The Silent Flight of Owls and the Forests They Need

Why can owls fly without making a sound?

The secret behind an owl's ability to hunt in the night forest without alerting its prey lies in the very structure of its wings. Most birds' wings generate a rushing sound as they cut through air, but an owl all but eliminates it. Even other raptors such as black kites or hawks produce a wingbeat sound clearly audible to the human ear, yet an owl can pass directly overhead without any sense of its presence. This difference comes down to the minute shape of the feathers that make up the wing.

This silence is no accident — it is thought to be a critical evolutionary adaptation for a nocturnal hunter that relies on hearing to locate prey. If an owl's own wingbeats were loud, they would drown out the faint sounds its prey makes, making it impossible to pinpoint a sound source accurately in the dark. In other words, silent flight is both an "offensive" mechanism that raises hunting success and a "hearing-protection" mechanism that keeps the owl's own ears useful.

An owl's hearing itself is also highly developed. In many species, the position and shape of the left and right ears are slightly asymmetric, allowing the owl to build a three-dimensional picture of a sound source from the tiny differences in timing and volume that reach each ear. The dish-shaped arrangement of feathers ringing the face, called the "facial disc," is also thought to funnel sound toward the ears much like a parabolic antenna. Silent flight and pinpoint hearing work together, like the two wheels of a cart, to make hunting in darkness possible.

Serrations: the sawtooth structure on the wing's leading edge

Along the leading edge of an owl's primary feathers (the outermost feathers of the wing) sit small, sawtooth-like projections called "serrations." This structure breaks up the airflow into finer streams, shrinking the turbulent vortices created when the wing meets the air. When large turbulence passes over a wing unchecked it generates vibration noise, but serrations scatter that sound energy into high frequencies outside the human hearing range, making the resulting wingbeat far harder to hear.

The spacing and angle of serrations differ subtly from species to species and are thought to be optimized for each owl's body size and flight speed. Some researchers point out that these projections do more than just "comb" the air — they also delay flow separation (the point where airflow detaches from the wing surface) over the top of the wing, helping the owl maintain lift while gliding at low speed. This low-speed capability also underlies the owl's ability to weave between trees and hover almost motionlessly above its prey.

Velvety surface: the down covering the feathers

The feathers covering an owl's wing surface are also unusual, with a fine, downy texture reminiscent of velvet. This "velvety surface" absorbs the friction noise generated when feathers rub against one another during flapping. In addition, the barbules (the fine branching structures that make up a feather) grow straight and maintain even spacing between adjacent feathers, further suppressing the noise created when feathers touch.

Most birds' feathers have a smooth, hardened surface built to resist wear and dirt during flight. An owl's feathers, by contrast, are soft, prioritizing silence over waterproofing or smoothness. As a result, owl feathers repel water poorly, and prolonged rain can leave the bird losing body heat quickly — a vulnerability that reflects the evolutionary trade-off made in exchange for silence.

Wing area and wingbeat frequency

An owl's wing loading — the ratio of wing area to body weight — tends to be lower than that of other raptors of similar size. A relatively large wing area means the owl can generate enough lift with fewer wingbeats. Fewer wingbeats mean fewer chances for feathers to rub against each other, further reducing the total noise produced. In short, an owl's silence rests not only on surface structures like serrations and the velvety texture, but also on the overall proportions of the wing itself.

These oversized wings also suit a slow, gliding style of flight. In the final instant before reaching its prey, an owl has been observed spreading its wings wide and dropping altitude almost as if sliding through the air, flapping barely at all until just before landing. It is a flight style that prioritizes reaching prey silently and precisely over sheer speed.

Silent-flight structures applied to human technology

An owl's wing structure has long been studied as a leading example of "biomimetics" — applying mechanisms found in nature to engineering. The idea of borrowing functions that organisms perfected over hundreds of millions of years of evolution has been put to practical use well beyond owls, from the water-repellency of lotus leaves to the drag-reducing skin of sharks. Among these, an owl's silent-flight structure is especially broadly applicable because it speaks directly to the pressing modern problem of noise.

Application to shinkansen pantographs

The best-known example is its application to noise reduction in shinkansen (bullet train) pantographs, the current-collecting devices on the roof of the train. To reduce the air resistance and noise a pantograph experiences at high speed, designs referencing an owl's wing cross-section and feather-edge shape have been studied and adopted. Because the shinkansen travels at nearly 300 km/h, even the smallest bump or shape difference on the vehicle's surface generates significant aerodynamic noise. The pantograph is especially prone to producing sound, and along with tunnel-entry noise countermeasures, it has been an area of ongoing shape optimization for many years.

