The anatomy of silence: how do owls fly without being heard?

Owls are known to be deadly and very skilled predators, with the ability to perform flights surprisingly quiet when compared to other birds. These characteristics not only allow a stealth approach to the prey, but also preserves the hearing acuity necessary for the detection of sounds generated by small animals in the environment, attributes especially relevant for species that hunt in the dark. (SAGAR, 2017). 

The ability to fly silently may vary between species of owls. Larger species, such as the great horned owl (Bubo virginianus), tend to use a hover strategy during the final approach of their prey, usually small mammals, which requires a high level of silence both to maintain stealth and to preserve your hearing sensitivity, essential for acoustic tracking. On the other hand, smaller owls, such as the burrowing owls (Athene cunicularia), which often feed on insects, tend to adopt quick attack strategies, such as landing in fall or capture in flight, contexts in which absolute silence may have less ecological relevance (PAYNE, 1962; JOHNSGARD, 2002).

For about 80 years, several studies have sought to answer a question: What are the unique morphological adaptations present in the wings of owls? According to Graham (1934), the configuration of owls' feathers has three main features: a row of comb-like structures on the edge of their cranial wings (known as leading-edge comb); a flexible fringe on the tail edge (trailing-edge fringe) and a back surface of feathers with velvety texture, similar to fluff, which acts on sound dispersion. These elements, together, reduce the aerodynamic turbulence during the flight and attenuate the propagation of sound, giving the bird a remarkable stealth when approaching the prey. 

Caption. Main features of the owl wings: (a) fringes on the back edge of the wing, (b) velvety surface at the bottom and center of the wing, and (c) comb-shaped structure on the front edge. Credits: SAGAR, 2017.

Caption. Barn owl (Tyto alba) planar phase, followed by (b) wing opening and attack movement with the claws. (c) Physical details on the edges of the wings and on the upper surface of the wings, which are focus of studies to understand the silent flight of owls. Credits: JAWSORSKI, J. W.; PEAKE, N., 2020.

Serrated structure of the cranial margin of the wing (Leading-edge comb):

When the air reaches the front of the wing - called the attack edge - it usually follows the aerodynamic shape of the wing, as with other birds or aircraft. However, some of this air may accumulate along the wing surface as it moves towards the end. This build-up creates turbulence and air swirls (known as vortices), which generate vibrations and noise. In owls, this attack edge has a special structure: a series of small serrated projections, similar to a comb, adaptation that prevents the accumulation of air, fragmenting the flow into small currents and reducing the formation of vortices. As a result, the flight of the owl becomes much quieter, even during the beating of the wings (GRAHAM, 1934).

Caption. Photograph of a wing prepared for the experiments. (a) Full wing, (b) attack edge hooks, (c) microscope image (magnification 4 ) of the wing hooks. Credits: GEYER, 2017.

Flexible fringe on the caudal margin (Trailing-edge fringe):

The feathers of owls have a fringed caudal (or escape) margin, different from the smooth edge observed in other birds. These fringes arise because, in this region, the small structures that normally keep the bars together (hooklets) are absent. This results in loose and jagged ends. The fringes vary from 1 to 4.5 mm in length, being larger at the base of the pen and smaller at the tip and their density also decreases along the pen, from about 5 to 2 fringes per millimeter (GRAHAM, 1934). 

These structures fulfill two fundamental functions in the quiet flight of owls. The first is the unification of the feathers: the fringes present on the edges of the primary feathers fit into adjacent feathers, forming a continuous flight edge, a factor that reduces the number of aerodynamic discontinuities in the wings, reducing sources of noise generation. The second function is sound attenuation: the presence of fringes modifies the airflow along the wings, dispersing sound waves and significantly reducing noise generated during flight - with reductions that can reach up to 10 decibels, depending on conditions and angle.

