Hi Matthew
Mute Swans are built for cold temperatures becsuse they are Northern hemisphere swans. Southern hemisphere swans such as Australian Black Swans cannot suvive in colder temperatures.
Although we cannot give you precise temperatures, we can state that swans or other waterfowl cannot survive any great length of time in iced-in waters. Ice is the greatest danger as food resources are scarce to non-existent which is why supplemental feeding is critical.
Ice prevents the birds from eating and drinking. Starvation and dehydration can occur. These and other stresses can induce illnesses such as avian flu and pneumonia.
Furthermore, if the ice should break and the birds cannot get out of the freezing water, they can die from drowning and hypothermia.
Lastly, ice can cause the birds to freeze on to the ice or prevent their escape from predators. Predators can walk across stretches of ice to attack and kill birds or other wildlife trapped on the ice.
It is for these reasons and more that birds migrate. It is captive waterfowl that are in the most danger during extreme cold temperatures.
Starvation can only take a few days to render the birds ill or dead. This is why we tell pond owners to use aerators/de-icers during the winter months. These devices move the water and prevent ice. When using aerators and de-icers, they must be used consistently at the first sign of a freeze and you must ensure that any moving parts, straps, ropes or other possible entanglement parts are placed in an area or housed in a compartment that are inaccessible to the birds. We hope that this information is of benefit. The Regal Swan Foundation