if both ybs face same way in nest they are same sex, if both face opposite ways they are one of each, this is approx 90% accurate, dont know if this helps? cant remember if its the cocks or hens that usually face towards back of nestbox,? i have also found that when ringing say with rings 11 and 12, i ring the bigger yb with the 1st ring, again approx 90% of the time the first one rung will be the hen