I think your darker one may be single barred (B/b+) and not B/B which is what you want for a golden cuckoo. If you keep the darker one and he is indeed B/b+ then half of his female offspring will be unbarred.
My theory also makes sense with you saying he looked like a girl chick. Girl GCMs are B/- (they have one barring gene because females only carry one copy of that gene) and so if he was B/b+ he would have a smaller head spot and be darker as a chick (just like a female)
just my 2 cents
My theory also makes sense with you saying he looked like a girl chick. Girl GCMs are B/- (they have one barring gene because females only carry one copy of that gene) and so if he was B/b+ he would have a smaller head spot and be darker as a chick (just like a female)
just my 2 cents