Thursday, January 31, 2013

Owls (Strigiformes)

While we wait to see what Mom and Dad Decorah are going to do, I thought I'd write a blog to celebrate owls. I collected this information from a number of sources, but I especially want to acknowledge the Owl Pages for their help. Owl Pages also has a facebook page. See the bottom of the page for all of my sources.

Owls are a large and diverse group of mainly nocturnal birds. Over 200 species  are distributed among 27 genera worldwide, ranging in size from the tiny Elf Owl (weighing as little as 31 grams or one ounce) to the massive Eurasian Eagle Owl and Blakiston's Fish Owl (weighing as much as 4.5 kilograms or ten pounds). Despite differences in size, wingspan, color, plumage patterns, and habitat, owls share some common characteristics, including:
  • Facial disks
  • Large forward facing eyes  
  • 'Soft' feathers
  • Round heads, distinctive facial markings and/or ear tufts
  • Talons and a 'hawk-like' beak.
Owls have a number of adaptations that help them survive and thrive after dark, including highly specialized hearing, vision, and plumage.

Hearing
Although owls are renowned for their ability to see at night, their highly specialized hearing plays a crucial part in helping them catch prey in low light and under heavy snow cover.

A sound localization map. The owl's medulla maps sounds to a three
dimensional space that allows owls to locate even hidden prey with a high
degree of accuracy. See bottom of page for image credit. 
Ear tufts are not ears. An owl's ears are located on the sides of its head, under the feathers of its facial disk. The owl's facial disk, which splits into left and right sides down the middle of its face, acts as a sort of parabolic microphone, funneling sound into the ear on 'its' side of the owl's head. In addition to magnifying sound, an owl's facial disk helps provide directional hearing. An owl can move its facial disk feathers to change the disk's shape, guiding more sound into one ear than the other. Some owls can establish left/right prey directionality based on differences in sound wave arrival times just 30 millionths of a second apart.

Directional hearing doesn't end with the owl's facial disk. Many owls have asymmetrically set ear holes - that is, one ear opening is higher than the other - which helps them determine the vertical location of their prey. In this case, sound waves arrive in either the upper or lower ear first, establishing the prey's vertical location. The combination of x and y-axis locations are believed to create a three-dimensional mental image of the space where the sound source is located, enabling the owl to strike even hidden prey with a high degree of accuracy.

Picture this:

An owl perched in a bare oak tree hears a mouse rustling under snow in a wintry nighttime forest. The sound reaches her right ear slightly faster than her left ear, so she knows which direction the rustling is coming from. She turns her head until the sound reaches both of her ears at the same time, signaling that the mouse is right in front of her. Now she knows the general location of her prey, but she needs to pinpoint its location more accurately in the darkness. 

 Sound reaches the owl's lower ear hole first, which tells her that the mouse is below her line of sight. She dips her head until she hears the sound equally in both ears. Her prey is deep in the snow. She launches into the air, flying silently thanks to her highly specialized feathers. 

 Talons
Barred Owl with Prey. Photo by By NaturesPhotoAdventures (Own work)
See bottom of page for full attribution.
Owls have four sharp, curved talons specialized for sensing, catching, and killing prey. In addition to special filoplume feathers that help some species of owls sense when they've made contact - a useful adaptation for hunting in the dark - the underside of an owl's foot is covered in a rough, frictional surface that helps owls grasp and hold prey. Many owls live in cold weather, and feathered or 'booted' feet also protect them against the cold and bites or scratches from their prey.

Like many birds of prey, an owl's feet appear massive in comparison with the rest of its body. When perching, two talons are directed forward and two behind. A flexible joint allows three talons to swivel forward during flight. When striking, owls spread their talons wide in a rough oval shape, increasing their chance of success.

Once they've made contact with prey, owls knead and crush it with their large, powerful feet, using their sharp, hooked beaks to grip and tear tissue.

About two feet from her target, she brings her feet forward, spreads her talons, and strikes.  Success!  The mouse can't escape her rough feet and sharp, curving talons. She quickly dispatches it, crushing it and wolfing it down head first. 

