Although there are no guarantees, swans found in the Northern Hemisphere are built for cold temperatures, even on the tundra. So, as long as the mother swan does not leave the nest/eggs for any great length of time, her body heat should keep the eggs protected.

The problem arises when the mom takes a 30 minute break and the eggs either get too cold or the weather changes quickly and the eggs are subjected to extreme heat one day, cold the next and then heat again. All of their survival and viability depends on the mother swan staying on the eggs and protecting them from extreme temperature and weather conditions. The Regal Swan