The mum swan could be injured or suffering from some type of illness. You need to call the RSPCA or a local swan sanctuary. They need to capture the family and have all swans checked by a licensed veterinarian. If they think the cygnets are old enough to care for themselves, then they may not need to capture all swans.

However, the mum swan needs to be checked to help her heal if there is an injury or illness. If it is an illness, then it could spread to the other swans. So, it is critical the mum swan is captured, examined and if necessary, treated before her return to the habitat. The Regal Swan