I live in a region with cold winters (ca. -40 C) and many quite small birds (sparrows, crows, ducks) stay for the winter.
How can the birds survive this?
As I understand it, smaller bodies will be less able to produce and retain heat. Despite this, birds seem to cope much better than humans, sitting in the cold all winter and even swimming in cold water. In comparison, humans lose mobility in their muscles when cold, and can suffer severe frostbite causing them to lose fingers, toes, or limbs from relatively short exposure to the same conditions.
What specific adaptations allow birds to cope so well in these conditions?