Have you ever heard how quiet owls are when they fly? They are very stealthy, giving them a distinct advantage over their prey. Owls can also pick up sounds that are almost inaudible to the human ear while hunting. However, great gray owls (Strix nebulosa) live in cold environments, often with snow on the ground, which can make hunting by ear more difficult, because their prey, such as voles (Cricetidae), hide beneath the snow and the sounds they produce could be dampened by the icy covering. Christopher Clark from University of California, Riverside, USA, James Duncan from Discover Owls, Canada, and Robert Dougherty from University of Washington, USA, wanted to know how the sounds produced by prey are affected by the presence of snow and how the owl overcomes these acoustic challenges to hunt.
First, the team wanted to find out how snow affects how sound carries. They went out into the field during the winter (February) in Manitoba, Canada, to identify locations where the owls had been hunting, and found seven holes in the snow produced by owls as they retrieved their food. Then, they dug 40 cm deep holes near the owls’ hunting sites and placed a waterproof loudspeaker at the bottom; they also placed an acoustic camera 1–1.5 m above the snowpack and 1.2–6 m from the loudspeaker. The team then played a recording of the sounds produced by a meadow vole (Microtus pennsylvanicus) digging beneath the snow through the loudspeaker, while gradually scraping the snow away in layers, recording the volume and location of the sound relative to the speaker at six snow depths. They used this information to simulate how the owl would perceive the sound after it traveled through the snow.
They found that snow does in fact act as a muffler for sound produced by rodents buried beneath it, especially for high-pitched sounds, such as when the voles are communicating with one another. As the snow was removed, the sound level increased and the location of the sound also appeared to move, with the sound source appearing to be displaced farthest to one side of the speaker when the snow was deepest, moving closer to the speaker as the snow was removed until it appeared to come directly from the speaker when all the snow was gone. The team suggests that owls could overcome this challenge by positioning themselves well above the snow, either on a perch or flying high, to reduce the likelihood of being misled by the distorted sound position. And it seems that the great gray owls have already come to the same conclusion as they often hover directly above their prey before plunging into the snow, to increase their accuracy.
The work done by Clark and colleagues highlights how the snow creates a sound illusion by bending the path of the sound – much like light is bent when passing through a glass of water, making a straw appear bent – directly affecting how great gray owls target food beneath snow cover. The birds have also evolved to fly extremely silently, diminishing the noise produced by their own flight, allowing them to overcome this sound illusion and hear voles digging beneath the snow while they hover above. One could say that these birds are the ninjas of the sky.
