Tuesday, May 20, 2014

The Great Horned Owl: Greatest Adirondack Predator?

GHOWbyTerryHawthorneWhen you ask most folks, which animal is the greatest hunter in the Adirondacks, they’ll usually say “fisher” or “bobcat”, or some other charismatic predator, but I believe the great horned owl may be the most efficient predator that has ever lived on earth period.

Its approach to hunting is based on a combination of stealth, remarkable powers of prey detection and location, and the application of strength all out of proportion to its size. Victims of a Great Horned Owl’s silent aerial attack typically are not aware of the owl’s presence until they are within the vice-like grip of the owl’s talons.

Birds do not migrate because of colder winter weather. In fact, feathers are much more effective insulators than mammalian fur, and when combined with the internal heat generated by their higher metabolism, birds are better equipped to withstand cold weather than mammals are. What drives birds to warmer climates is the lack of food available to them in the winter.

UtahSteve0001a-WoodallSeed eaters congregate around our feeders. Osprey and loons, whose principal diets are fish, leave to find open water. Raptors who prey on small mammals depart, because many of the animals they hunt in summer are hibernating, while many of the non-hibernators conceal their movements by constructing tunnels under the snow cover, covert paths to get from dens and nests to food sources. If you’ve ever been snowshoeing in winter, and felt yourself suddenly sink through, you stepped in one of these tunnels.

With their scent masked, and the sounds of their movements muffled by the snow, the prey of predators like bobcat, fisher, fox and coywolf may survive winter, if they have stored or can locate enough food. These predators, in turn, with their food supply diminished, become desperate and may target our pets more in winter than they would in summer.

Then there is the great horned owl. It’s not so much the size of the great horned owl. Snowy owls are bigger. A great horned owl weighs between two to four pounds, has a wing span of 36 to 60 inches, and stands from 18 inches to 2 feet tall. As with all raptor species, the female is the larger of the two sexes. What distinguishes the great horned owl is a combination of sensory abilities which will subject its prey to arguably the greatest weapon ever developed by natural selection. They don’t have to leave in winter.

Vision Quest

The visual acuity of these nocturnal hunters is much greater than our own. As with people, the forward position of the eyes provides excellent binocular vision. The owl makes its living as a night hunter, and the need for enhanced light collection requires large pupils, whose apertures react to light sources independently of each other. In fact, their eyes are so large, that were our eyes of comparable proportion to our bodies, they’d be the size of grapefruits. The eyes are plug-shaped, and are fixed snugly into the eye sockets in the skull. As a result, owls cannot swivel or “roll their eyes” as we can, or look peripherally without moving their head. While humans have seven neck vertebrae, allowing us to move our heads 180 degrees, raptors have fourteen,and the owl directs its vision by rotating the head 270 degrees, through its lateral and medial axis.

GHOW_MacKenzie_July_2012_kevin_fOur eyes have about 120 million “rods” on the retina. These are for detecting motion, as well as shades of light and dark in low light. The rods are located around and outside the “fovea”, that central area of the retina where there are about 7 million color receptive “cones” clustered. An interesting experiment: stand in an area of ongoing but sporadic activity, like a meadow bordered by shrubs and saplings, an area frequented by song birds and rodents. Note that you are more adept at picking up motion peripherally, than when the motion is in front of you, because our rods are outside the field where the cones are located. The owl’s fovea, on the other hand, is covered with rods as well as some cones, so they are much better able to detect motion in low light in any direction, than we are.

Like most predators, owls have a limited number of two types of color receptive cones, best at resolving light of medium wavelength (“green” area of the color spectrum) and shorter wavelength (blues). Color is less important to predators like owls, than detecting motion. Humans, whose ancestors were tree and savanna living frugivores, that is, creatures whose lives depended on locating fruit, have three types of cone, adding longer wavelength (red), for greater color resolution.

Hearing Your Environment

The great horned owl’s “ear tufts” are not ears at all, but feathers which aid in camouflage, and may also indicate their mood. The owl’s asymmetrical ears are hidden under the dark edges of the facial disk, which is split by the beak and the forehead. Owls can hear noises ten times fainter than our hearing permits, enabling them to locate prey they cannot see, for example, prey scurrying through the snow tunnels they excavate under snow pack or prey obscured by brush.

