There’s little about a Snow Bunting that isn’t perfectly suited to life in the deep freeze. These winter wanderers are outfitted like little polar explorers, with a natural down parka of dense white feathers that cover even the birds’ ankles and base of the bill—and keeps their exposure to cold at a minimum.

https://www.allaboutbirds.org/are-snow-buntings-north-americas-hardiest-songbird/