Macro photo of bird feather barbs and barbules. This parakeet feather shows the barbs of a feather that have lines of barbules on both edges. The barbule filament structures zip together to shape the feather and to hold its surface rigid. The feather is made of a substance called beta-keratin. While preening, a regular activity, the bird's beak and tongue can re-zip any places where the feathers barbs have come apart. Sick and distressed birds can sometimes be recognised by the state of their feathers because they have ceased to maintain them. A healthy birds feathers should be glossy and show few breaks.