How Do Owls Fly Silently?

Bird Adaptations for Flight

The body of the bird is specifically designed for flight.  For the bird to gain lift and maintain flight, it must be lightweight.  This is mainly achieved through its hollow bone structure.  Also, many of the bones in the bird’s skeletal system are fused together, which helps to reduce weight and create a rigid support system.

Another means of weight reduction are the reproductive organs, which are kept small until they are needed during the breeding season.

The respiratory system of the bird is proportionately much larger than a mammal’s, about one-fifth of its body volume, whereas the average mammal is one-twentieth, which allows it to maintain the energy expended during flight. [Stanford University]

How Birds Fly

Its wings are its key component when it comes to flight.

The bird has strong breast muscles that it uses to push its wings downwards.  With the wings fully extended, this motion pushes the air downwards, generating lift.  The hinged wing then folds up on the upstroke.  The action is similar to pumping your legs on a swing to gain momentum.  The pressure is lower above the bird’s wing because the air travels faster above the wing than it does below. [University of Illinois]

The air that rushes over the birds’ feathers creates turbulence, which in turn creates sound audible as it soars through the air.

How Owls Fly Silently

The noiseless flight of the owl can be credited to three physical attributes of its unique wing structure: the soft serrated edges of its leading wing, the flexible fringe on the trailing edge of the wing, and the soft downy feathers at the top of the wing.

The leading wing feathers, also called the stealth feathers, are serrated, which play a small part in the reduction of noise during flight, but it’s the fringe at the trailing edge of the wing that allows for the greatest noise reduction by breaking up the sound waves that are produced as the air rushes over the wings during flight.  In addition, the entire top of the owl’s wing is covered in a velvety downy surface, which acts as a sound absorber.  These things in tandem change the aerodynamics leading to the suppression of the sound that would normally be generated and radiated from the owl’s wings. [Jaworkski]

Silent Flight Makes Owls Efficient Predators

An obvious benefit to silent flight is the owl’s ability to sneak up on prey without being heard. The hearing of its prey, such as mice and voles, is acute between 2-20kHz. The gliding motion and flapping of the owl’s wings generate noise at low frequencies below 2kHz, making the owl silently invisible. [Lilley, G]

The silent flight also benefits the owl while hunting because it allows it to hear more efficiently.  Soundless flight and the position of its asymmetrical ears help the owl to more accurately identify from where a sound is emanating, which means quickly and efficiently pinpointing the position of its prey.

Owl Flight Could Be a Model for Quieter Commercial Flight

Understanding the exact mechanisms used in silent flight can be used as a model in any noise-reducing application where the surface is subjected to flow fields such as aircraft and submarines, of which the military implications are obvious.  Developing this technology could also help to solve the issue associated with commercial aircraft noise at major airports and the development of silent wind turbines. [PubMed]

Resources

Bachmann, T; Frontiers in Zoology
Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia.
2007; 4:23
http://www.ncbi.nlm.nih.gov/pubmed/18031576

Stanford University
Adaptations for Flight
http://www.stanford.edu/group/stanfordbirds/text/essays/Adaptations.html

University of Illinois Department of Physics
How Do Birds Fly?
http://van.physics.illinois.edu/qa/listing.php?id=760

Jaworski, J; American Physical Society
Vortex Noise Reductions from a Flexible Fiber Model of Owl Down
http://absimage.aps.org/image/DFD13/MWS_DFD13-2013-002069.pdf

Lilley, G; American Academy or Aeronautics and Astronautics, Inc.
A Study of the Silent Flight of the Owl
ICASE, NASA, Langley Research Center, Hampton, Virginia
http://arc.aiaa.org/doi/abs/10.2514/6.1998-2340