It is possible, but you would first need to look at the carcass to see if there was any visible trauma. Missing parts could indicate a predator such as fox, mink, or snapping turtle. If there was no trauma, it might indicate that the cob may have drown the bird or that the bird died from natural causes such as age or illness. Usually, Blue Herons will kill young cygnets, but they defend themselves with their long sharp bills. Are there any visible damage to the swans? Without having seen any trauma injuries to the Heron or having been there to see what it caused its demise, it is very difficult to blame the male swan. It is possible that the swans saw the bird die or killed by a predator that they are agitated and are much more protective of the cygnets and aware of their surroundings. The Regal Swan