How do rabbits survive winter?

In summer they turn brown, but in winter they turn white.

That's true of some wild breeds, but not pet rabbits.

Generally speaking, mammals (including rabbits) adapt to cold weather by burning more protein and carbohydrates, and growing thicker fur.

Pet rabbits that live outdoors might benefit from a slightly different diet than indoor bunnies (like alfalfa hay, instead of just grass hay, and a different kind of pellet).

But pet bunnies don't adapt to freezing cold weather: they get hypothermia and die. Even if it's not that cold, rain and wind can lead to hypothermia.

If you have outdoor bunnies, you should make sure they have proper shelter. In the winter, you may have to bring them inside. See the related questions below for more info and helpful links.