Great Horned Owl
Copyright ©  DickDaniels (carolinabirds.org)
General Information on Owls (1):
Owls are placed in the Order Strigiformes and based on several characteristics are related to members of the Order Caprimulgiformes * - the nightjars, oilbirds, and potoos. Recent molecular data suggest that owls may have some affinity to the hawks (2, 3). The total world owl species is 204, of which only 19 are found in the US and Canada.
*For more information please see the Whip-Poor-Will, a nightjar, that we discussed in the September 2015 Part 2 Bird Blog.
The first recorded owl fossil is from 60 million years ago - Ogygoptynx wetmorei - from Tiffany, CO. The largest recorded owl species, that lived in Cuba during the arrival of the Ice Age (2-3 mya; Ornimegalonyx oteroi), was 3 feet tall - twice the size of the Great Horned Owl. This large owl was a weak flyer and probably hunted on foot. Try to picture a 3-foot owl running on the ground in pursuit of – perhaps, you? How big were its beak and talons?
Great Horned Owl: Bubo virginianus
The Great Horned Owl is called the “tiger of the skies’. It will savagely attack any intruder that threatens their eggs or young. Allan Ecker, author of Owls of North America, documents cases of attacks on people near their nests. The author of this reviewed book, although wearing a helmet and full-face mask, did not encounter aggression during the 3 months he observed an active nest – “Not all owls are created equal.” He says.
1.Owls of United States and Canada; A Complete Guide to their Biology and Behavior. Wayne Lynch. John Hopkins University Press. Baltimore, MD 2007.
2. Hackett, S.J. 2008. A phylogenetic study of birds reveals their evolutionary history. Science 320(5884):1763-1768.
3. Winkler, D.W., S.M. Billerman, and I.J. Lovette. 2015. Bird Families of the World. Lynx and The Cornell Lab of Ornithology.
Report of the book, "Owls of United States and Canada; A Complete Guide to their Biology and Behavior." (2007) By Wayne Lynch. John Hopkins University Press. Baltimore, MD.
Emphasis is placed on the Great Horned Owl (i.e, considered here as a Species Account as well as a book report) and information is placed in the following 4 topics (for your preference click on selected topics):
A. The beak and feet of owls. How do they differ from other birds?
What appears as a small beak nestled in facial feathers or bristles is a very large, strong, and sharply hooked beak. For example, the Great Horned Owls has a 1.5-inch beak and the Snow Owl supports a 2-inch beak. The edge of its beaks is used to dismember prey.
The primary weapons of attack and capture, though, are the owl’s feet and talons. Of the four toes, the outside one can face forward or backward. Nice trick for capturing and holding prey? The positioning of toes of most birds of prey (including the Bald Eagle) is 3 forward and one backward. The osprey prefers the arrangement of 2 forward and 2 backward. Why does the owl show such flexibility with one of its toes?
B. How do owl feathers differ from other birds?
Wings of Owls:
Bird wings are categorized into 4 types. The owl’s wing is of the high-lift type, characterized as big and rounded. This large wing surface area allows for low speed flight (soaring and gliding). The large feathers at the wing end (primary wing feathers) flare and separate into slots. These slots further enhance the lift of the bird while in flight.
The Great Horned Owl’s large wing measures 388 square inches (8 X 10 inch paper sheet is 93 sq. in.), following third behind the Snowy and Great Gray Owl. Owls have a relatively low wing load*. That is, the owl’s body weight relative to their wing area is low. This low wing load also enhances lift and slow flight.
(*Wing load = the amount of body weight carried by each unit of wing areas.)
So why would an owl want to fly slowly? Slow flight provides more time to scour the landscape for a dinner. Slower flight decreases noise that is normally caused by airflow over the wings. This quietness increases their efficiency to detect faint sounds made by hidden prey. And, the prey is less likely to notice a silent bird slowly flying overhead.
And that is not all – the structures of the wing feathers are modified to decrease flight noise:
Stiff comb-like fringe on the leading edge of wings
Soft hair-like fringe on trailing edge of wing – softens air flow and lessens flutter (some owl species)
Fine, velvety down present on upper wing surface of flight feathers. Wing feathers without this down create noise during flight maneuvers, when feathers slide over one another.
