I would have thought the egg laid after "mating" would be fertile if the cockerel is doing the job correctly. I laugh at my young silkies who haven't a clue yet, one youngster grabs the ladies by the head and then wiggles his bottom on the floor facing her!!! Bless he will soon get it right (men are so slow at these things!).
Not all eggs will be fertile especially this time of year
Sue