Three months ago I decided to fill my incubator after a fox took a few chicks and 2 roosters. After about a week I discovered that one side of the incubator was 5 degrees warmer than the other. I decided to split the difference and hope for the best. I think one of my old hens did not have fertile eggs, and I think several eggs were not successful due to the temperature issues, but 8 hatched. All 8 are pullets! Not sure if this is connected to female embryos being more hardy, or just luck (good for once)as I still have plenty adult roosters.