Feathers have a unique structure that contributes to their low terminal velocity. They consist of a central shaft with many barbs branching out from it. These barbs are further divided into barbules, which create a fluffy, three-dimensional structure. When a feather falls, the air trapped between the barbs and barbules creates a cushion that resists its downward motion. This structure effectively increases the feather's drag coefficient, allowing it to reach a lower terminal velocity compared to a smooth, solid object of the same size and density.
For birds, a low terminal velocity is advantageous as it enables them to control their descent during gliding and landing. This level of control assists birds in landing safely and precisely on trees and other surfaces. It also assists in maneuvering in different environments and flying more efficiently, conserving energy during long flights.
Additionally, some birds use feathers with reduced barbules to create specialized feathers for specific purposes, such as rapid flights or displays.