Dorsal surface of the feathers with velvety texture (Downy Wing Surface):

The wings of the owls have a velvety surface formed by thin structures called penoles, located at the distal end of the barbles of the feathers. These pendulums make the surface of the wing soft and porous, being able to overlap with other ribs, creating an effect similar to a velvet. Studies have shown that this structure helps to smooth the airflow, reducing the separation of the air layer on the wing surface and thus decreasing the generation of noise during flight. 

Despite these apparent benefits, there are still doubts about the evolutionary function of this structure, because not all nocturnal birds or owls have elongated pendulums. Some diurnal species, for example, have these structures poorly developed, which suggests that other factors, such as type of prey or hunting strategy, also influence their presence.

Caption. Morphology of the velvety surface. (a) SEM image of the velvety surface on the wing of a tower owl (Tyto alba). Top view. (b) SEM image in higher resolution showing the overlapping radiates of various beards. Bottom view. (c) Side view of the velvety surface with indication of thickness and angle parameters.  Scale bars: (a) 1 mm, (b) 200 μm and (c) 100 μm.. Credits: WAGNER, H. et al., 2017.

Caption. Owl of the species Strix sp. varies in flight, highlighting A) comb on the attack edge, B) fringes on the fins and C) velvety dorsal surface. Credits: : CLARK, C. J.; LEPIANE, K.; LIU, L., 2020.

Studies with the silent flight of owls have growing importance not only for biology and ecology, but also for engineering. By understanding the physical and morphological mechanisms that allow this sound suppression, researchers have developed bio-inspired technologies such as drone wings and quieter wind turbines, contributing to innovations in several areas that seek to minimize the acoustic impact without compromising performance (JAWORSKI; PEAKE, 2020; SAGAR, 2017).

Therefore, the investigation of the silent flight of owls represents a link between evolutionary biology and technological applications, revealing how solutions refined by nature can inspire new forms of harmonious coexistence between performance and sound discretion.

Author: Fernanda Vitória Marinho - GEAS Brazil Regional Representative Center-West

Revision: Iago Junqueira  - GEAS BRASIL partner by The Wild Place

Savage Panel  July/2025

BIBLIOGRAPHICAL REFERENCES:

CLARK, C. J.; LEPIANE, K.; LIU, L. Evolution and ecology of silent flight in owls and other flying vertebrates. Integrative Organismal Biology, v. 2, n. 1, p. 1–32, 2020. Disponível em: https://doi.org/10.1093/iob/obaa001.

GEYER, T. F.; CLAUS, V. T.; SARRADJ, E. Silent owl flight: the effect of the leading edge comb. International Journal of Aeroacoustics, v. 16, n. 3, p. 115–134, 2017. Disponível em: https://doi.org/10.1177/1475472X17706131.

JAWORSKI, J. W.; PEAKE, N. Aeroacoustics of silent owl flight. Annual Review of Fluid Mechanics, v. 52, p. 395–420, 2020. Disponível em: https://doi.org/10.1146/annurev-fluid-010518-040436.

JOHNSGARD, P. A. North American owls: biology and natural history. 1. ed. Washington, D.C.: Smithsonian Institution Press, 1988.

NORBERG, R. Å. Hunting technique of Tengmalm's Owl Aegolius funereus (L.). Ornis Scandinavica, v. 1, p. 51–64, 1970. Disponível em: https://doi.org/10.2307/3676334.

PAYNE, R. S. How the barn owl locates prey by hearing. Annual Cornell Lab of Ornithology, v. 1, p. 151–159, 1962.

SAGAR, P.; TEOTIA, P.; SAHLOT, A. D.; THAKUR, H.C. An analysis of silent flight of owl. Materials Today: Proceedings, v. 4, n. 8, p. 8571–8575, 2017. Disponível em: https://doi.org/10.1016/j.matpr.2017.07.204.

WAGNER, H.; WEGER, M.; KLAAS, M.; SCHRÖDER, W. Features of owl wings that promote silent flight. Interface Focus, v. 7, n. 1, 20160078, 2017. Disponível em: https://doi.org/10.1098/rsfs.2016.0078