Underfjädrar

Underfjädrar, a Swedish term translating to "under-feathers" or "down feathers," refers to the soft, fluffy plumage found beneath the contour feathers of birds. These feathers are characterized by their loose structure, lacking a central shaft or a tightly interlocking vane. Instead, they consist of a cluster of barbs that radiate from a central point, creating a structure that traps air effectively.

The primary function of underfjädrar is insulation. The trapped air acts as a thermal barrier, helping birds maintain their body temperature in varying environmental conditions, from cold climates to hot weather. This insulating property is crucial for survival, especially for young birds and those living in harsh climates.

The structure of underfjädrar is adapted for this purpose. The barbs are short and flexible, and the barbules, which would normally hook together in stiff feathers, are either absent or poorly developed. This lack of interlocking allows for the creation of a voluminous, airy layer.

Underfjädrar are distinct from other types of bird feathers, such as flight feathers (remiges and rectrices) which are designed for aerodynamics, and contour feathers which provide the bird's outer shape and coloration. The distribution and density of underfjädrar can vary significantly between species, depending on their habitat and lifestyle. Birds in colder regions or aquatic birds often have a denser layer of underfjädrar for enhanced insulation.