Thank you for your answer. Unfortunately, there was visible injury--she appeared to be decapitated. I don't know if this was because she was lingering too long with her neck under water, or if the culprit was not a snapping turtle, but a fox (although I did find her in the water, near the shoreline). I will remain hopeful that the father will continue to care for these cygnets. So far he remains protective of them and lets them feed first when I bring out waterfowl food.