We are sending this question to The Regal Swan veterinarians and will try to provide an answer in the near future. In the meantime, have you had a necropsy done on the cygnets to see if they are being poisoned by something in the water, i.e., algae, botulism toxin, or if it is genetic or an illness, etc.? We would strongly suggest that if another cygnet expires, you have it necropsied. The Regal Swan