Chapter 4 of 23

Built for the Night

Silent wings, sharp ears and eyes that gather light

About 7 minute read

A Barn Owl is one of the most finely tuned night hunters on the planet. Almost every part of its body seems to have been shaped for the job of finding a small animal in the dark and grabbing it without being noticed. In this chapter we will go through the main tools of the trade, one at a time.

Wings that make no noise

Barn Owls fly in near silence, and there is nothing mysterious about how they manage it. It comes down to the design of their feathers and the way the wings are built.

The front edge of the main wing feathers is shaped like a little comb, with tiny stiff hooks that break the air into gentle streams instead of a rush of noise. The back edge of the wing has a soft fringe that smooths the air as it leaves the wing. The upper surface of the feathers is covered in a velvety layer of tiny hair like structures that trap a cushion of air. Even the legs and feet have very fine feathers to keep turbulence down.

The wings are also large for the weight of the bird. Engineers call this a low wing loading, and it means the owl can fly very slowly without stalling. A hunting Barn Owl often drifts along at walking pace, or even hangs in the air on the faintest breeze. That slow flight gives the bird time to listen, and being silent means the owl can hear its prey while the prey cannot hear the owl.

There is a price to pay for all this softness. Barn Owl feathers are not very waterproof. In a heavy shower, a Barn Owl can gain around a quarter of its body weight in water, and a bird that is completely soaked can end up over three quarters heavier than normal. A wet owl cannot fly properly, and it certainly cannot fly quietly. That is why Barn Owls sit out the rain in a dry roost, and why a run of wet weather in the nesting season can be so dangerous for them. They also carry very little body fat, so they cannot simply sit out a long spell of bad weather on their reserves.

Hearing like nothing else

If the wings are clever, the ears are astonishing. The Barn Owl's heart shaped face is a satellite dish. The stiff, curved feathers around the edge of the disc scoop up sound and bounce it towards the ear openings, which sit inside the disc just behind the eyes. A Barn Owl can even change the shape of the disc a little, rounding it out when it is alert and relaxing it when it dozes.

The two ears are not the same. One sits slightly higher than the other, and they are shaped differently. That means a sound reaches each ear in a slightly different way, and the owl's brain compares the two to work out exactly where the sound came from, both left and right and up and down. The result is that a Barn Owl can pinpoint a sound in front of it more accurately than any other animal that scientists have tested. Experiments with tame owls have shown that a Barn Owl can strike a moving target in a completely dark room, using its hearing alone. Its hearing has been measured as many times more sensitive than that of a pigeon.

Is a Barn Owl bothered by noise? Only a little, as it turns out. Barn Owls can get used to any regular noise. There are nests in busy farmyards, in church towers with working bells, in barn conversions with a family living below and even next to a firing range. What upsets a Barn Owl is the unexpected, especially if the bird has nowhere to hide. A noisy place with a good hiding spot can suit them perfectly well.

Eyes for dim light

Barn Owl eyes are large, dark and packed into the front of the face. They look forwards, which gives the owl good depth perception, and they are shaped more like tubes than balls. That shape works like a telephoto lens, and it is held firmly inside a bony ring in the skull. The retina is full of the light sensitive cells known as rods, which work best in dim light, and the pupils open very wide.

Because the eyes are fixed in place, a Barn Owl cannot roll them around the way we do. Instead, it moves its whole head. That is why you see a watching owl bobbing and swaying its head from side to side. Each little movement gives it a slightly different view, and by comparing them, it can judge how far away something is. It is one of the endearing things about owls, and it is a fine sight when you see it through binoculars.

How good is owl vision, really? Better than ours in low light, certainly, but not the superpower people often suppose. In near darkness, a Barn Owl sees a brighter and more detailed picture than we do and is probably very good at noticing anything small that moves. But it cannot see in total darkness, and on very dark, cloudy nights or in thick fog, it relies mainly on its ears and on its memory of the landscape.

Barn Owls seem to pay little attention to things that stay still. Stand quietly and you may find that an owl completely ignores you, and there are plenty of stories of a bird landing on a gatepost next to a motionless person. As soon as you move, the bird notices and is off. Barn Owls also seem unbothered by ordinary artificial light, including floodlights and airfield lights, though they dislike lights that swing about, because the moving shadows unsettle them.

A neck that turns and feet that grip

An owl can twist its head around by about 270 degrees, which is three quarters of a full turn. It cannot spin all the way round like a film monster, but it can look straight behind itself. This is possible because owls have many more neck bones than we do, roughly twice as many, and special arrangements of blood vessels that keep blood reaching the brain even when the neck is twisted.

The feet are strong and armed with sharp, curved talons. Like many owls, a Barn Owl can swing its outer toe backwards or forwards, so that it grips with two toes at the front and two at the back, or with three at the front, depending on what it needs to do. The soles are covered in tiny rough bumps that help to hold slippery prey. And there is the little comb on the middle claw that we met in chapter 2, a built in grooming tool for the face.

A body that loses heat

One more thing is worth knowing, because it explains a lot about the bird's behaviour. Barn Owls evolved in warmer countries than ours. Their soft plumage is not a great insulator, and they cannot store much fat. In Britain, in cold air, a flying owl loses heat quickly, and it burns a lot of energy keeping warm. Many of the habits you will read about later, from perching on fence posts in winter to squeezing between straw bales at night, are ways of coping with that.