Great gray owl can find and catch voles hiding under up to 50 centimeters of snow cover. Thanks to the work of a team of researchers, we finally know how these raptors can pull off such a feat.
As owls soar over snow, owls use their broad faces to locate muffled sounds from their prey, according to the new study, published Nov. 22 in Proceedings of the Royal Society B.
“Snow is known to absorb sound,” says study leader Christopher Clark, an ornithologist at the University of California, Riverside, who conducted a series of sound-measuring experiments in the province of Manitoba, Canada, this year.
Prior to this study, researchers believed that raptors focused on the ultrasound emitted by rodent vibrations. But it seems owls can also pick up deeper sounds, such as those made by voles digging tunnels in the snow.
Although owls’ ears are often thought to be on top of their heads, they are actually closer to the center of their faces. This is bordered by a ring of feathers that reflect sound and direct it to the animal’s ears.
The larger an owl’s facial disc, the better it is able to hear lower frequencies. According to Clark, the great gray owl, which is found throughout the northern hemisphere, has the largest facial disc of any owl species.
“We think the reason their facial discs are so large is that they are more sensitive to low-frequency sounds. »
In February 2022, Clark and his colleagues traveled to the Manitoba woods and discovered seven new 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 speakers there. Due to the icy temperatures of up to -30 °C, the researchers had to contend with technical problems. “It was exciting work, the mission always went badly because of the weather,” says Clark.
The team then used an acoustic camera equipped with an array of microphones to record the various sounds in the environment. She then played white noise (a high-frequency noise) and recordings of a vole (a low-frequency noise) through the speakers.
By manipulating the layers of snow on the speakers, the team was able to estimate the impact of snow depth on sound frequencies. For example, the data showed that while a lot of white noise could penetrate 20-centimetre-thick snow layers, only low-frequency noise could penetrate 50-centimetre-thick layers; And it is precisely these sounds that owls can perceive.
Clark and his colleagues then studied acoustic mirages, a sound effect experienced by owls when hunting.
When the sound waves coming from underground hit the snow surface, their trajectory bends. Because of this phenomenon of sound distortion, called refraction, the sound appears to originate from a different location than where it is actually radiated: so unless the owl is directly above its prey, it cannot find it in the Locate the middle of the snow surface.
“It’s the same problem we have when we try to pick up objects underwater,” says Megan Gall, a sensory ecologist at Vassar College in Poughkeepsie, New York, who was not involved with the study.