The swan may have gotten off the beaten path (migratory) and was either chased or landed near you and your deck was the safest place for it during the night. Hopefully, you do not have any dogs, cats or wildlife that could have chased it. Once the swan got its energy and bearings, it probably went back to flying to its winter migration area. As far as shivering, if there was something wrong with the bird, it probably would not have been able to continue. Swans have many feathers and a great deal of down to protect their internal heat, so even though it is very cold, the swan will still stay rather warm. The Regal Swan