Penguin feather, coloured scanning electron micrograph (SEM). The penguin (family Spheniscidae) has the most densely packed feathers of any bird. Here, barbs are seen running diagonally, with hooked barbules perpendicular to the barbs. The hooks prevent the feathers from sliding relative to one another, increasing their effectiveness as insulators, as well as allowing the birds to become more streamlined in the water. A layer of oil is secreted onto the feathers to make them waterproof.