Bird feathers have evolved into a certain geometry where the drag of the wings are minimal while the weight of the bird itself is also limited. These two properties: lightweight and low drag friction, is achieved through an intricate microstructure.
The fractal nature of the feather can sometimes be confusing. In every scale order of magnitude, the structures appear to repeat themselves. It may be helpful to pay attention to the scale of every image. The largest image presented is only 1 mm across.