Feather sexing Black copper Marans and D'uccles?

[meganrebecs]

Anyone familiar with feather sexing? I have a few of these guys who just hatched last night. Saw a video somewhere on here which showed that you could sex the birds based on the length or their wing feathers if they were under 2 days old. Looks pretty staight forward....but then I heard it only works for specific breeds....and not bantams?

Any input? Does anyone know what breeds in particular can be feather sexed?

thanks!