The Ridges Sanctuary

Preservation, Education, and Research

Nature Notes: Owls and the Subnivean Layer

Posted on Feb 27, 2022 by Jeanne Farrell   No Comments Yet | Posted in Blog · Nature Notes · Ridges Outpost

By Jackie Rath

For the last few weeks, I have been serenaded to sleep by the hoots of a Great Horned owl sitting on a branch by my window. It’s nesting season here for the owls, and they are hooting with a whole new set of calls that I love to try and figure out when I am listening late at night. Mating season for owls in Door County occurs in late February to early March, and this Great Horned outside my house is marking its territory while looking for a mate.

This winter is my first winter here in Door County, as I moved to Ephraim early last spring. My first winter comes with a plethora of new information that I have never had the opportunity to learn about and see with my own eyes. One of the winter wonders that has fascinated me most is the subnivean zone that forms for creatures to survive the winter, especially from strong nighttime predators such as the owls. We often look out at snowy white fields and think about how serene and peaceful it looks. But underneath the snow, a whole other winter world is filled with animal activity that we can’t hear or even really see if you aren’t paying attention.

Think about how humans prepare for seasonal changes and what we do to stay warm in the winter months. We stock up for food, we turn up the heat in our living spaces, or we layer up to stay cozy. But what about the small creatures such as mice and voles that face harsh winter conditions and predators? How do they make it through to spring?

That is where the subnivean zone comes into play. It is a layer created by snowpack that is beneath (“sub,” Latin for under) the snow (“nix” or “nivis”). Snow of six inches or more creates a warm climate underneath that acts as a buffer from freezing weather and wind, and its temperature roughly stays around 32 degrees. The heat from the ground melts the snow which condenses against the snowpack, forming a thin ceiling of ice.  It also acts as a layer of protection for food stock and from predators. Normally, small critters lack winter camouflage against the white backdrop of snow. These tunnel systems allow them to duck down in their winter homes where larger predators cannot fit.

Subnivean Zone Illustration by Kristin Link (www.kristinillustration.com)

However, the subnivean layer does not cancel out predation for these critters. Predation in the subnivean zone can come from within, with predators able to follow the tunnels to their prey, such as a weasel or fisher. It can also come from outside of the layer, above the snow. Owls are one of the most fascinating predators in relation to the subnivean zone and one of the factors that make this winter formation so complex.

There are five species of owl (snowy owl sightings rare and only seen in January and February) found in Door County: Snowy Owl, the Great Horned Owl, the Barred Owl, the Eastern Screech Owl, and the Saw-whet. These owls make for very strong predators, especially in relation to the subnivean layer, and here is why:

Vision: Owls have far-sighted, tubular eyes. Instead of spherical eyeballs, owls have “eye tubes” that go far back into their skulls. The size of their eyes helps them see in the dark, and their far-sighted vision allows them to spot prey from yards away. Some owls (Great Horned, Saw-whet) are crepuscular, which means they are active at dawn and dusk, and have yellow eyes. Nocturnal owls, like the Barred owl, have dark eyes.

Hearing: Owls have super-powered hearing. They are capable of hearing prey under leaves, plants, dirt, and snow. Some owls have sets of ears at different heights on their heads, which lets them locate prey based on tiny differences in sound waves. This makes hearing movement under the snowpack much easier and helps them locate their prey.

Talons: Owls have extremely strong and sharp talons and are strong enough to pick up larger prey and crush them. They can quickly break through snow or a layer of ice to retrieve their meal.

Feathers: Owl flight is silent. Unlike most birds, owls make virtually no noise when they fly. They have special serrated feathers that break turbulence into smaller currents, which reduces sound. Soft, velvety down further muffles noise, making prey vulnerable to silent owl attacks.

These captivating birds of prey are known for their distinct calls, nocturnal habits, and silent flight. Combined, these characteristics make predation of the subnivean zone an easy feat for them in the winter months.

Saw whet owl Coco, at Owl-O-Rama. Coco is a former avian friend at Open Door Bird Sanctuary.

The next time you are stepping or skiing through snow, think about the subnivean zone and watch for tracks/holes leading into their tunnel systems. Keep those little creatures’ homes in mind when shoveling and plowing.

Animal tracks at the snowshoe trail leading to a hole at The Ridges Sanctuary.

Join us for the 6th annual Owl-O-Rama at The Ridges, on March 4th and 5th! On Friday, start with our Owl Prowl: a presentation on owl species in Door County, followed by a hike on our Logan Creek Property to hear them calling to one another. On Saturday, head to our Workshop to build an Eastern Screech Owl nest box, and then stop by the Owl Meet & Greet with Open Door Bird Sanctuary at our Nature Center!

For more information on Owl-O-Rama events, including program times, locations, and fees, visit www.ridgessanctuary.org or call (920)-839-2802.

Sources:

Hooghuis, Sarah. “Birding at Home: Who’s Hooting?” Audubon Vermont, 10 Feb. 2021, https://vt.audubon.org/news/birding-home-whos-hooting.

Lukes, Charlotte. “Door to Nature: Barred Owls and Barn Owls.” Door County Pulse, 7 Jan. 2020, https://doorcountypulse.com/door-to-nature-barred-owls-and-barn-owls/

Lukes, Charlotte. “Door to Nature: Christmas Bird Counts.” Door County Pulse, 19 Nov. 2021, https://doorcountypulse.com/door-to-nature-christmas-bird-counts-2/

Mattson, Craig. “Beneath the Snow: The Subnivean Zone.” Schlitz Audubon, 20 Jan. 2020, https://www.schlitzaudubon.org/2019/12/20/beneath-the-snow-the-subnivean-zone/


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