Research into quieter drones

More recently, an owl's wing structure has also been referenced in research aimed at reducing the flight noise of drones used for urban delivery and inspection work. Adding fine serration-like projections to the edge of a propeller has been shown in multiple studies to reduce the high-frequency sound that makes up most of the rotor noise. As drones increasingly fly at low altitudes close to people's living spaces, this is a field where practical application is expected, partly out of consideration for the noise experienced by nearby residents. As drones see wider use in logistics and disaster surveys, the importance of technology to suppress flight noise is only likely to grow.

Drone noise is not simply a matter of discomfort. Noise considerations for neighbors often constrain night and early-morning flights, and effects on wildlife have also been pointed out. If noise-reducing technology matures, it could ease constraints on drone operating hours and flight routes, expanding their use in socially important applications such as disaster search operations and deliveries to depopulated areas. It is striking that the structure of a single bird's wing is involved in solving such practical challenges.

Spillover into ventilation fans and blowers

Efforts to reduce noise using an owl's wing structure as a reference are also under way in everyday appliances. Manufacturers have developed ventilation fans and blowers with fine unevenness added to the blade edges, or with finely textured surfaces, to suppress the whooshing sound produced during rotation. Quieter HVAC equipment has become more important as homes become more airtight, and in that sense the structure carried by a single species of bird contributes, indirectly, even to the comfort of our living spaces.

Potential application to wind turbine blades

Wind turbine blades, where noise is often a source of concern, are also being studied at a research level for possible quieting inspired by an owl's wing structure. Wind power is expanding as a renewable energy source, but the low-frequency noise generated by large rotating blades has sometimes been cited as affecting the living environment of nearby residents. Incorporating serration-like microstructures into the blade edge could, in theory, scatter sound energy toward higher frequencies and reduce perceived noise. Practical application requires balancing this against the blade's overall strength and generating efficiency, so the idea remains at a research stage for now, but it continues to draw interest as a technology that could help reconcile the spread of renewable energy with residents' living environment.

Owls cannot build their own nests

Being able to fly silently boosts an owl's hunting success, but its survival faces another major constraint: it cannot dig a nest hole with its beak or claws. Unlike woodpeckers, which can carve holes into wood, owls can only use tree hollows that already exist. This constraint ties the distribution of the species itself tightly to the distribution of forests containing tree hollows.

How do tree hollows form?

A tree hollow forms naturally when fungi enter a spot on the trunk or branch where the bark has been damaged or a limb has broken off, gradually rotting the core of the wood into a hollow cavity. Healthy young trees rarely develop hollows; only thick, old, large-diameter trees produce a cavity large enough for an owl's body. Large trees remaining in shrine and temple forests or in traditional rural landscapes (satoyama) have become some of the few precious breeding sites available to owls.

Forming a tree hollow takes decades, sometimes centuries. First the tree suffers some kind of wound; decay fungi then colonize the wound and slowly break down the interior over time. During this process the tree itself often continues living, so it can look like a perfectly healthy large tree from the outside while its interior is quietly hollowing out. Plantation forests, which are typically cut and replanted on a cycle of just a few decades, simply never allow this much time to pass.

The "wounds" that trigger hollow formation come in various forms: a large branch snapped by a typhoon, a scar left by a lightning strike, a woodpecker hole that widened over time, or bark stripped away by deer and other animals — most arise incidentally through natural forces. People cannot deliberately wound a tree to create a hollow; a hollow can only be produced by waiting for nature to take its long course. This quality — that a hollow can only be made by time itself — is exactly why tree hollows can never be fully replaced by artificial nest boxes.

Forest conditions a nesting site needs

A tree hollow alone is not enough. Because owls hunt along forest edges or within the forest itself, a nesting site also needs surrounding forest space that is easy to fly through, sparse forest floor suited to hunting, and a mix of land use — fields and grassland scattered near the forest — around the nesting area. It is not enough to leave a single large tree standing; the entire forest structure, including hunting grounds, needs to be intact.

More specifically, within a radius of a few hundred meters to a few kilometers of a hollow-bearing breeding tree, there needs to be grassland where small animals such as mice can live, and open forest space with good visibility. In young plantation forests where trees grow too densely for the undergrowth to even be visible, an owl cannot efficiently locate prey. Conversely, in completely open farmland with no trees at all, there is nowhere to hide and no nesting tree to use. Owls need a mosaic forest landscape combining large trees with open spaces — diversity is essential to their habitat.

Owls aren't the only ones using tree hollows

Tree hollows serve as an important living space for many creatures besides owls. Japanese flying squirrels, bats, tree-hollow-nesting birds such as great tits, and insects that use hollows as shelter often take turns using the cavity of a single large tree at different times of year. Such hollows are called "microhabitats," and they are increasingly recognized as an essential element in measuring the overall biodiversity of a forest. Thinking about owl conservation, in that sense, means thinking about the conservation of an entire community of forest creatures connected through tree hollows.

