Recently, I led two groups to northern Minnesota to observe winter finches and northern owls. On both trips, we successfully spotted several winter specialty species, including “evening grosbeaks,” “pine siskins,” “common redpolls,” and “white-winged crossbills.” We were also searching for northern owl species, such as the great gray owl (Strix nebulosa). On both trips, we successfully observed at least one great gray owl.
The great gray owl is the tallest owl species, but despite its height, it is not the heaviest or most powerful owl in North America. That distinction goes to the snowy owl and the great horned owl, both of which have larger and more powerful talons. Despite its size, the great gray owl weighs only 2.5 pounds compared with the great horned owl at 3.5 pounds and the snowy owl at 4.5 pounds.
One evening, we watched a great gray owl hunting just before sunset. It perched on impossibly small twigs at the tops of trees, intently looking and listening. I took the opportunity to educate the group about what appears to be a simple behavior: looking and listening for prey.
Great gray owls hunt small mammals, such as mice and voles. In northern Minnesota, however, snow covers the ground during winter, making hunting more challenging. The simple act of looking and listening might not seem extraordinary, but let’s take a closer look at how a great gray owl hunts.
The great gray owl has the largest facial disk of any owl, consisting of hundreds of specialized feathers that function like a parabolic dish, funneling sound to the bird’s ears. In most owl species, the ears are located below and behind the eyes. Great gray owls have asymmetrical ear openings – one positioned lower and the other higher – which allow sound to reach each ear at slightly different times, helping the owl precisely locate its prey. Interestingly, the density of feathers increases closer to the ear openings. While this might seem counterintuitive – why add obstacles in the path of sound? – the structure actually enhances the owl’s ability to capture and focus sound effectively.
The larger facial disks help great gray owls pick up low-frequency sounds more effectively than high-frequency ones, aiding them in locating prey beneath the snow. However, assumptions about how the owl hears a vole under the snow can lead to misunderstandings.
If you’ve ever been outside during or right after a heavy snowfall, you’ve likely noticed how the landscape sounds different. Recent experiments reveal that sound doesn’t travel well through snow, which acts as a natural muffler. Low-frequency sounds are less affected than high-frequency ones, making them easier to detect. The great gray owl’s larger facial disks are particularly adept at picking up low-frequency sounds, giving the bird an advantage when locating prey beneath deep snow.
The problem is that sound doesn’t travel directly through snow. Instead, it bends and shifts as it passes through the snowpack, altering its origin by as much as five degrees from the actual source. This phenomenon, known as an acoustic mirage, makes it difficult for an owl perched at a distance to accurately locate the source of the sound.
To compensate for this shift in sound, the great gray owl employs a unique hunting behavior. After identifying the apparent location of prey from its perch, the owl flies at a low altitude toward the sound’s source. At the last moment, it swoops directly above the location and briefly hovers. Using its large facial disks, the owl listens again before plunging down. Low-frequency sounds traveling directly upward through the snowpack experience less distortion, allowing the owl to pinpoint the exact location before diving through the snow to reach its prey.
Once again, something in nature that seems simple turns out to be far more complex upon closer examination. It’s yet another example of the intricate wonders of the natural world. Until next time …
Editor’s note: Stan Tekiela’s NatureSmart column appears twice a month in the Eden Prairie Local News. Tekiela is an author, naturalist, and wildlife photographer who travels extensively across the United States to study and capture wildlife images.
You can follow his work on Instagram and Facebook. He can be contacted via his website at www.naturesmart.com.
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