The bird's body is covered with a layer of feathers called the contour feathers. Only the feathers covering the body have the interlocking barbs on the end. On the overlapping part of the feathers, there are no barbs. The unbarbed section of the feather traps more air, which provides greater insulation. Strong flyers, such as swallows, have longer, stronger barbs than do poorer flyers.
Young birds have a very efficient insulation against the cold.
Even before the layer of contour feathers develops, the young birds are covered with down. This light, loose structure traps a lot of air near the bird's body. A covering of down feathers is warmer than a heavy layer of contour feathers. In cold climates, some species of birds have a layer of down feathers underneath their contour feathers.