Recently I was leading a couple groups to northern Minnesota to see winter finches and northern owls. On both trips we were successful in seeing a number of different winter specialty species such as Evening Grosbeak, Pine Siskins, Common Redpoll, and White-winged Crossbills. We were also in search of northern owl species such as Great Gray Owl (Strix nebulosa). On both trips we were successful at seeing at least one Great Gray Owl.

The Great Gray Owl is the tallest or longest owl species but despite their overall height, they are not the heaviest or most powerful of the owls in North America. That award would go to the Snowy Owl and to the Great Horned Owl. Both of these owls have larger and more powerful feet / talons than the Great Gray. Despite their overall size, the Great Gray Owl only weighs 2.5 pounds compared to 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 would perch on impossibly small twigs at the tops of trees. It would look down intently looking and listening. I took this opportunity to educate this group about what looks like a seemingly simple behavior of looking and listening for its prey.

Great Gray Owls hunt small mammals such as mice and voles. But in the far reaches of northern Minnesota, snow covers the ground during winter. So, this simple act of looking and listening doesn’t seem like a big deal. So, let’s take a closer look (pun not intended) at the simple act of a Great Gray Owl hunting.

The Great Gray Owl has the largest facial disk of all owls. The facial disk is hundreds of specialized feathers on the bird’s face that act like a parabolic dish and funnel sound to the birds’ ears. In many owl species, the ears are located below and behind the eyes. They have asymmetrical ear openings which means they have one ear opening that is located low and the other is higher. This allows incoming sound to reach each ear opening at two different times. What is more interesting is that the density of feathers increases the closer to the ear opening. This would seem counter intuitive because why would you want more obstacles in the pathway to your ear openings.

The larger facial disks help to pick up low frequency sounds better than high frequency sounds. This helps the bird to locate the prey under snow. But here is where we can get into trouble when we assume a few things such as the owl hearing a vole under the snow.

If you have ever been outside during or right after a large snow event you most likely have experienced how everything sounds different when the landscape is covered in snow. Recent experiments show that sound doesn’t pass through snow very well. Snow muffles sound and experiments show that low frequency sounds are less impacted than high frequency sounds when traveling through snow. The larger facial disks of the Great Gray Owl help pick up low frequency sounds making them better at finding prey under deeper snow.

The problem with this is, sound doesn’t travel directly through the snow either. Sound is bent and shifts as it passes through the snowpack, shifting the origins of the sound by as much as five degrees relative to the actual source of the sound. This is called an acoustic mirage. So, an owl that is perched at a distance wouldn’t be accurate in assessing the source of the sound.

In order to compensate for this shift in sound, the Great Gray Owl has a unique hunting style or behavior. After it locates the prey location from its perch, it will fly off the perch in a low altitude approach to the apparent sound source. At the last moment the owl will swoop up directly above the location of the sound and briefly hover. Using its large facial disks, it once again listens before dropping down. Low frequency sounds passing directly up through the snowpack has less distortion or shifting of the sound allowing the bird to pinpoint the exact location before the bird plunges down through the snow reaching for the prey.

So once again, something in nature which seems so simple turns out to be way more complicated once it is examined closer. This is just another example of the complexity of nature. Until next time…

Stan Tekiela is an author / naturalist and wildlife photographer who travels the U.S. to study and capture images of wildlife. He can be followed at www.Instagram.com, www.Facebook.com and www.Twitter.com. He can be contacted via his web page at www.NatureSmart.com.