Is there a specific reason that you need to know the gender at this particular moment? If not, we would suggest that you wait until at least 6 months of age. You can take them to the veterinarian and get a DNA blood test (which is the most reliable means of sexing), but it is not worth the risk of the parents being upset and trying to harm or reject the cygnets. Once the birds are older, the less protective the parents will be and if they choose to reject them, the young birds will have learned most of their knowledge about being swans from their parents and will be able to survive on their own.

Sometimes, the first cygnet (male) hatched will tend to stay with dad while the mom takes care of the rest of the brood. The Regal Swan