It's a commonly held misconception that squirrels hibernate. They do spend longer sleeping in their dreys (nests) up in the trees, keeping warm and saving energy, especially during bad weather when they might sit it out for days at a time.They come down regularly to forage for food throughout the winter and even breed through December and January, as well as in the summer.

The cheeky red chap in the picture prefers to stick his head in a box of nuts and pretend that the weather's fine really.