The chest and body feathers consist of soft-billowy plumes. Have you noticed how puffy an owl’s breast looks? This soft plumage and cryptic feather patterns are shared with their distant relatives, the nightjars.
Protection provided by cryptic feather patterns and coloration is achieved in three ways:
Camouflage colors of grays, browns, tans and black match their habitat background.
Disruptive designs, such as dark bars and streaks, and light colored spots break up the owl’s silhouette, concealing their outline.
Less important but sometimes helpful is counter shading. When crouched on the ground, light colored feathers on their undersides offset their shadows. This adaptation is more important in ground roosting nightjars and Burrowing Owls.
Feathered ear tuffs are not used to aid in hearing. Their function is for camouflage. These ‘ears’ help break up the owl’s outline and can mimic the top of a broken branch. The Great Horned Owl’s ear has yet another function – communication. If you see their ears DOWN, the owl is agitated (remember we discussed their aggression toward intruders of their nests and young). Ears UP is good – the owl is probably watching a potentially tasty meal. A happy owl.
General Color Variation in Owls:
Darker plumaged owls are found in moist environments. Why? Because melanin, a dark pigment, absorbs and concentrates the sun’s warmth, thereby drying a wet owl better than lighter colored ones. (Gloger’s Rule – Constantin Gloger, a Polish scientist in the mid 1800s, studied the relationship between climate and color variation in birds.)
The Great Horned Owl with the widest distribution of any owl in North America shows the most color variation in plumage of any species:
Darkest: Pacific Northwest – Moist region
Lighter and more brownish: Eastern regions of NA
Pale and greyish: Arid prairie regions of NA
Most pale: Subartic Canada (this color version of the Great Horned Owl is sometimes mistaken as a juvenile Snowy Owl)
The sexes of the Great Horned Owl cannot be distinguished based on plumage variation. However, the Snowy and Barn Owls are sexually dimorphic – that is, the male and female plumages differ between the sexes.
How do owls hear and see?
Modified feathers ‘fine tune’ owl acoustics; they hear more effectively than other birds. Have you noticed the face of an owl? The face looks like a disk. This shape is the result of loose feathers arranged around the face - overlying a second disk of stiff and compacted feathers attached to a specialized flap of skin = facial ruff. This concave wall of feathers is formed behind the ear. The facial ruff probably works as an amplifier that collects sound waves.
A more complicated concept in owl hearing is how they pinpoint sounds. Similar to our hearing the owl can pinpoint sound by determining the difference of loudness heard by each ear. And, unlike our ears that are symmetrical, some owls have asymmetrical ones due to skull and soft tissue aberrations. This asymmetry works because it increases the distance between ears, thereby decreasing the time lag of sound, which improves the owl’s ability to localize sound.
Owls have BIG eyes. The Great Horned Owl sports the biggest eyes – about the same size as humans. But, we have 50X more mass than this owl. So, the owl’s big eyes (relative to body size) permit large images to be recorded on the retinas. And, large pupil dilation increases the amount of light received by the retina. However large and fluid-filled globular eyes take up a lot of room in a skull – especially in a small owl skull (as compared to our skull size). So, what trick has owl evolution devised to overcome this problem? Tubular eyes. Yes, tubes for eyes. Such unique shaped eyes weigh less and take up less space. Unfortunately, a loss of peripheral vision results in this trade. And these tubular eyes cannot swivel as round ones do. An owl’s eyes always face forward resulting in only binocular vision. To offset this problem, owls can rotate their heads. We have all noticed how an owl appears to nearly rotate their heads in 360 degrees. They do not actually go all around the full circle, only up to 280 degrees. At this magic limit the head snaps back SO fast that it appears as though the head has spun the full circle.
Even with these elaborate ear and eye modifications, owls apparently do not see or hear any better than humans. However, owls have adapted behavioral strategies for night hunting – a highly developed spatial memory. They memorize where obstacles are located such as a large tree in their potential line of flight.
How do owls eat, live and breed?
The Great Horned Owl has a rapacious appetite and it will eat about anything that moves – a “night shark” (Ray Cromie). Nine species of owls have served as meals for this large owl. Baby alligators are on their menu in Florida. Raptors such as the Peregrine Falcon, nestling Red-tailed Hawks and Ospreys, and a young Harris Hawk are noted as feasts for this hungry owl. Even the most ferocious of raptors, a Northern Goshawk and its young are notable examples of the killing power of this owl. Snowshoe hares, as large as 3 pounds, are a favorite dish, but due their large size, the head and feet are discarded before lift-off.
