The secrets of silent flight

Tawny owls are rarely seen in daytime but are supremely adapted to night-time hunting.  Although they have good night vision they hunt mostly by sound.  They can hear the footsteps of a mouse in the dark and have evolved silent flight so the prey doesn't hear the hunter's approach and so they can still hear it during the attack.

Like most owls, tawnies have large wings which allow slow flight and slow wingbeats but much of the secret of the silent flight is in the feather design.  Last summer I found two moulted tawny owl primary flight feathers in the garden and I recently took a closer look at them.  

They have three special features which are different from other bird feathers.

The leading edge of the feather shows serrations which are tiny comb-like structures that modify the airflow over the wings to reduce noise although the exact mechanism is not known.  The serrations are apparently most well developed on the 10th primary (P10) which directly meets the airstream but are also present on P7-9 (I don't know which feather this is).  Interestingly serrations are much more developed in nocturnal owls (such as the tawny owl) than in diurnal owls.

For comparison here is the leading edge of the owl feather (L) compared with a primary flight feather from a wood pigeon (R).

The upper surface of the owl's feather has a velvet-like covering which is thought to deaden the sound of the feathers moving over each other as well as stabilising airflow to allow slower flight.  The velvet was a bit patchy on this feather but bear in mind it was a discarded moulted feather that had been in use for a year or so.

Here is a comparison of the owl feather and the pigeon feather.

The trailing edge pf the owl's feather is fringed.  The fringes prevent separation of the airflow between adjacent feathers, again reducing turbulence and reducing noise.

Here again is a comparison with a wood pigeon feather.

The amount of fringing varies along the length of the feather although I don't know if that is a function of wear.  This is at the feather tip.

The feather photos were taken with a Canon 100mm macro lens plus stacked Raynox DCR-150 and DCR-250 conversion lenses, giving +12.8 dioptres in addition to the macro lens.

You can read a lot more about the science of owl feathers in a Royal Society publication here.  You can watch a fascinating short video of a pigeon, a peregrine falcon and a barn owl flying in an acoustic laboratory here.