The facial dish directs sound to the ears, and the right ear is positioned higher under the disk than the left ear, causing sounds to reach one ear a fraction of a second before it reaches the other. The owl tilts and turns its head until the sounds coordinate, and because the owl cannot change the direction of its eyes without rotating the head, the owl is at that moment, staring directly at the location of its prey. It’s almost not fair!

Think about the incredible means of experiencing any environment that develops through natural selection: because the owl’s very survival depends on locating prey which is more often than not hidden, they have evolved an auditory system which allows them to pinpoint the location of prey they may not be able to see. Similarly, the wolf and bear depend on their sense of smell to locate food sources which they often can’t see, just as the snake detects prey by “tasting” the air and in the case of pit vipers, detecting heat. The bat uses a type of sonar to detect mosquitoes, and the list goes on and on. For us, our senses of hearing and smell have been blunted by living within the protective environment of civilization, and we depend primarily on our vision. When it comes to natural selection, the old saying goes “use it, or lose it”.

Silent Flight

Think of the large Adirondack birds you’ve seen, for example, wild turkey, grouse especially, or any type of water fowl. One of the features they share, is how much noise their wings make while in flight.  Owls are not fast fliers, with a top speed of about 40 mph. However, like virtually all owls, the great horned owl’s flight is silent, possibly due to “flutings”, features of the primary wing feathers, which are serrated and resemble comb’s teeth.

While most large birds cause noisy turbulence as the air moves over their pumping wings, the owl’s feather fluting breaks the sounds into thousands of smaller sounds not audible to the ears of mammals. In addition, while other birds preen their wings to hook the ends of their feathers together, so they can fly more efficiently, owls do not.  What they lose in speed as a result, they make up in stealth, because the dissipation of the turbulence is further enhanced. As the owl drops from its perch and glides towards the sound, silent flight also enables the continuous and increasingly more accurate audible triangulation of the prey’s location as the owl closes in.

The Ultimate Predatory Weapon

Great horned owls have thick legs and heavy talons. Like our thumbs, the outside talon is opposable, which allows an owl to switch between a perching posture, with three toes in front and one in the rear, and a hunting and grasping posture, with two toes in front and two in the rear. The bottom of an owl’s foot is sensitive in the way the palms of our hands are, so that they can feel the mammal’s spine, allowing the owl to frame the spine with its talons as it strikes.

If a strong athlete tries to crush your hand in a muscular grip, the crushing power in his fingers and palm is about 65 to 75 lbs. per square inch. The crushing power in a Great Horned Owl’s talons is reputed to range from 200, to an incredible 500 lbs. per square inch, ten times on average stronger than the grip of a typical human hand, so once the talons sink through the prey’s back, most prey are killed instantly. They quite literally may not know what hits them. The great horned owl is the “Jack the Ripper” of the animal kingdom, waiting in silence to deliver an overpowering attack.

Prey

Most raptors try to approach their prey from the rear, flying or gliding in, grasping the animal with their talons. They continue pumping their wings and flying, so that the animal is literally swept off its feet. Great horned owls are the weight lifters of the raptors, and can fly while carrying prey equal to 1.4 times their own weight. If their prey is not killed instantly by the puncture and pressure of the talons, horny pads under the owl’s toes allow the owl to control the struggling prey, as the owl flies to a secure perch. Smaller prey are swallowed whole, while larger prey are torn into manageable chunks, using the beak and talons. The forward talon on the great horned owl has a serrated edge, like a ginsu knife, which helps the owl dismember large prey by sinking its talons, reaching down with the beak, and lifting its head up and back.

The great horned owl eats everything from skunks—raptors have a very limited sense of smell—to housecats, whose disappearance is blamed more often than is deserved, on our Adirondack coywolves. Think about it… if you live in a wooded area, and you’re losing cats, odds are it isn’t the coywolves. Muskrats are a favorite, as are rabbits, squirrels, weasels, minks, martins, bats and anything else they can catch.  They are a main predator of the crow, and take other birds including hawks and smaller owls. Like all mammal eating raptors, they are major controllers of rodents. Although it is only the size of a red tailed hawk, great horned owls are extremely aggressive, and have been known to drive even bald eagles from their nests.

