Owl Tuesday: Great Grey Owl

April 17, 2012 • 9:46 am

The Great Grey Owl,  Strix nebulosa, is a large owl found throughout the northern hemisphere:

Photograph by Jan-Michael Breider, Wilhelmina, Sweden, 2004

Its large facial disc helps focus sound, and its feathers are configured so it flies relatively silently, both to avoid alerting prey and, as this amazing BBC video shows, to avoid interfering with its uniquely keen sense of hearing, crucial in detecting prey:

Wikipedia says this about the “snow plunge”:

They then can crash to a snow depth roughly equal to their own body size to grab their prey. Only this species and, more infrequently, other fairly large owls from the Strix genus are known to “snow-plunge” for prey, a habit that is thought to require superb hearing not possessed by all types of owls. Unlike the more versatile eagle and horned owls, Great Gray Owls rely almost fully upon small rodents, with voles being their most important food source.

Here’s another video showing a relatively tame male:

The World Owl Trust has this fascinating information about the eyesight of owls:

Because of their predominantly nocturnal tendencies, owls have evolved several physical adaptations which facilitate catching prey in the dark. All owls have large forward facing eyes giving good stereoscopic vision, vital for judging distances. Indeed, owls have the most forward facing eyes and hence the best stereoscopic vision of all birds. In smaller species the head often appears flattened so that the eyes can be as widely spaced as possible to increase the stereoscopic effect. This is often further enhanced by bobbing or weaving the head to give a differing perspective known as the parallax effect.

The eyes are very large, those of a Snowy Owl weighing as much as our own. They are modified in nocturnal species to improve sensitivity in low light intensities. They are tubular, rather than round, giving a relatively large cornea in proportion to the overall size of the eye and enabling more light to enter the eye. The light passes through the pupil (which can be closed by the iris to a small pinprick in bright light or opened so wide that virtually no iris is visible at night) to the lens. This is large and convex, causing the image to be focused nearer to the lens hence retaining maximum brightness. One drawback is that owls are long sighted and cannot focus on objects which are too close. Tactile bristles around the beak partially compensate for this. The tubular shape also gives a comparatively large retina size which is packed full of light sensitive rods, about 56,000/square mm. in the Tawny Owl. These rods are far more sensitive than cones at low light levels. The phenomenal light gathering properties of the owls eye is further enhanced in many species by a reflective layer behind the retina, called the tapetum lucidum [JAC: cats have this, too!], which reflects back onto the rods any light that may have passed through the retina without hitting one the first time. Tawny owls would appear to have the best developed eyes of all the owls, indeed of all vertebrates, being probably about 100 times more sensitive at low light levels than our own.

As well as rods, all owls possess colour sensitive cones in their eyes. Although having fewer light sensitive cones than humans, they can probably detect colours to some extent. They are certainly not blind in daylight and some, like the Eagle Owl, have better day time vision than us. Our night time vision, however, is better than some diurnal Pygmy Owls.

Owls are unable to move their eyes in the sockets because of the size and tubular shape. To compensate, they have a deceptively long flexible neck which enables them to turn their head 270° in either direction horizontally and at least 90° vertically.

17 thoughts on “Owl Tuesday: Great Grey Owl

  1. Gorgeous beast!

    When camping in the BWCA Wilderness in northern Minnesota as an 18-year old (with only one other companion, also an 18-year-old) I was awakened (heart in mouth) by a great grey owl calling in a tree directly above us. Whew!

    1. Living on the rural edge of San Jose CA, we had a diverse collection of wildlife. From golden eagles to hummingbirds, it was a birders paradise. However, no bird so impressed me than this owl. I knew one was out there, I had heard the call in the still of the night, but never saw it. Then one night, with a full moon, a full grown one flew by and I saw it in silhouette. Its wingspan was tremendous, I guess about 180 cm. Simply a magnificent bird.

  2. Actually, the stereoscopy of owls is probably not very good, though in fact they are the only birds in which disparity sensitive neurons are well established. These are neurons that are sensitive to the differences in the images each eye receives and are the primary neural units that ultimately lead to stereoscopic depth perception.

    It’s also probably the case that the presence of stereoscopy in owls is a fortuitous by-product of being almost completely unable to move their eyes thanks to their awesome adaptations to nocturnal conditions which caused both their eyes and their ears to take up so much of their skull that the muscles needed for eye movements can’t fit.

    Also, their eyes don’t actually face forward, but are turned outward such that their optical axes diverge by about 30-60 degrees (depending on species)!

    An excellent paper that reviews the utility of binocular vision in birds is:
    Martin, G. What is binocular vision for? A bird’s eye view. Journal of Vision, 2009

  3. What a gorgeous bird! I saw a juvenile great horned up close once when I was hiking the north side of the Catalina range in Az. Even though it was clearly a juvenile, it was about the size of an adult red tail hawk. It was only about 7 feet up in a scrub oak, and did not seem at all perturbed by me walking by and stopping to observe him.

    Unfortunately, that was back before the days of digital cameras.

  4. Blog problem: I clicked on the distribution map to enlarge it, left a comment, and it’s stuck on the map page, not here on the main page for this topic.

    1. Here’s that comment I made on the enbiggened map page:

      I’ve peered closely at the distribution map wondering if we have these owls here (“here” = Victoria, BC) but alas, Vancouver Island is not green at all. What is the ecological problem with V.I., as far as Strix nebulosa is concerned?

      I also wonder about the isolated sub-range in central California. Can that be?

  5. I remember a number of years ago during a particularly cold stretch of winter, one of these guys was spotted and photographed in a residential area of Massachusetts – quite outside of its normal range, apparently.

    Like Emperor penguins, it’s easy to underestimate how large they can get. Small dogs and cats beware…

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