Vision
An owl's field of vision. Image used permission of
The Owl Pages. Check them out - it's a great site!
Owls have very large eyes, accounting for one to five percent of total body weight depending on the species.  Like many predatory animals, their eyes face forward, increasing the range of binocular vision. Binocular vision, or seeing an object with both eyes at once, makes depth perception possible and aids owls in everything from capturing fleeing prey to accurately landing on a branch in the middle of the night. According to the Owl Pages, the field of view for an owl is about 110 degrees, with about 70 degrees being binocular vision.

Like many nocturnal animals, owls have tubular eyes and an abundance of rod cells, which are excellent at collecting light but don't see color well. Consequently, owls have monochromatic vision and cannot roll or move their eyes within their sockets, but must swivel their heads to visualize their surroundings. The swiveling radius of the owl’s head is around 270°, so it can see behind itself without turning around.

However, owls don't see well in the dark just because their large eyes are good at gathering light. Their fixed tubular eyes give them a long axial eye length - that is, there is an unusually long distance between the cornea and the retina, called the Posterial Nodal Distance or PND. The longer the axial length of the eye and PND, the larger the image projected onto the retina. The owl's larger retina also has room for a lot of photoreceptors, giving it heightened visual acuity since each photoreceptor processes a smaller portion of the image. In short, an owl's long, tubular eye projects a large image over a large, dense field of photoreceptors, enabling it to see detail and depth even in very low light conditions.

Finished with her mouse, the owl flies back to her favorite perch. A human might not see the branch and would be unable to judge depth and distance in the darkness, but her highly specialized eyes can see clearly even at night. As she approaches it, she raises her body into a nearly upright position and flares her wings to slow down. With feet outstretched, she has enough momentum to keep moving forward until her rough feet grasp the perch. She hears something in the distance and swivels her head to look at it. 

Feathers and Markings
Comb or fringe-like structures on the edge of an owl's
primary feather. By Kersti (Own work). See bottom
of page for full attribution.
Many birds have stiff primary or flight feathers, but owls have a special comb-like or fringe-like leading edge that helps them fly almost completely silently.

When birds fly, air rushes over their wings, creating lift and turbulence. The stiff edge of a normal bird's wing produces a larger area of turbulence, which makes a whooshing noise. However, the comb-like soft edge of an owl's wing breaks down the turbulence and muffles the sound of air rushing over the wing surface.  The lack of owl-related flight noise helps owls hear prey while keeping prey from hearing their approach.

The plumage of owls is generally cryptic - that is, the colors and patterns of their feathers camouflage them.  For example, Snowy Owls live in the tundra and are primarily white, while Great Horned Owls live in the woods and are brown, grey, and white. Changes in cryptic patterns can even be seen within species inhabiting different ranges: Great Horned Owls that live in birch forests tend to be a little lighter than those that live in darker oak or maple forests.

Although cryptic plumage aids camouflage, many owl species have facial and head markings: face masks, ear tufts, and brightly colored irises. Wikipedia states that these markings are more common in species inhabiting open habitats and are thought to be used to signal other owls under low light conditions.  Other people have suggested that, like cryptic patterns, tufts and facial markings serve to camouflage owls.

The owl lifts off her branch. Sub-nivean voles and mice flare briefly in her awareness as they scrabble beneath the snow. Her directional hearing, acute vision, muffled flight, powerful talons, sharp beak, and cryptic plumage make her a formidable  predator as she flies silently through the dark woods to her nest. 

We have four owl cams to watch:
We're getting brief glimpses of Snowflake and Dan, the female and male at Valmont, and the Great Horned Owls at EagleCrest have been observed mating:

Eggs should begin to appear in our nests in the next week or three. The earliest date they've appeared at Valmont is February 8th (in 2008) and the latest is February 25th (2007). We hope you enjoy watching them!

Things that helped me learn about this topic:

Image credits
  • Medulla map: Solis, Michele M, and Perkel, David J(Jan 2006) Neuroethology. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0003380]
  • Owl feather: By Kersti (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons
  • Barred Owl: By NaturesPhotoAdventures (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
  • Owl Vision: The Owl Pages: http://www.owlpages.com/articles.php?section=owl+physiology&title=vision
Posted by at
Labels: ,