The owl cannot digest all parts of its prey so a nice tight wad of ‘indigestibles’ is made into a pellet. Their thick and muscular gizzard grinds the food allowing for smooth nutrients to flow ‘south’, leaving the pellet. The Great Horned Owl can regurgitate a 3-4 inch pellet.
Owl Family Life:
The Great Horned Owl is a year round resident. Although very territorial in the breeding season, this owl will share common territory with others in the winter – crossing paths only occasionally. This large owl breeds every 2-3 years, skipping every third year. Day length triggers hormonal changes and pair stimulation activities. Light change is not detected visually as we might expect, but by a small amount of light penetrating the skull received by receptors in the brain’s hypothalamus. Chemical messages are then sent to the pituitary gland that releases these important hormones.
Pair bonding of Great Horned Owls then begins: they notice each other; they roost together; they vocalize; they preen and nibble each other's sensitive facial feathers; and they become a couple. They may even bill-fence to keep each other in check. The male carries out an elaborate ground display: he crouches in front of the female; he fluffs; he sways; and he struts. All the while, the female is evaluating his condition. She is looking for ‘healthiness’ based on his feathers and antics. The duration and strength of his calls are important, as well. If he is good, his low frequency hooting can be heard up to 3 miles. Now a couple, they may ‘sing’ duets* or use vocalization to signal fright, to threaten an enemy, and to beg food from its partner.
(*Duets may last up to an hour. The female starts the duet with a 7-note song for 3 seconds. The male answers with a 5-note response for 3 seconds.)
Owls do not build their own nests – they take abandoned stick nests of corvids (crows and ravens) and hawks. If the nest breaks down during nesting activities, the owls do not refurbish it with new sticks. This lack of renovation of a heavily used nest may explain why the female does not sit on the nest with her nestlings – it is too fragile. The Bald Eagles bring in new nest material to maintain nest strength for both the nestlings and parents.
The Great Horned Owl has 2-3 eggs and the female incubates them while the male provides her food – usually 3-5 vole-sized animals per day, calling to announce his arrival.
Each chick is provided, while in the shell, an egg tooth and a specialized muscle on its neck. Using this tooth and muscle the unhatched chick creates a crack in the egg, called starring. To free itself from the egg, the chick will then ‘wiggle’, kick, squirm, stretch, and chip. In 1-3 days the chick greets the world wearing a coat of white down. In two weeks a thicker coat of multi-shaded down, that is distinctive for each owl species, replaces the white down.
The female feeds the young, as provided by the male. Even though adult owls eat their food in one gulp, the newly hatched chicks are fed small morsels. In two weeks the owlets are big enough to swallow a small rodent, whole. What do these young owls do when they are not eating or sleeping? They look around, pick at their toes, nibble on debris, preen, stretch, and flap their wings. The female observes from her nearby branch.
As the little owls mature they begin to explore, moving to nearby branches. Each day they move out further. At this stage they are called ‘branchers’. This strategy is effective in avoiding nest predators – perhaps because the female does not stay in the nest with them for protection (as does the Bald Eagle)? At the age of 7-10 weeks flight training school begins. For 2-3 months the adult owl continue to feed these ‘branchers’ as they learn hunting skills. A fine line in the sand is drawn, when adults stop feeding their youngsters. This natal dispersal is a once in a lifetime movement of the juveniles at summer’s end. It may occur abruptly or slowly, perhaps mediated by hormones.
Non-breeding owls are called ‘floaters’. They lead a secretive life and for that reason are considered ‘shadow populations’. These owl populations get squeezed between territories of breeding owls – at their boundaries. They do not vocalize, they are very sneaky and they may move over large spaces to avoid attacks from the breeding owls.
Ornithologists define cyclical invasions of owls as ‘irruptions’. They are not migrations but short movements that benefit their survival. A successful breeding year may create a high density of owls that decreases food availability. The owls will move to find more abundant prey. Or they may move in response to sever weather conditions in their territory – for example, a thick snow cover may prevent detection of prey.
Book Report/Review by: Patty Wainright
Reviewed by: Michelle Brown, Michael LaCombe, Marty