system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How do birds (ducks, geese etc.) not freeze when swimming and dunking their heads in ponds in the wintertime when it’s - degrees out?
gpt
They can be hurt by exposure, but they have evolved methods of (usually) avoiding it.  For one, water birds have a thick layer of feathers covered in oil that repel water.  This prevents the cold water from touching their skin directly, except on their feet.  

The main things birds have to worry about is damaging their feet through exposure to cold water since they aren't protected by feathers.  Birds have to limit their time in cold water to avoid loosing too much heat.