As long as the lake is not frozen or boiling, it probably does not matter. There are several other factors which are more important. The first, is whether the swan species is from the Northern or Southern Hemisphere. Northern birds are prepared better for cold weather while southern birds are better equipped for warmer weather. Then, the abundance of food and nesting resources, shelter from inclement weather and scarcity of human and predator disturbance are all factors that the birds must consider. The Regal Swan