The feather is a complex structure made, like the nails and part of the beak, of keratin and composed of a pipe (the calamus), planted in the epidermis and extended by a rachis which is the central axis of the feather. On both sides of the rachis are the vexillae, each of which is made of rows of barbs composed of hundreds of barbules which are linked to one another by thousands of barbicelles, tiny hooks that look like small hooks that ensure the cohesion of the whole vexilla (Figure 1).
Modern birds have seven types of feathers whose morphology depends on the function they perform: vibrissae, filoplumes, down, and four types of closed, rigid feathers (Figure 2).
The latter are the semi-feathers, contour feathers, remiges and rectrices, the latter two being the functional instruments of flight itself. Most of the others have a protective, insulating or demonstrative function except for the vibrissae, which are sensory organs located around the beak that facilitate prey location.
Notes & References
Cover image. [Source: Ashlyak at ml.wikipedia, CC BY-SA 3.0, via Wikimedia Commons]