Even hollows in the same tree can serve different creatures depending on the size, depth, and height of the entrance. A small entrance limits access to small birds and small bats, while a wider opening allows medium-sized birds and mammals like owls to use it too. In this way, the greater the variety of hollow shapes and sizes, the richer the range of creatures a forest can support. As a single large tree develops multiple hollows over the years, the tree itself comes to function as a small hub of its own ecosystem.

Blakiston's fish owl and the crisis of vanishing nest trees

Among Japan's owls, Blakiston's fish owl, found in Hokkaido, symbolizes the crisis created by dependence on tree hollows combined with changing forest environments. It is one of the largest owls in Japan, with a wingspan said to reach up to 180 cm. Because it is so large, it needs an especially large hollow to fit its body, further limiting the trees suitable for nesting. Blakiston's fish owl is believed to require trees over 300 years old to nest, and such trees have rapidly disappeared due to large-scale logging and expanded plantation forestry.

A population that fell to just 70 birds

In the 1970s, the population of Blakiston's fish owl fell to roughly 70 birds, and extinction became a real possibility. In response, the Environment Agency (now the Ministry of the Environment) launched a breeding and protection program in 1984, advancing measures such as winter feeding and nest box installation. According to Ministry of the Environment materials, the estimated population had recovered to 165 birds as of March 2018. It is currently classified as Critically Endangered (CR) on the Ministry's 4th Red List, still placing it in the highest-risk category for extinction.

Deforestation is not the only factor behind the decline. River channel modification has reduced fish populations (Blakiston's fish owl feeds mainly on fish and crayfish), while traffic accidents, collisions with power lines, and river structures that make it harder for fish to swim upstream are also thought to have played complex, combined roles. As a result, the protection and breeding program covers not only the conservation of nesting trees but also river environment improvement and traffic accident countermeasures as part of a comprehensive effort.

Traffic accidents in particular have long been a focus of countermeasures, since fish owls tend to fly to well-lit roads near feeding stations. Efforts include reviewing the placement of feeding stations and installing warning signs at accident-prone stretches, in coordination with local road authorities and government. The case of Blakiston's fish owl clearly shows that protecting wildlife requires attention not only to habitat itself but also to the risks that arise where that habitat meets human living spaces.

Support from nest boxes and protected areas

In areas where natural tree hollows are scarce, artificial nest boxes support owl breeding. Around 200 nest boxes and 12 feeding stations have been set up across Hokkaido, led mainly by the Ministry of the Environment, and are considered a direct factor in the population's recovery. The Wild Bird Society of Japan has also independently established wild bird sanctuaries since 2004; it currently maintains 14 sanctuaries covering roughly 1,800 hectares across Hokkaido that preserve habitat as it naturally exists, and within these areas 11 breeding pairs regularly fledge chicks.

Nest boxes have proven highly effective as an emergency measure, but experts generally agree they should be understood only as a "bridge" until natural tree hollows can recover. A nest box decays within a few decades, whereas a large tree with a natural hollow continues functioning as part of the forest for far longer. The ultimate goal is to restore forests where breeding trees can renew themselves across generations without human intervention.

Owls in central and southern Japan, and satoyama nest box projects

While not facing as severe a risk of extinction as Blakiston's fish owl in Hokkaido, the common owl species (Ural owl) found widely across central and southern Japan faces the same structural problem: a shrinking supply of large trees in satoyama (traditional rural landscapes). Large trees once dotted satoyama across Japan, protected as sacred groves or estate woodlands, and functioned as nesting sites for owls. Since the postwar period of rapid economic growth, however, residential development and the abandonment of satoyama management have fragmented these networks of large trees in many areas.

Nest box networks supporting satoyama conservation

In recent years, conservation groups around the country have expanded activities that support owl nesting by installing nest boxes as a substitute for old trees. For example, the Yatsugatake Nature Club in Nagano Prefecture launched its "Owl Project" in 2004 and has installed more than 20 nest boxes across a 10 km by 5 km area, conducting ongoing surveys and protection work. Groundwork Nishikinu in Tochigi Prefecture likewise runs a conservation network for nesting environments that positions the owl at the top of the satoyama ecosystem.

What these efforts have in common is that they don't stop at installing nest boxes: they combine regular surveys that record owl breeding conditions with outreach to local residents. The presence of owls tends to make people feel the richness of nature close to home, and observation events for children and collaborations with school education are held in many areas as a result.

Know-how has also accumulated around nest box design. Whether an owl actually uses a box depends heavily on the size and depth of the entrance, the installation height, and the tree species and direction it faces. Improvements have accumulated over years of observation, including designs that make it harder for predators such as crows and martens to raid the nest, and sloped roofs that prevent rainwater from pooling. This accumulated knowledge is itself the product of dedicated citizen science.

