Are your bird showing active breeding? Parasites will keep their attention off breeding but if you see breeding going on I would think it is something else. Of course it could be the hens not the cockerels. I am watching my Dorkings pretty close to see if I see these issues. So far the eggs are fertile but we are in the Southern/coastal part of SC.Hmmm on the parasites. I will check. It's about time to dose them with Ivermectin, though I haven't really seen anything, it surely won't hurt. The temps only are in the low 30's here at night, and the roo seems active enough, and since I've already hatched eggs from him and these same girls I am befuddled, unless like you said, it's either parasites or the light.