**5. Feathers**

Owl feathers are unique in both structure and use. Owls are more heavily feathered than any other bird, even having feathered eyelids and, in many species, feathered feet and toes. In 2017, David H. Johnson (Executive Director of the Global Owl Project) and a small group of volunteers systematically plucked and counted every single feather from the remains of a recently deceased female Great Horned

#### **Figure 8.**

*The primary flight feather of the barn owl (*Tyto alba*) shows the serrated leading edge (fimbriae) and the wispy trailing edge. Photo: Alan Sieradzki.*

Owl (*Bubo virginianus*). This painstaking exercise, taking 46 man-hours of work, resulted in a total count of 12,230 feathers. Full details of this feather count exercise are planned to be published shortly [Johnson, D.H. personal communication].

It is, however, the unique structure and form of the wing feathers, allowing the owl almost silent movement through the air, that are the most remarkable. The owls' nearsilent flight can be attributed to three wing feather adaptations unique to owls—(1) a comb-like leading edge to the primary and secondary flight feathers (fimbriae); (2) a fine, wispy, fringe-like trailing edge to the flight feathers (**Figure 8**); and (3) a velvety covering on the upper surface of the wing and a shiny, down-covered underside [19, 20]. The large wings of these birds, resulting in low wing loading (calculated by the weight of the bird divided by the surface area of both wings) [21], and a low aspect ratio, contribute to noise reduction by allowing extremely slow and buoyant flight. Also, the owl's wing feathers can separate from each other in flight, allowing the air to flow over each of the individual flight feathers. With all other birds, air rushes over the surface of the wing creating turbulence which, in turn, produces noise. With an owl's wing, the comb-like serrations on the feather's leading-edge break down the air into little groups of microturbulences. This effectively muffles the sound of the air rushing over the wing surface, which is further dampened by the velvety coating on the wing's surface and allows the owl to fly silently [22]. A recent study has shown that there is a direct correlation between the size of the facial disc in relation to the length of the comb-like serrations, suggesting that species that rely more on their auditory system for locating prey also have the more silent flight [23]. This also suggests a dual purpose in the need for silent flight, the need for stealth, allowing the owl to approach prey undetected and the need for self-masking, enabling the owl to locate prey by sound while in flight [23]. Such is the effectiveness of the owl's unique wing feathers for silent flight, that the international aeronautical industry is now investing heavily in the research and development of wing design based on the owl's fimbriae towards solving the aerodynamic noise of aircraft [24].

Other uniquely structured feathers of the owl are their auricular feathers. In almost all owl species, the facial plumage forms a parabolic dish with a facial ruff. The centre

*Designed for Darkness: The Unique Physiology and Anatomy of Owls DOI: http://dx.doi.org/10.5772/intechopen.102397*

of the ruff is formed by tightly packed feathers, with thick rachis and dense webbing. Such feathers are also found on the pre-aural flaps which cover the ear openings, and in the region of the beak. The facial ruff made up of auricular feathers, collect and amplify sounds, and direct them to the ear openings [25]. Three different types of auricular feathers occur in the facial disc of the Barn Owl. One type covers the reflector feathers of the disc and dominates the general appearance of the facial ruff. A similar smaller type of auricular feather is situated at the pre-aural flaps. The third type of auricular feather (semi-bristle) is found in the region of the beak and functions as a mechanoreceptor [26].
