How Do Owls Fly Silently? The Secret of Owl Feathers

Where flight noise comes from

When any bird flies, it makes noise in two main ways. Air flowing over the wings forms swirling eddies, or turbulence, especially at the sharp trailing edge of the wing, and this produces a whooshing sound. Feathers also rub against each other as the wing flaps, adding rustles and clicks. A flock of pigeons taking off, with their loud clapping and whistling wings, shows how noisy flight can be.

Owls have evolved several features that tackle each of these sources. British aircraft engineer R. R. Graham described the main ones in a well-known 1934 paper, “The silent flight of owls”, and scientists have been studying them ever since.

Four features that silence an owl’s wings

1. The comb on the leading edge

The front edge of the outermost flight feather on an owl’s wing carries a row of tiny, stiff, hooked points, like the teeth of a comb. These serrations cut the oncoming air into many small, gentle swirls instead of a few large, noisy ones. Researchers believe they also help keep the airflow smooth over the wing at the slow speeds owls fly when hunting. Most other birds have a smooth, hard leading edge.

2. The soft fringe on the trailing edge

The back edges of an owl’s flight feathers end in a soft, ragged fringe of loose barbs instead of a crisp edge. The trailing edge is where the airflow leaving the wing makes most of its noise. A soft, porous fringe lets the air pressure even out gradually, so the eddies are weaker and quieter. A 2013 study by engineers Justin Jaworski and Nigel Peake showed mathematically how a flexible, porous edge like this can greatly reduce trailing-edge noise.

3. The velvety feather surface

The upper surfaces of an owl’s flight feathers are covered in fine, hair-like extensions that give them a velvety feel. This soft layer:

• Damps the rustling of feathers sliding over each other during each wingbeat.
• Absorbs sound produced by the airflow, much like the soft surfaces of a recording studio.

Together, the three features mainly reduce high-pitched sound, above about 2 kilohertz. That is the range where small mammals such as mice and voles hear best, and also where owls themselves listen for prey.

4. Big wings and slow, gliding flight

Owls have very large wings for the weight of their bodies. A European barn owl, for example, has a wingspan of about 80–95 cm (31–37 in) yet usually weighs only about 280–420 g (10–15 oz). Engineers call this a low wing loading.

Large, light wings let owls fly slowly, glide often and flap gently, with fewer, softer wingbeats. Noise from airflow rises steeply with speed, so a slow-flying owl is quieter to begin with. Hunting barn owls quarter low over fields at a walking pace, then drift down onto prey with hardly a sound.

Why silent flight matters for hunting

Silent flight gives owls two advantages at once:

• Stealth. Many prey animals have sharp hearing. A vole or mouse that hears an approaching wing has a chance to dive for cover. A silent owl arrives before it can react.
• Listening while flying. Owls hunt largely by sound. Barn owls can catch mice in complete darkness using hearing alone, judging the direction and distance of tiny rustles. If their own wings were noisy, they would drown out the sounds they are tracking.

These skills, along with soft plumage and excellent night vision, help explain what owls catch, described in What do owls eat?

Not all owls are equally silent

Silent flight is most developed in owls that hunt mammals at night, such as barn owls, tawny owls and long-eared owls. Some other owls are noisier:

• Fish owls and fishing owls of Asia and Africa lack the comb-like leading edge. Their fish prey cannot hear them through the water, so silence brings little benefit.
• Day-active owls, such as the northern hawk-owl, rely more on speed and surprise than silence, and their flight is less muffled.

The tawny owl, a common woodland owl in Europe, is a classic silent hunter. It drops from a perch onto mice and voles on the forest floor, often guided by sound alone.

The downside: owl feathers and rain

Soft, velvety feathers come with a cost. They are less waterproof than the tightly knit feathers of most birds, and they soak up water easily. Wet feathers are heavier and noisier. Barn owls in particular struggle to hunt in heavy or prolonged rain, and long spells of wet winter weather can leave them hungry. This is one reason barn owls often roost and nest in barns and other sheltered buildings.

How engineers copy owl wings

Because owls are so quiet, engineers study their wings to make quieter machines. Well-known examples include:

• Bullet trains. In Japan, the 500 Series Shinkansen, introduced in the late 1990s, used a pantograph (the arm that touches the overhead power line) with small serrations inspired by owl feathers to reduce wind noise.
• Fans and ventilation. Several manufacturers make fan blades with serrated or wavy edges based on owl wings, which run more quietly.
• Wind turbines and aircraft. Researchers test serrated and porous trailing edges on turbine blades and aircraft parts to reduce noise for people living nearby.

Owl flight shows how small details of feather design can solve a big engineering problem. For more on how feathers and wings work in general, see How do birds fly?