Feathers have three main functions – insulation, display and flight. Each feather has a central rachis with rows of barbs on either side. Those intended for insulation and in some for sexual display, have the barbs remaining loose and fluffy. Flight feathers on the other hand have barbs that are hooked together to form a continuous vane.
The images by Johnny Wee of the Olive-backed Sunbird (Cinnyris jugularis) (above left) and the Orange-bellied Flowerpecker (Dicaeum trigonostigma) (above right) are excellent examples of male feathers for sexual display.