Squirrel nests are warm, waterproof, and very protective. They work hard to layer twigs, leaves, and materials taken from humans to make walls that are as impenetrable by prey as possible. Their nests aren't like birds' nests that are open from the top. Squirrels' nests are like caves that are closed from the top and sides with just a tiny opening to get in and out. Their nests are so well insulated they block winter winds, snow, ice, rain, and just about anything else that nature can throw at them. Squirrels nestle inside and their body heat is retained by the insulation, making a tiny little climate-controlled haven that's perfect for their families.