Barn owl: A deadly hunter

Hidden beneath the barn owl's graceful, ghostly exterior is one of nature’s most finely tuned killing machines.

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Barn owl hunting at dusk

The barn owl has provided some of Springwatch’s most memorable moments. But hidden beneath its graceful, ghostly exterior is one of nature’s most finely tuned killing machines. Dominic Couzens reveals all.

A wavering, ghostly shape appears out of the gloaming, floating silently over a field. Your heart misses a beat and your senses seem to burst and freeze at the same time. It’s the barn owl, one of Britain’s most charismatic and sought-after birds. An encounter with this species is both thrilling and a little unsettling at the same time.

But why? To be honest, the pale plumage, small eyes and somewhat severe, heart-shaped face tend to cut the barn owl adrift from the friendly, ‘wise old owl’ persona of children’s literature – the one exuded, for example, by the tawny.

Instead, the barn owl appears more ghostly, more other-worldly and, especially when flying low along a beat on the edge of a field, it just seems to look more mechanical than other owls and birds. If you agree with this last impression, then you might be interested to know that it is closer to reality than you might have first thought.

Complex wiring

The barn owl isn’t a machine, of course, but within that head is an extraordinarily complex web of senses, a battery of detectors and wiring that would do credit to any modern electronic creation. In fact, it’s such a busy information highway that there probably isn’t enough room for wisdom.

The barn owl has to make its living in an exceptionally difficult way – catching fast-running animals that hide in dense foliage in the dark – and so it requires concomitantly extraordinary abilities.

Sharp ears

Its sight is excellent in low light, but its true headline ability is its hearing. Indeed, there is a case to be made for the barn owl having the most acute and strongly targeted hearing of any animal in the world.

Some years ago, American scientists performed a remarkable experiment with this species. They light-proofed a 14m by 4m room and covered the floor with dead leaves to a height of 5cm. Then they introduced a barn owl and a number of deer mice into the controlled conditions.

They allowed the bird to acclimatise and then turned off all the lights, making it completely dark. Amazingly, they found that the owl could still hunt successfully without using its eyes; all it needed to make a kill was to hear the rustle of the mice in the leaves. Furthermore, when the owl, flying blind, struck, it almost never missed.

Silenced feathers

There are several astonishing aspects to this discovery, but let’s deal with the least obvious first.

If we accept that the barn owl more or less always hunts in flight, then we have to appreciate that, if it is to hear so subtle a sound as the rustling of a mouse, its own flight must be silent. This is quite a feat and achieved by a number of adaptations to the bird’s plumage.

Firstly, all of the wing feathers have a soft, downy surface, which eliminates the noise of them rubbing together. Secondly, the primary wing feathers (those that form the wing tip) have a fringe guarding their leading edge that is a bit like a comb and helps to order the flow of air smoothly over the wing.

Thirdly, the trailing edges of the wing feathers also have soft, hair-like fringes, which apparently damp down any turbulence created by the streams of air that meet behind the wing. These latter features are hard to understand without a knowledge of aerodynamics, but at least we can admire the fact that they do work.

Then, of course, we can marvel at the accuracy of the barn owl’s hearing – it found the small mice in a big room. It would be worth carrying out an experiment to see whether a human could catch a mouse by hand in complete darkness (do have a go and let me know how you get on).

How barn owls hear

The difference between this bird and a human is in the way it detects and locates sound. Here, once again, the barn owl has some remarkable adaptations.

In terms of actually picking up noise, studies have shown that its heart-shaped facial disc – such a clear and distinctive feature of the species – is made from stiff feathers that reflect any sound towards the ear openings on the sides of the head, thus amplifying the overall signal.

This is the principle of the parabola, where an umbrella-like reflecting surface, such as a satellite dish, concentrates widely separated waves into a point. The importance of the facial ruff is such that if the feathers are removed experimentally, the barn owl can no longer catch prey in the dark. With it, however, this bird can detect sounds that are too quiet for us even to pick up.

Selective hearing

Now, here’s a truly scary fact. Those in the know about the barn owl reckon that this bird’s auditory senses are so good that it can be selective while hunting.

This owl is so adept at distinguishing between frequencies that it is thought it can tell whether a rustle or squeak in the vegetation below is being made by a woodmouse or a bank vole. There is a distinct probability that it can also tell whether it is an adult or juvenile, or perhaps even a male or female – and who knows what else?

In our ignorance as observers, we might think that all the owl requires when it’s hunting is to find something – anything – to eat. However, a barn owl leaving its roost at dusk may have other ideas, equivalent to us thinking: “I fancy a Chinese tonight.”

Pinpoint precision

However, that’s not all. One of the barn owl’s most extraordinary adaptations is related to how it can pinpoint sounds: in other words, telling precisely where a noise is coming from.

Humans can do this quite well by assessing the minuscule time difference that the sound waves register when they reach each of our two ears. If the sound is to our left, for example, we will hear it in our left ear before our right ear, albeit only a tiny fraction of a second earlier – but still enough for our senses to appreciate the difference.

The barn owl can do this, too. However, its hearing has an extra dimension. The ear openings on either side of its head are not opposite each other; the one on the left is slightly higher than the one on the right. This means that a sound coming from directly below will be picked up by the right ear before the left ear, and thus the vertical difference will be accentuated.

Background disturbance

Now, this is all very well in the quiet of a blacked-out room, but the barn owl faces a very different set of conditions in the wild. Its world will normally be crowded by extraneous noises and interruptions, such as wind and the sounds of other animals and people.

Once again, we should sit back and be amazed that this bird can cope with such confusion – think how difficult it is for us to concentrate when there is a busy soundtrack in the background.

 

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