Great gray owls can find and catch clams hiding under up to 50 centimeters of snow. Thanks to the work of a team of researchers, we finally know how these birds of prey can achieve such a feat.
According to the new studypublished on November 22 i Proceedings of the Royal Society Bas they soar 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 ultrasound emitted by rodent vibrations. But it seems that owls are also able to pick up lower-pitched sounds, such as those made by mice when they dig tunnels in the snow.
Although owls’ ears are often thought to be on top of their head, they are actually closer to the center of their face. This is surrounded by a ring of feathers which reflect sounds and channel them towards the animal’s ears.
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 face plates are so large 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 speakers there. Due to freezing temperatures, which reached -30°C, the researchers had to deal with technical problems. “It was exciting 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 mouse (a low-frequency sound) through the speakers.
By manipulating the layers of snow on the speakers, the team was able to assess the effect snow depth had on sound frequencies. The data revealed, for example, that while much white noise could 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 the acoustic mirage, a sound effect that owls experience when hunting.
When the sound waves from the underground hit the surface of the snow, their path is bent. Because of this phenomenon of sound distortion, which is called refraction, the origin of the sound appears to come from a different place than where it is really emitted: therefore, unless the owl is directly above its prey, it fails to locate it in the center of the snow extent.
“It’s the same problem we have when we try to pick up objects underwater,” says Megan Galla sensory ecologist at Vassar College in Poughkeepsie, New York, who was not involved in the study.
