Why can a lumpy bird sit in the cold? What keeps her warm?

Air conducts heat very poorly. In houses, people make frames with two or three glasses. There is air between the glasses, which prevents heat from escaping from the house.
When it gets cold outside, the birds find a calm place and sit there, fluffing their feathers. Thus, they increase the amount of air between the feathers, which does not allow heat to pass through. In a calm place, the wind will not blow air out of the feathers. The bird keeps itself warm, and the air between the feathers prevents heat loss.