Feeding on the ground is dangerous, as it may expose the owl to possible ambush. This is sometimes unavoidable, as when great horned owls kill skunks or domestic cats which may be three times their weight. Some folks ask us how it is even possible for the owl to kill such a relatively large animal. Well… consider this…. even if you feed your cat every day, it’s still a predator… it is what it is. When you let it outside, it goes hunting, picking through the ground cover, trying to flush rodents, etc. If it doesn’t look up and spot the owl, the first hint that something is wrong, may be its last thought in life, when it feels the owl’s talons penetrate its back, crushing its spine.

The owl “mantles” over the dead cat, spreading its wings as a warning to other raptors that it is guarding its prey and will fight to retain it, as it starts to tear the cat’s carcass open, so that it can fly the dismembered pieces to a safer location. What if the bobcat or fox comes out of the brush behind the owl while it is thus engaged?

Habitat and Camouflage

Great horned owls are extremely adaptive, and have the broadest range of any owl in the Americas. They are found in many different habitats, from sub-arctic tundra and prairie to mixed deciduous and coniferous forests, to mangrove swamps and rain forests, and natural selection tweaks the color of their feathers to fit in and hide in almost any habitat.

In addition, the symmetrical shape of an owl’s head, with its shorter, broader neck, assists the owl in remaining hidden from potential prey or predator. With the 270 degree neck rotation, the owl may survey its surroundings without moving its torso or legs, and with its less clearly defined profile, compared to, say, a hawk, the owl is much less likely to be noticed by other animals. I have observed well camouflaged screech owls basically disappear by simply shutting their eyes.

Territory, Courtship and Nesting

The male great horned owl defends a territory ranging in size from a square mile to about 4 square miles, with the availability of prey and the location of other great horned owl territories being the main limiting factors. Other males are forcefully excluded from their territory. They tend to be monogamous with their mates, and begin hooting back and forth in October or November.

There is something comically humorless about owls, in the sense that I’ve never seen an owl do anything that appeared to be playful, or fulfilling any desire for amusement. While we often see red tails chasing each other and diving as a part of courtship, owls appear stiff and serious, always businesslike. Even their courtship is almost formal, with the male bowing and presenting food, and the rubbing of beaks.

GHOW_chick_DebM_060112_aGreat horned owls nest very early in the season and the female can often be seen incubating her eggs in February, covered with snow, while the male is out hunting and providing for both of them. Just as this “winged tiger” rules the night, its diurnal counterpart, the red tailed hawk, handles the day shift, preying on many of the same animals. Their territories may overlap, but great horned owls nest earlier in the winter, and since they do not build nests, but rather tend to appropriate the work of others, may take over last year’s red tail nest. Great horned owls may also take over a large squirrel’s nest, or use any cavity, such as in a tree, on a ledge, etc.

On occasion, these raptors may take each other’s chicks, and a great horned owl may attack a red tail’s nest while the hawks are roosting, but generally the two birds of prey will coexist in the same territory.  If you are hiking during the late winter- early spring nesting season, and you are threatened by a great horned owl, or, for that matter, by a northern goshawk, take it to heart and turn around, or go around in a wide circle. They are warning you away from the area of their nest, and they are the only birds of prey who have on occasion actually attacked people.

Great horned owl egg clutches average two or three, laid over a number of days, which the female incubates for about 5 weeks, continuing the incubation until the hatched chicks are about two weeks old. Chicks are born sequentially, so there’s a real advantage to being the oldest, in terms of relative size, and the ability to shove aside your smaller siblings when food is offered. Chicks begin crawling out of the nest to perch on nearby branches, at 6 weeks, about a week before they start to fly. The young owls will stretch and exercise their wings.

Their initial flight may succeed, or they may flutter to the ground, and begin calling to their parents. The parents have no way to pick up the young owl, so they will feed it on the ground until it flies successfully, or a predator discovers it and ends it life.

If you’re camping, or hiking, and you discover a great horned owl chick on the ground, try to locate the nest, by looking at branches, tree hollows and ledges around you. If you find the nest, throw a sweatshirt or towel over the young owl, and gently gather it up. This should protect your from its talons. Then, climb up and return the chick to its nest. If there is another chick present, don’t be surprised if that young owl boots its sibling out of the nest… again. One way to create more food for yourself, is to kick your sibling out of the nest. If you’re uncertain what to do, call us at 855-Wolf-Man.

