In the Mediterranean forest inhabits a great diversity of animal species that fulfill different functions in the ecosystem. Each species has its strategies to hunt or avoid being hunted by others. These strategies include a kind of arms race related to the senses of sight and hearing .
The prey try to camouflage themselves and acquire adaptations that allow them to have wide ranges of vision around them, to avoid being surprised by predators. These also tend to take advantage of camouflage to avoid being seen, and have a frontal vision, with a much smaller range, but in which the images captured by each eye overlap, allowing them to calculate distances with great precision.
Besides sight, a good sense of hearing is another essential part of dynamics, and as a consequence, the best defense is to make as little noise as possible. Prey with a keen sense of hearing better hear an approaching predator, and the quieter predator more successfully avoids being overheard. And vice versa.
It is difficult for animals that live and move on the ground to develop systems that allow them to walk silently in a temperate forest, full of leaf litter. Apparently, hunting birds, such as raptors, have it easier. However, if you have a keen sense of hearing, the flapping of wings is an easily heard sound . And the small mammals, habitual prey of the raptors of the forest, have very good hearing.
As an adaptive response, owls, owls and barn owls , nocturnal raptors, have developed the ability to perform silent flights . And not only is it inaudible to humans, but more importantly, it’s also inaudible to their prey.
In general, the sound that a bird generates when flying has two causes. One is the friction caused by the rubbing of the feathers , similar to the rustling that occurs when rubbing the taffeta fabric. The other cause is given by the mere nature of movement through a fluid, such as the atmosphere, and the turbulence caused by the body in said fluid. The noise is stronger the greater the turbulence, it is the noise we hear when moving a long object quickly.
Owls and barn owls fly at a moderate speed. One might think that a slow flight is, by necessity, quieter, since it generates less turbulence, but the truth is that these animals make much less noise when flying than any other bird at the same speed.
And it is that these nocturnal birds of prey have certain adaptations that allow them to reduce these noises.
It is common in biology that an organ that initially has a function acquires new functions during the evolutionary process. In the case of owls, the wing feathers are not only used to fly —and to maintain temperature, to waterproof the body… and for the rest of the usual functions that feathers have in most birds—, they also serve to achieve silent flight.
A pen is made up of several parts. The central axis, which supports the structure, is called the rachis , and the divisions that come out on both sides are called barbs, which in turn branch into barbules . The first primary feathers of the owl’s wings have the barbs directed upwards, and the barbules are hooked in the anterior part. These structures form a serrated ridge on the leading edge of the wing. The upper surface of the feathers is also velvety , and the back of the feather has long, thin barbules that generate a trailing edge of the wing similar to the hairs of a brush.
Such particular adaptations, on the one hand, soften the friction between the feathers, reducing noise, and, on the other hand, thanks to these structures, the wings of owls and owls do not cut through the air, but rather they ‘comb’ it , avoiding turbulence. The velvety texture favors the air to travel over the surface of the wings more smoothly, and has a better outlet. In this way, they also reduce the sound caused by the friction of the wings against the air.
Jaworski, J. W. et al. 2020. Aeroacoustics of Silent Owl Flight. Annual Review of Fluid Mechanics, 52(1). DOI: 10.1146/annurev-fluid-010518-040436
Lilley, G. 1998, junio 2. A study of the silent flight of the owl. 4th AIAA/CEAS Aeroacoustics Conference. 4th AIAA/CEAS Aeroacoustics Conference, Toulouse,France. DOI: 10.2514/6.1998-2340
Sarradj, E. et al. 2011. Silent Owl Flight: Bird Flyover Noise Measurements. AIAA Journal, 49(4), 769-779. DOI: 10.2514/1.J050703
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