Nature up close: Short-eared Owls

By “Sunday Morning” contributing videographer Judy Lehmberg.

I’ve spent much, much more time looking for owls than I’ve spent actually seeing them. Most of my looking has been in Montana, Idaho and Wyoming for great gray owls. I figured I would have a decent chance of finding them because they are big and, unlike most owls, active during the day. The only time those searches have been successful is after someone else found one and gave me an idea where to look.

Even so, I still spent a long time looking over and over the same area, finding nothing, until once I looked up to find a female great gray owl looking down at me. She seemed quite unconcerned by my presence. She sort of glanced at me once but most of the time she seemed to look right through me -- an experience I will never forget.

Asio flammeus, the scientific name of the short-eared owl, means an eared owl with flame shapes on its plumage. They are widespread over the world -- 600,000 in North America, and at least three million worldwide. Short-eared owls nest on the ground, which is unusual for owls. They also occasionally nest in groups, a rare exception for a normally solitary bird.

Short-eared owls are creatures of open grasslands where they can be found gliding between the occasional tree, looking in the grass for prey, or twisting 180 degrees as they dive to the ground when they spot a mouse.  Watching owls fly is a rare treat, as most are nocturnal, but they often become active at dusk and dawn. Seeing an owl perform its hunting maneuvers stimulates a person to reflect on the evolution of its hunting adaptations, a never-ending arms race between predator and prey.

Owls are aided in their hunt by their sharp senses, especially their hearing and sight. Their eyes are large and tubular-shaped with a higher density of rods in their retina. Rods allow for better night vision, as does the tubular-shaped eyes.  The longer the distance between the rods in the retina and the lens, the more magnified the image is. Big eyes with lots of very sensitive rod receptors allow owls to see in low light better than their prey, mostly voles and mice. The tubular shape of their eyes gives them an advantage in improved night vision but it keeps them from being able to more their eyes. They compensate for that by being able to swivel their heads 3/4 of the way round in either direction when they wish to see something beside or behind them.

Along with other owls, the short-eared owl’s hearing is superb. Nocturnal owls must detect sound emitted when their small prey make rustling noises in leaves and grasses. Their ears are asymmetrical; one is higher than the other.  This placement allows owls to zero in on their prey. In turn, the owls must suppress the sounds they make as they sail through the air.

Birds like whistling swans get their name from their wing’s sound as they fly; owls have evolved to fly quietly. Their feathers suppress sound in several ways; the leading edge has serrations that muffle the flapping and change the aerodynamics to reduce noise. The soft trailing edge of the wing also reduces sound, and their soft downy breast feathers act to absorb sound. They even have soft feathers on their legs. They do not emit much sound in the upper frequencies (the ones voles and mice use for their own communications, above 20,000 hertz), so the rodents can’t hear the owls flying towards them. Owls don’t fly silently, but the low frequency sound they do make is undetectable by their prey.

There are some differences between various owl species’ wing shapes, which result in how well they are adapted for flight. The “aspect ratio” is the ratio of wing length divided by average wing width. For birds that soar for long distances, like albatrosses, the ratio may be 18, with very long, skinny wings. The short-eared owl has an aspect ratio of 8.9, which is larger than all of the other North American owls, which average around 5. Longer wings are valuable for long distance fliers, and short-eared owls migrate long distances from their nesting to wintering areas.

They also prefer to hunt in open grasslands, flying a little faster when hunting than other owl species. The shorter aspect ratio of barred owl wings is useful for maneuvering in heavy woods, where slow flight, tight turning and wide wings are an advantage.

“Wing loading” is a measure of how much weight is supported by each square centimeter of wing area, and is relatively low for broad winged owls in comparison to many other birds (0.33 grams/cm2 for short-eared owls). In comparison, the wing loading for a barn swallow is 0.14 grams/cm2, and 0.73 grams/cm2 for an American coot.

Short-eared owls are commonly seen in Canadian and Northwestern states during the summer breeding time, but are usually only seen in the winter in the rest of the U.S. A person can count herself lucky to watch one of these efficient predators as it sails over the grasslands or hunts in snow-covered meadows.

It is not a coincidence that designers of future stealth airplanes are studying owls to try to mimic their near-silent flight. Maybe the owls can teach us something. After all, they have been flying quietly for millions of years.

       
Judy Lehmberg is a former college biology teacher who now shoots nature videos.

       
For more info:

• Judy Lehmberg (Official site)
• Judy Lehmberg’s YouTube Channel

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