very good post this , i have a mix of darkness and natural yb in 2 sections.and like you ALF have noticed the same ,not all affected ,but when cleaning out ,finding the odd flight or tail feather in both sections .mine were put over end of March first round .also handled a fellow club members birds in the week , and most on the sixth flight .i have assumed like Tony C ,its down to the weather ,not too fussed ,but will be wary of which go to the last races