Unfortunately, nature is not very kind to young species of any kind. It is truly survival of the fittest or we would be overrun with too many animals on the earth. Many species have a large number of young due to the limited survival rate of their young as well as their young being part of nature's food chain.

Any number of situations could have caused the demise of the cygnets. They include:

Predation

Drowning in severe storms as the young cygnets do not have the adult feathers to repel water, remain warm and dry.

Illness from catching cold because they can't remain dry or warm.

Salt water is another killer. It is one thing for the parents to stay in the area with brackish water for a limited amount of time, but no swans other than the South American Black-Necked Swans have the propensity to filter salt from the water. This high salinity may be too much for the young cygnets and this could also cause their demise.

High concentrations of insecticides, pesticides, fungicides or other chemicals caught in channels and not flushing out of the area could also pose a toxic situation to all swans involved.

Hopefully, the lone cygnet will make it. Good luck. The Regal Swan