Hi Sue An abrupt change in habitat was probably the cause. If there was no visible trauma, then you can rule out predation as this can be a major threat in a new setting. Was the swan being fed by people to supplement his diet? Starvation or poisoning could also be a problem in a new habitat. The swan may not have known where or how to find food or may have eaten something it should not have eaten. There could also have been stress related problems from being suddenly removed from his home. He may have missed the duck but that probably was not the cause of death. Do you know how old the swan might have been? Old age may have been a factor. Without a necropsy, it is very difficult to know if there was poisoning, starvation or illness. We would strongly suggest that if an animal is going to be abruptly removed from its home, that a companion be taken with it and they both be placed in a pen to introduce them to their new habitat and human caregivers. The pen should be 1/2 in water and 1/2 on land with a feeder inside so the birds know where and how to feed. The pen should be enclosed top to bottom to protect from predators digging or climbing into the pen. The birds should stay in the pen for 2 weeks to acclimate themselves to the new habitat. The Regal Swan