I keep finding outdated threads, but in case anyone finds this, just wanted to say that I am 2/2 so far with feather sexing of my BCMs. I have 10 in the incubator, so will see on the next batch. My first was an extra BCM sent from the hatchery (Cackle). His wing feathers started becoming coppery on day 5 (or maybe that was just when I noticed!). They are now almost 4 weeks and he is definitely a cockerel. None of the others (pullets) had the copper on the wings.
On day 2 of my own hatch, one of the chicks started with copper on the wings. I guessed (rightly), that he was a cockerel too. It is now day 7 and his comb is pointier, taller and pinker. I can almost guarantee that he is a cockerel.
Anxious to see if my suspicions are right in the next batch, due in 2 weeks. I am leaning very strongly (95%) that feather sexing appears to be quite easy with the BCMs. Granted, they must be of good parentage, as questionable breeding may result in lots of copper leakage. However, copper leakage on wing feathers seems to point directly to cockerels.
Just my opinion. Wish someone above had actually posted their results.