If you have a local veterinarian or veterinary laboratory, you can have a necropsy (autopsy for animals) conducted. If this is not possible, you may want to look at the carcass and see if there is any visible trauma, i.e., was it attacked by a predator? If so, you should see visible tearing and slashing of the skin or tissue. If there are no signs of trauma, it could have died from being beaten by a larger swan. Usually, the only trauma in this case would be that you would notice the swan unable to move or walk due to neurological damage. If there was a viral, fungal or bacterial infection, the necropsy would be able to ascertain and usually, not always, describe a possible culprit. The same thing could be found in the case of a poison or other toxicological cause by conducting a toxicity test by the lab.

All of this is to say, that a necropsy and many tests may or may not be able to ascertain the cause of the swan's death if it was healthy and a predator was not the cause of its demise. Even if the swan was sick, poisoned or had some other neurological indicator such as starvation, drowning, etc., a necropsy may or may not give the exact cause of death, but it could rule out a myriad of causes. The Regal Swan