Walking the dog at dusk out on the Cambridgeshire fens mid-May, lots of swallows around, meadow pipits, yellowhammers, the inevitable wood pigeons, collared doves, starlings and blackbirds, a few LBJs (little brown jobs), chaffinch, house martins, robins, (barely glimpsed, but certain) goldcrests and more. Heading along the lode thought I saw a little egret out of the corner of my eye, but turned to see a beautiful barn owl (Tyto alba) in the lowering sun circle the fields, hunting small mammals, worrying the skylarks on their nests.
Barn owls are well known as silent fliers. Their wings are huge compared to their body size and mass, they are also curved. Both characteristics are evolutionary adaptations to their hunting technique. They can move very slowly with barely a flap even in the lightest of updrafts almost hovering over prey they have sighted. But, it is the structure of the feathers which makes them much quieter than other raptors allowing them to hear prey without the background noise of their own wings.
The barn owl’s wing feathers are soft which smooths airflow, reducing noisy turbulence. In addition, the leading edge of their foremost wing feather (the 10th primary) is fluted appearing to have tiny barbs (that have tiny barbs upon them) that break up the airflow hitting the wing and again reduce noisy turbulence. It might be that the flutings raise the frequency (pitch) of the sound above that audible to prey and perhaps the owl too.
Research at Cambridge University into the silence of owl wings might be used to reduce noise pollution from wind turbines, details here.