or something. She said no but she could sex them by wing sexing I kinda hesitated for a moment(knowing better the whole time in the back of my mind) but i said: "O'yeah, alright, I want 5 pullets and you to show me how to do this anyway." She went though about twenty or so looking and showing me differrences the whole time. She said ther you go 5 little pullets. I left there with a smile I got me another bump(fix) you know. Well....3 wks. later I'm starting to get that confused feeling about my pullets, so I say: "I'm game" I'll give them another wk. or so to tell more about my PULLETS. Well she was darn near on the money at 6 wks I had 4 perfect light barred roos 1 little Dominique pullet. I got my lesson anyway you can't sex Barred Rocks by fast feather sexing, Unless you do it backwards heck I don't know anymore. And bless her poor heart she sure should have made a killing off that cage full of roosters selling them as pullets.
The life and trials of catdaddyfro
Remember to eat your Peanut Butter and Bananna sandwiches. They're good for ya.