Why Are Females Larger Than Males?

I don’t know that there’s any conclusively proven theory for this, but I suspect it has to do with the division of labor, and either way, I love to speculate, so here goes. Great horned owls have not evolved dramatically in millions of years. If we could ask Mother Nature why, she’d probably scratch her head, and say, “what exactly would we fix here? …… like requiem sharks, they’re perfect predators.”

All birds are essentially descended from theropod dinosaurs, and the earliest “birds”, for example, archaeopteryx, one of those “missing link” creatures, shared features of both. In any case, raptors certainly did evolve, from ancestors who themselves probably evolved in eco-systems where the raptors themselves had to be wary of their own predators. During the nesting season, female raptors tend to incubate the eggs, while the male is out hunting. When you’re hunting, speed and agility are more important than size, but when you’re defending the nest against attacks by larger predators, which of the females survived? The big girls did, and whenever they had female offspring, they’d pass those “big girl” genes down to their female chicks. Or, maybe not.

Digestion & Other Yucky Stuff

Owls lack a crop, the throat pouch where other birds store food prior to digestion. Food goes down the esophagus to the proventriculus, a stomach-like organ, where enzymes, acids and mucus begin to break the food down. Next stop is the gizzard, or ventriculous, which separates out the indigestible parts, like bones, teeth and fur, which will be regurgitated later as grayish-white, sausage-shaped pellets or “castings”, within about 12 to 24 hours of feeding. The act of casting signals that the bird is ready to feed again.

Like reptiles and amphibians, birds have only a single intestinal waste vent, the cloaca, from which they expunge a fecally acidic white paste and a clear urinary fluid. The reproductive organs are also concealed within the cloaca, which is why, aside from the fact that females are considerably larger, you can not tell a raptor’s sex without a blood test. Owls often perch on pine branches high above and along the Wildlife Refuge trail, so look for the tell-tale castings under the trees, or the whitewash fecal spray on the trunks and branches.

Longevity and Mortality

Once they get past that first critical year, great horned owls live about 15 years in the wild. Starvation tends to be the number one killer of wild animals, and in captivity, with starvation off the table, great horned owls may live 30 to 35 years.

Adult great horned owls have no serious predators, though they may occasionally be killed by golden eagles, or northern goshawks. Foxes, bobcats, coyotes or domestic cats, may kill an owl caught out in the open while mantling over prey. The number one cause of death among young owls is starvation, while others are taken by hawks. Many owls are struck by cars, while diving at prey attracted by roadside apple cores and other food waste. Please, don’t litter, but if you throw organic waste out the car window, get it away from the road.

Great horned owls are occasionally shot by farmers, or poisoned when they eat rodenticide killed rodents. If you insist on using pesticides and rodenticides, understand that you are not only killing bugs and rodents, but the predators which eat them as well. Our use of poisons is not only ultimately self-defeating, but poisons will eventually end up in our drinking water. If you have a rodent problem inside, get a cat. Outside, build nesting boxes for barred owls, screech owls, kestrels and barn owls. They’ll control your rodents.

Photo: Artemis, the female held by Wendy, became tangled in barbed wire shortly after first leaving the nest in North Carolina, photo by Terry Hawthorne. Utah, the male held by Steve, was struck by a car in Utah, photograph by Bill Woodall. Two shots of great horned owl chicks, pre-fledge, by Kevin and Deb MacKenzie.

 


Steve Hall

Steve and Wendy Hall run the Adirondack Wildlife Refuge and Rehab Center in Wilmington. They've been rehabbing and releasing wild animals for over 35 years, specialize in predators, keep wolves as the cornerstone of their educational program, and have lived in the Adirondacks for the past 13 years. The Adirondack Wildlife Refuge became a non-profit about 4 years ago.

Visit www.AdirondackWildlife.org to learn more.




18 Responses

  1. Peter Brownsey says:

    I think the Eagle is The Greatest Adirondack Predator. We live on Stillwater Reservoir and see Bald Eagles hunting all year. They hunt loons, ducks, fish, mice,owls, snakes, cats and most anything that they can pick up.

