Owls are known as silent predators of the night, capable of flying just inches from their prey without being detected. The quietness of their flight is owed to their specialized feathers. When air rushes over an ordinary wing, it typically creates a “gushing” noise as large areas of air turbulence build up. But the owl has a few ways to alter this turbulence and reduce its noise.
First, the leading edge of the owl’s wing has feathers covered in small structures (hooks and bows) that project out from the wing. One hypothesis is that these serrations break up the flowing air into smaller flows that are more stable along the wing. Furthermore, this change in airflow patterns also appears to reduce the noise of the flowing air. The wing’s serrated leading edge appears to be most effective at reducing noise when the wing is at a steep angle—which would happen when the owl is close to its prey and coming in for a strike.
These smaller airflows then roll along the owl’s wing toward the trailing edge, which is comprised of a flexible fringe. This fringe breaks up the air further as it flows off the trailing edge, resulting in a large reduction in aerodynamic noise. Then, any remaining noise that would be detectable by the owl’s prey is absorbed by velvety down feathers on the owl’s wings and legs. These soft feathers absorb high frequency sounds that most prey, as well as humans, are sensitive to. All together, these feather features enable owls to remain undetected when they fly.
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This summary was contributed by Ashley Meyers.
“Barn owl wings differ from those of other birds and aircraft… The distal wing of barn owls resembles a slightly cambered plate…over which the airflow normally tends to separate, especially at low-speed flight… Serrations at the leading edge prevent a separation and increase the lift by generating a turbulent boundary layer over the airfoil upper surface… The turbulent boundary layer delays leading- and trailing-edge flow separation to higher angles of attack…similarly as shown in experiments by Soderman (1972) at higher Reynold’s numbers.
As mentioned above, Neuhaus et al. (1973) showed that serrations influence the noise generation only at steep angles of attack. One explanation might be that in cruise flight conditions the stagnation point at the leading edge results in a relative low air flow through the serrations due to the low air stream velocity. At sharp angles of attack, however, serrations comb through the air like a plough through a field. By their bending and orientation, serrations induce tiny vortices running over the dorsal wing surface as shown by Ito (2009). This phenomenon increases the lift and reduces the noise of barn owl wings during flapping flight and striking, which is extremely useful for the owl.” (Bachmann and Wagner 2011: 200)