How Barn Owls Hunt in the Dark: Hearing and Silent Flight

The barn owl (Tyto alba) is the most widespread land bird on Earth, found on every continent except Antarctica, from English farmland to Australian wheat belts. It is 32 to 40 centimetres long, spans a metre or more across the wings, and weighs only 400 to 700 grams, which is less than a pigeon-sized body with a much larger wing would suggest.

In 1971 Roger Payne published the experiment that defines the species. He released barn owls in a completely dark room with a mouse on a floor of dry leaves, and the owls struck the mouse accurately by sound alone. When he dragged a wad of paper on a string instead, the owl hit the paper. The bird was not seeing; it was triangulating.

That ability, combined with flight that produces almost no sound above the frequencies rodents hear, is why the barn owl is the standard animal for research on sound localisation and the standard answer to how an animal can hunt on a moonless night.

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The face is a sound dish. The heart-shaped facial disc is a ruff of dense, stiff feathers that the owl can adjust with muscles, and it funnels sound toward two ear openings hidden under the feathers at the disc's edges. For the frequencies a vole makes moving through grass, roughly 3 to 9 kilohertz, the disc adds in the order of 10 to 20 decibels of gain and sharpens direction.

The ears are not level. The left opening sits higher on the skull and points slightly down; the right sits lower and points up. A sound from above reaches the right ear a fraction louder, a sound from below the left, so the loudness difference between ears gives elevation. A sound from the side reaches the nearer ear a few microseconds earlier, and that time difference gives horizontal bearing. Eric Knudsen and Masakazu Konishi showed in 1979 that barn owls combine the two to locate a sound source to within about one to two degrees in both axes, the most precise passive hearing measured in any animal.

The wings are the other half. A comb of stiff serrations on the leading edge of the outer primaries breaks up the airflow, a velvety pile on the upper surface damps vibration, and a soft fringe on the trailing edge smooths the wake. Together they push most of the flight noise below 2 kilohertz, under what a mouse hears well, and under what would mask the owl's own hearing. Low wing loading, a big wing on a light body, means the owl can quarter a field at walking pace without stalling.

Barn OwlGreat Horned Owl

A barn owl hunts by quartering: a slow, buoyant flight two to three metres above rough grassland, field margins, and ditches, with sudden hovers when it hears something. It may also still-hunt from a fence post. When it commits, it swings its feet forward, spreads the talons, and drops onto the sound with its head drawn back and the facial disc still tracking the prey until the last moment. Small mammals are swallowed whole; an adult needs three or four a night.

The indigestible remains come back up as a pellet, usually one or two a day, a compact grey wad of fur and clean bones. Because the bones are intact, pellets collected from a roost are used by ecologists and school classes to census the small mammals of a whole farm without a trap.

Breeding tracks vole numbers. A pair nests in a hollow tree, a barn, a cliff cavity, or a nest box, laying four to seven eggs at two-to-three-day intervals from March onwards. Incubation takes about 30 days and the chicks hatch in sequence, so a brood contains owlets of visibly different ages and the youngest starve first if food runs short. In a strong vole year a pair may raise two broods. The cost of this fast strategy is a short life: most barn owls die in their first year, many on roads, and the average adult lives only a few years, though wild birds have reached 15.

Barn owls are rodent control that runs on grass. A pair with a brood removes well over a thousand voles, mice, and rats in a season from the few square kilometres around the nest, taking them from exactly the habitat where crops and stored grain are at risk. Growers have noticed: vineyards in California and oil palm plantations in Malaysia install nest boxes by the hundred, and studies in both have measured lower rodent damage around occupied boxes.

The same diet makes barn owls a sensitive indicator. Second-generation anticoagulant rodenticides accumulate in the rodents the owls eat, and surveys in Britain and North America have found residues in most barn owl carcasses tested. Loss of rough grassland to intensive farming and the conversion of old barns into housing have removed both hunting habitat and nest sites across much of Europe. Where nest boxes and grass margins have been put back, populations have recovered quickly, which is the clearest evidence that habitat, not the owl, was the limit.

Barn owls are also prey. Great horned owls in North America and eagle owls in Europe take them regularly, and nests in open barns are raided by raccoons, martens, and crows. The barn owl's night shift is partly a way of staying out of the day's traffic.

A barn owl does not see the mouse. It hears where the mouse will be and arrives there without a sound.

Engineers have been copying the wing for a decade. Serrated leading edges and fringed trailing edges modelled on owl feathers are in trials on wind turbine blades, drone rotors, and aircraft landing gear to cut noise, and the barn owl's auditory map in the midbrain was the model for early neural circuits that localise sound in hearing aids and robots.

The behavioural lesson is simpler. The owl's advantage is not a better signal from the mouse; it is less noise from itself. By removing its own sound, it both avoids alerting the prey and keeps its own sensors clean. Reducing the noise you generate is often cheaper than amplifying the signal you want.

And it is one of the most rewarding nocturnal animals to find, because a barn owl is white, slow, and hunts open ground where you can see it. The notes below are the field version.

• Where and when

  Rough grassland, field margins, river meadows, and marsh edges at dusk and the hour after, and again before dawn. In winter and when feeding chicks they hunt in daylight too, especially late afternoon after rain.

• What you will see

  A pale, almost white bird with a big head and no visible neck, flying low and buoyantly like an enormous moth, turning back on itself and hovering briefly. No other owl in open country looks that pale from below.

• What you will hear

  Not a hoot. A barn owl gives a long, harsh screech in flight and a snake-like hiss at the nest. Chicks begging from a barn in June sound like a kettle through a wall.

• Sign and etiquette

  Pellets under a roof beam or on a ledge mean a roost; white splash on a wall below a hole means a nest. In the UK barn owls are Schedule 1 protected: do not approach an active nest, use lights on a roost, or play calls.

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