2023-05-26 10:19:00
Great gray owls can find and capture voles hiding under up to 50 centimeters of snow. Thanks to the work of a team of researchers, we finally know how these raptors can accomplish such a feat.
According to the new studypublished on November 22 in Proceedings of the Royal Society Bwhen hovering above the snow, owls use their broad faces to locate the muffled sounds emitted by their prey.
“Snow is known to absorb sound,” says the study leader, Christopher Clarkan ornithologist at the University of California at Riverside, who this year conducted a series of experiments aimed at measuring sounds in the province of Manitoba, Canada.
Prior to this study, researchers believed that raptors focused on the ultrasound emitted by rodent vibrations. But it would seem that owls are also capable of picking up lower-pitched sounds, such as those created by voles when digging tunnels in the snow.
Although owls’ ears are often thought to be on the top of their head, they are actually closer to the center of their face. This is bordered by a ring of feathers which reflect sounds and channel them towards the ears of the animal.
The larger an owl’s facial disc, the more it is able to hear lower frequencies. According to Clark, the great gray owl, found throughout the northern hemisphere, has the largest facial disc of any owl species.
“We think the reason their facial discs are so big is to make them more sensitive to low-frequency sounds. »
In February 2022, Clark and his colleagues traveled to the forests of Manitoba and spotted seven recent holes; these holes are dug by owls when they dive into the snow to hunt their prey.
The team dug an additional hole next to each existing hole, and placed loudspeakers there. Due to the freezing temperatures, which reached -30°C, the researchers had to deal with technical problems. “It was exhilarating work, the mission kept going badly because of the weather,” says Clark.
The team then used an acoustic camera, equipped with an array of microphones, to record the various noises in the environment. She then played white noise (a high frequency sound), and recordings of a vole (a low frequency sound) through the speakers.
By manipulating the layers of snow on the loudspeakers, the team was able to assess the impact that the depth of the snow had on the sound frequencies. The data revealed, for example, that while much white noise might pass through 20 centimeter layers of snow, only low-frequency sounds managed to pass through 50 centimeter thick layers; and it is precisely these sounds that owls are able to detect.
Clark and his colleagues then studied acoustic mirage, a sound effect that owls experience when hunting.
When the sound waves coming from underground hit the surface of the snow, their trajectory bends. Due to this phenomenon of sound distortion, which is called refraction, the origin of the sound seems to come from a different place from where it is really emitted: therefore, unless the owl directly above its prey, it fails to locate it in the middle of the expanse of snow.
“It’s the same problem we have when trying to pick up objects underwater,” says Megan Galla sensory ecologist at Vassar College in Poughkeepsie, New York, who was not involved in the study.
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