Any number of things could have killed the cygnets. We have seen fungal, bacterial and viral infections, insecticide or other chemical poisoning, egg sac peritonitis or the parents accidently stepping on them. So, without a necropsy, there is no way of knowing the cause of their demise. All swans need water to eat, especially the young. As long as the bowl is nearby it is okay, but the best solution is to have the food at the bottom of the bowl covered by water making sure that the food is always covered by the water. This will ensure that the cygnets wlll get water with their food. Their deaths do not necessarily mean that they died from something that they ate. So, don't beat yourself up over this. The Regal Swan