[mbaldw]

Hi Hedgehoggy,

thanks for your kind words regarding my website – most appreciated.

So perhaps I can conclude that the parents are dumping their youngsters in my garden while they go off hunting!

Given that you appear to have fledglings in your garden, maybe the owls did roost there. As I understand the situation, the owls leave their nest and move out into nearby trees and bushes where they wait (often highly vociferously) for their parents to return with something good to eat.

What I find so amazing is that their flight is sooooo eerily silent.

That is very cool. I had always been told (and read) about owls being silent in flight, but I was still amazed the first time I saw a Tawny owl fly off without making a sound. The ‘secret’ to their typically silent motion lies in their skeletal anatomy and their feathers.

The skeleton is very light; many of the larger bones are hollow and air-filled. In conjunction, there are several aircells situated around the body (some of which integrate into the hollow wing bones) that provide buoyancy. Additionally, the majority of the flight muscles attach to the keel (breast bone), which takes their weight out of the wings. The skeletal adaptations, coupled with broad wings means that owls have what aeronautical engineers call a low “wing-loading”. In other words, owls have a large wing area for the weight of the bird – the larger the wing-loading, the more effort is required remain airborne. It also controls how easily birds (e.g. raptors like buzzards) can soar; the higher the wing-loading, the larger the turning circle and thermal pocket required to gain height. Anyway, with a low wing-loading, the bird can fly buoyantly and with ease through the sky. So, Tawny owls -- which have a wing-loading of 0.40 grams per sq-cm -- can fly rather effortlessly, with few flaps of their wings. Compare this to, say a grouse, which has a wing-loading of 1.34 g/sq-cm and needs to flap like crazy to say in the air*. Basically, fewer noisy flaps and more quiet gliding (it saves energy too).

It’s not just the anatomy of the skeleton and wing that explains the silent flight – the feathers are perhaps the most important component. Each wing has ten primary and ten secondary feathers numbered descendently – so, the tip of the wing has primary ten, running down to primary one at the bend of the wing; here secondary one is situated, running to secondary ten. Secondary ten is separated from the tail by three tertial feathers. It is the primary ten at the edge of the wing that we’re particularly interested in, because each barbule has a stiff, hair-like projection at the tip – together, these form a comb-like fringe (ca. 2.5mm wide) along the feather’s leading edge. In addition to this ‘comb’, the primary and secondary feathers on the trailing edge of the wing possess a soft, hair-like, fringe and the wing itself is covered with a soft, velvety down. These three features combine to moderate the passage of air over the wing, helping to reduce the turbulence and hence noise. This may all sound great (and it certainly helps the owls move about unheard, unless the feathers are worn, wet or being moulted), but as A. A. Wardhaugh points out in his Owls of Britain and Europe:

“Unfortunately for owls, these sound-deadening adaptations cause more drag or resistance to the air during flight.”

At any rate, knowing how it’s done, doesn’t make it any less spectacular to witness

Cheers,
Marc.

* The values I’ve given for wing-loading are those presented by Mikkola (1983), who also quotes Brull’s 1964 study, giving a wing-loading for the Barn owl of 0.29 g/sq-cm. However, in his Owls of Britain and Europe, A.A. Wardhaugh gives the wing-loading of the Barn owl as 3 kg/sq-m. By my reckoning, this equates to a wing-loading of 3 g/sq-cm by Wardhaugh’s reference – an order of magnitude greater than Mikkola/Brull! I haven’t checked up the morphometric details to work it out for myself yet!