We are sorry for your loss. There could be any number of causes. If there is no visible signs of outward trauma, the only definitive way of knowing would be to have a necropsy conducted.

She may have been sick with a bacterial, fungal, viral or parasitic infection or she may have gotten into something that was toxic and poisoned her. Is there any lead or organophosphate fertilizers, insecticides, or poisonous plants in the habitat? If a necropsy is conducted, you might want to have a toxicology report conducted.

If she was not sick, she could have choked on food. Again, anything could have caused her death. Without a necropsy, there is no definite way of knowing. The Regal Swan