Friday, 20 July 2012

On silent wings

I have fond memories of late summer afternoons spent counting and identifying plants on a piece of wet meadow in north Norfolk. On some days I would watch the barn owl quartering the meadow around me, on the wing early because of rain the night before or because there were hungry chicks to feed in the nearby barn. Every now and then the barn owl would pass within a few feet of me while I sat motionless and intent on some plant or other. The thing that still strikes me about these close encounters was the silence of the owl as it drifted by.

The silent flight of the barn owl is something that many have commented upon. When you compare the flight of the barn owl with a bird like a pigeon – which, incidentally, is about the same weight – the difference is striking. How the owl manages to minimise the noise that it makes while flying is all down to some clever adaptations in its wing feathers.

The first of these can be seen with the naked eye if you examine the leading edge of the wing closely; you would need a bird in the hand to do this. The outer flight feather (a primary) and the smaller feather that covers its base (a primary covert) is modified into a comb-like structure which is comprised of a series of gently curving serrations. These are formed from the feather barbs, their modified structure very different to the barbs found on the other flight feathers. Research suggests that the serrations alter the flow of air over the wing and reduce the amount of wing noise.

A second adaptation can be seen on the trailing edge of the flight feathers. Here the barbs are loose, producing a softer fringe to the feather and again reducing the amount of wind noise. The final adaptation, the downy nature of the upper surfaces of the flight feathers, serves a similar purpose. Measurements of flight noise across a range of bird species show that barn owl flight does not generate the frequencies of ultra-sonic noise that are a feature of other birds. This is important for a predator of small mammals which have hearing that is especially sensitive in the ultra-sonic range. There is another advantage of this adaptation for the barn owl, in that it locates much of its prey by ear. Small mammal squeaks, together with the sound of them moving through vegetation, would be lost to the owl if it drowned them out with noisy flight. For a bird like a pigeon there is no selection pressure in favour of silent flight, so it is only in the small mammal-eating owls that we find this adaptation.

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