Are there signs of any trauma? This could indicate a predator or being hunted by man. If there are no signs of trauma, then the only other possibilities would be the intense weather. The birds could have died from drowning, starvation or not being able to keep warm and dry, thus dying from illness. Swans and other birds can become frozen to the ice or land. Not being able to move for hours can significantly hamper circulation, thus facilitating quicker body heat dissipation. Is there anyone in the area that you could report the deaths to see if they can necropsy the birds to determine the cause of their demise? The Regal Swan