  2. Steve Hall Steve Hall says:

    Hi Peter: Bald eagles are more scavengers than predators, but as with all other creatures, there are exceptions born of circumstance and opportunity. The Golden eagle, for example, is much more of a pure predator than the Bald Eagle. Same can be said of all owls. On the lighter side, Ben Franklin, who was quite a naturalist, upon learning that the Congress was preparing to designate the Bald Eagle as the national symbol, had this to say:

    “For my own part I wish the Bald Eagle had not been chosen the Representative of our Country. He is a Bird of bad moral Character. He does not get his Living honestly. You may have seen him perched on some dead Tree near the River, where, too lazy to fish for himself, he watches the Labour of the Fishing Hawk (osprey); and when that diligent Bird has at length taken a Fish, and is bearing it to his Nest for the Support of his Mate and young Ones, the Bald Eagle pursues him and takes it from him.

    “With all this Injustice, he is never in good Case but like those among Men who live by Sharping & Robbing he is generally poor and often very lousy. Besides he is a rank Coward: The little King Bird not bigger than a Sparrow attacks him boldly and drives him out of the District. He is therefore by no means a proper Emblem for the brave and honest Cincinnati of America who have driven all the King birds from our Country.” Ouch!

    More on bald eagles, Franklin and the Philosophers Camp at Follensby at http://www.adirondackwildlife.org/Bald_Eagle.html

  3. Jesse B says:

    Thank you Steve for this interesting and informative article. It’s great to read about animals less ‘charismatic’ than deer or black bears, but are just as critical to a healthy Adirondack ecosystem. I hope you consider writing more of these profiles in the future.

  4. Kevin "MudRat" MacKenzie MudRat says:

    Nice article again, Steve. Intriguing facts that help us appreciate the intricacies of these beautiful creatures. There’s nothing quite like being glared at by Artemis or one of the others!

    • Steve Hall Steve Hall says:

      Thanks, Kevin. The photos of the great horned owl chicks above, were taken by Kevin and Deb MacKenzie, as have many of the photos at the Adirondack Wildlife Refuge website.

  5. Bob says:

    Very nice article. What is a coywolf? Did you mean Eastern Coyote?

  6. Steve Hall Steve Hall says:

    Bob: We call the Eastern Coyote the “coywolf”, because it’s a hybrid between the western coyote and the Eastern wolf, like the wolves you’d see in Algonquin Park. More information at http://www.adirondackwildlife.org/Coywolf.html

    • Paul says:

      Steve,

      Why don’t they then use the same species name as the wolf (lupus) and use some type of sub-species designation?

      If the wolf and the coyote can mate and produce fertile offspring then they seem like the same species to me?

  7. A says:

    Powerful and majestic birds!!

  8. David Gibson Dave Gibson says:

    Thanks, Steve, wonderful and comprehensive. Calling in a great horned owl one cold winter night was one of the most chilling things I’ve ever done. It landed so close to me that I dropped my “hooter” and plunged through the snow to my house, never to try that trick again with so formidable a predator.

  9. Steve Hall Steve Hall says:

    Dave: Dang!… it worked!… priceless. Steve

  10. Dan Crane Dan Crane says:

    I always thought humans were the greatest Adirondack predator, or just the greatest predator period. I guess it all depends on the definition of “greatest” though.

  11. Steve Hall Steve Hall says:

    That’s why I said “greatest”, Dan. We may be the most frequent predator, but we’re the dumbest predator.

  12. LAURIE says:

    Hi. We were camping at the Adrirondak Loj Lean Tos this past weekend and heard an animal kill at night – the last few cries of a bird. Right before that we had heard several owl calls and also what we guessed was a Coyote (but we just don’t know). The next morning we found the remains of a yellow beliied sap sucker… just feathers and beak. Is this a possible owl kill? Do owls typically prey on this type of bird? What was this bird doing at at about 11PM?

    • Steve Hall Steve Hall says:

      Laurie: Owls do prey on woodpeckers. One of our wolves once led us to the remains of a pileated woodpecker. All that remained was the head, and we found the casting pellet of a great horned owl near the head. The owl had obviously devoured the wood pecker on a tree branch above, and later regurgitated the pellet, which would contain bones, etc.

  13. Carrie says:

    Steve, We have a great horned owl by our employment and 3 of us have been attacked by he/she. We are unsure as to why since it’s early fall and there shouldn’t be babies and there’s plenty of food sources for he/she. Would you help us understand why this owl is after us?
    Thanks
    Carrie