The answer would be NEVER. The male mute will kill the cygnet. Unless the male is a very young bird (under 1 year of age), you could not safely introduce the female until she is at least 1 year of age as the young male may not show aggression, but could physically hurt her trying to mate with her. How old is the male swan? What is the gender of the cygnet? How do you know the genders, i.e., has DNA Sequencing been conducted? Cloacal sexing is the least reliable if the swans are very young. The Regal Swan