Such citizen-led activity also has value in enabling detailed, locally attuned monitoring that large-scale government protection programs alone cannot fully cover. Rather than applying a single nationwide standard, local groups that have observed the same area for years can tailor nest box specifications to the local forest conditions, climate, and predator species — a strength unique to them. Government agencies, citizen groups, and researchers each bringing information from their own perspective leads to more effective conservation measures.

Why owls are considered an "indicator species"

Because owls sit near the top of the food chain, a forest where owls can live is also proof that the entire ecosystem beneath them — small animals and insects included — remains rich. When owls, which prey on small mammals such as mice, insects, and even small birds, can breed stably, it means the vegetation and soil environment supporting all those prey species are healthy enough to sustain the ecological pyramid.

The diversity of owls worldwide, and Japan's own species

The order Strigiformes (which includes both the "true owl" family and the "barn owl" family) comprises roughly 200 known species worldwide and is found on nearly every continent except Antarctica. Sizes range from small species barely 5 cm long to large species with wingspans approaching 2 meters, but most share the basic traits of being nocturnal, excelling at silent flight, and using tree hollows or ground burrows as nests.

The main owl species found in Japan

Japan is home to several owl species: the common owl (Ural owl), about 50 cm long and found widely from Hokkaido to Kyushu; Blakiston's fish owl, Japan's largest; the smaller brown hawk-owl, a summer visitor; and close relatives that breed in tree hollows and under the eaves of shrines and temples from lowlands to mountains. While the forest height and hollow size each species prefers differ, they all share a dependence on "spaces created by time" — whether large trees or gaps in old buildings.

Owls seen near urban areas

In recent years, cases of owls and brown hawk-owls breeding in large trees remaining in suburban shrine groves and parks have been reported in various regions. Even close to human living areas, owls can get by perfectly well as long as large trees and a quiet environment are available. This suburban presence is drawing attention as a familiar case study in balancing development with nature conservation, and it has also become a popular subject for local nature observation events and birdwatching.

Breeding near urban areas, however, does not necessarily mean a stable population. If redevelopment progresses nearby and large trees are cut down, a nesting site can be lost in a single year. Many of the large trees remaining in urban areas have survived by chance, protected for religious or cultural reasons — as shrine or temple grounds, or as estate woodlands of old families — rather than deliberately preserved with owl conservation in mind. In that sense, owl presence near urban areas rests on a fragile balance sustained largely by chance.

Keeping forests intact for the long term protects owls

While the evolutionary achievement behind an owl's silent flight has extended its benefits into engineering fields, the tree hollows that sustain the species can only form over centuries, not the decades-long timescales of most human activity. No matter how many nest boxes are installed, a forest dominated by short-rotation plantation stands can never match a forest where large trees with true hollows are allowed to grow.

Installing nest boxes and designating protected areas are effective immediate measures against sharp population declines, but restoring forests where owls can live stably over the long term requires deliberately expanding zones where large trees are left uncut. In forestry, too, approaches that combine long-rotation management — retaining some trees over an extended period rather than clear-cutting everything at once — with selective logging (thinning out only part of the stand) are being tried in various regions to reconcile timber production with the conservation of large trees.

Such management methods may be less efficient for timber production than clear-cutting, but they deliver values that don't show up easily in numbers, such as reduced soil erosion and preserved biodiversity. A hollow-dependent species like the owl helps make these "hard-to-see values" visible as the concrete presence of a living creature. Dialogue between forestry stakeholders and conservation groups, seeking a balance between timber production and ecosystem conservation, is spreading gradually across forests nationwide.

Whether a forest still supports the quiet flight of owls is, in a sense, a mirror reflecting whether that land's forest has been kept healthy over centuries. An owl gliding through the night forest without a single sound of its wings quietly speaks to the depth of time in forests that most of us rarely get to witness.

What any one of us can do may seem limited, but small actions add up: joining a local nature observation event to learn about owl populations, donating to or supporting groups running nest box projects, or simply paying attention to how the large trees in a nearby satoyama or shrine forest are being managed. Each is a small contribution toward leaving forests with tree hollows for the next generation. A forest where owls can live quietly is also a forest where people can spend a long, rich stretch of time.

For more on why large trees can survive so long, see The Conditions Behind Thousand-Year-Old Giant Trees: Japan's Distribution of Ancient Trees Explained. For other creatures that support forest ecosystems, see Animals That Carry Seeds: How Jays and Bears Rebuild Forests, and the Future of Vanished Woodlands and Ecosystem Services of Bats: The Real Power of Pest Control and Pollination.