Saturday, October 18, 2008

Great Horned Owls – Who Knew?

This post comes on the heels of one I made yesterday, with a blurry photo of a bird overlooking my feeders. Seeing the Great Horned Owl brought up a lot of questions regarding this fairly common, but totally regal bird. As is my nature, I had to do some research. This is what I discovered:

Most of us will easily recognize this large, ‘eared’ owl. One of the largest of North American owls, about the size of a Red-tailed Hawk (18 - 25 in, 2-5 lbs, wingspan 3.5 - 5 ft), they are very bulky with large heads, short tails and those long, widely spaced ear tufts. They are generally brownish on the upperparts, spotted with darker brown, black and whitish. The throat is white, contrasting with the darkly splotched upper breast. The brownish-buff underparts are paler toward the belly and are barred with darker, horizontal lines. The legs and feet are thickly feathered with only the talons exposed. The eyes are large and yellow.

Coloration varies both individually and regionally. While these birds exist in nearly all habitats throughout North and South America, the most northern birds and those from desert areas tend to be much lighter in color; so much paler, as shown here, as to be mistaken for snowy owls, but with gray faces. Birds in the west and from tropical areas are a richer color with dark reddish facial disks, and those from Central American can be a quite dark, chocolate brown.
These birds generally don’t migrate, but the owls living in the most northern part of the species' range may migrate south for the winter.

Those ‘ears’, which break-up the bird’s silhouette and help with camouflage, are not actually ears at all; but rather tufts of feathers that can be lowered and raised at will. They are used in communication between birds; much like a cat or dog uses its ears to convey information; often flat when irritated and upright when inquisitive.

Their soft, loosely packed feathers, which feel like fleece, is superbly insulating, and keeps them nearly silent when flying. The leading edge of their wings are serrated, which breaks up air-flow and helps wind pass over gently, keeping the bird's flight noiseless.

This species is a perch-and-pounce hunter; it perches and waits for prey, then glides down silently and hits from behind. Although its short, wide wings allow maneuverability among trees of the forest, the resulting high wing-loading makes aerial foraging less efficient. However, this same fact allows this bird to lift much more than it’s own weight.

Eyes

Like many such preditors, owls have massive eyes relative to their body size. However, owls are not able to move their eyes within the socket and must turn their heads to change their view. They are equipped with extra vertibre which allows them to turn their head about 270 degrees; nearly full circle.

Contrary to popular belief, however, owls cannot see well in extreme dark and are able to see fine in the day

Although owls have spectacular binocular vision, they are probably not able to see as well close-up; they’re far-sighted and unable to see anything clearly within a few inches of their eyes. For this reason, they make use of special hair-like feathers, filoplumes, around their beaks and feet to ‘feel’ prey.

Owls have a third, opaque, eye membrane called the nictitating membrane. This membrane helps to clean the eye of material and protect the eye from the brightness of day or foreign objects at night.

Ears

The feathers of the facial disc are arranged like a shallow bowl. This shape acts like a satellite dish, to help funnel sound into the ear openings at either side of the head. Hearing in owls is highly sensitive and the ears are arranged in a way allowing the owl to localise a sound. Humans can tell whether a sound is closer to their right ear than their left; Great Horned owls use their assymetric ear openings to precisely triangulate the location and distance of sounds, thus enabling them to pinpoint the location of a noise. Scientists have discovered that owls can catch prey entirely by sound.

Owls have incredible senses of hearing, a trait that allows them to hunt at night. Their ears are located on the sides of the head, but are off-set, not symmetrical like human ears. These openings are also slightly tilted in different directions - often the right ear is longer and set higher up on the skull. Plus, owls have soft feathers that surround the openings which they can spread to make a funnel for sound to enter the ear. An owl's hearing is as good — if not better — than its vision.

Talons

Of all the owls, the great horned owl has the strongest talons, which take a force of about 30 pounds to open, and allow it to sever the spinal column of prey even larger than itself. These birds also have 500 pounds per square inch of crushing power in their talons. A normal man has about 60 pounds per square inch in his hands.

Like most birds of prey, the Great Horned Owl has four toes, two forward and two back. However, the outside, back toe is able to swivel forward; making a larger ‘mitt’ with which to catch prey. The middle toe’s tallon is especially sharp. The bird tears meat with its hooked bill, and pulls meat against this extra-sharp talon to slice it. Smaller prey, like mice are swallowed whole.

Prey

In fact, prey is quiet varried; from fat grasshoppers to mice, snakes, squirrels, and opossums. While they also catch smaller birds, they more often look for larger prey (which is more energy-cost effective) like hares and even porcupines. Like most birds, these owls have a poor sense of smell and are one of the few animals to regularly hunt skunks; to the point that their nests and the birds themselves often smell musky. Taxidermists sometimes have to deodorize them before mounting, because the bird smells so strongly of skunk.

Great horned owls are considered to one of the most voracious of all raptors. They have a prodigious appetite: one great horned owl may ingest several mice each night, thus potentially dispatching over a thousand mice each year; a great boon to human society. Although mammalian prey typically comprise more than three quarters of the diet, more than fifty species of birds have been recorded as prey. In addition to hunting small songbirds, water birds (especially coots and ducks), can be important prey; as well as grouse, herons, Canada Geese, swans, hawks (including Red-tailed) and other owls. It is a serious predator on nestling Ospreys and the reintroduction of Peregrine Falcons has been hampered in some areas by owls killing both adult and nestlings.

Sometimes this bird walks along the ground hunting small mammels, insects and reptiles, and will even wade into water for frogs, other amphibians, fish and crustatians.

The Great Horned Owl is a regular victim of harassment from flocks of American Crows, as well as other smaller birds. Crows congregate from long distances to mob owls, and may continue shrieking and diving at them for hours. The antagonism of the crows may be well earned, however, as the owl is probably the most important predator of crows and their nestlings.

In northern regions, where larger prey that cannot be eaten quickly are most prevalent, these big birds may cache and let uneaten food freeze and then thaw it out later; using their own body heat…sort of ‘incubating’ a meal.

Pellets

About six to ten hours after an owl has eaten, its stomach forms a packet of wrapped hard-bits. This pellet of fur, feathers, scales, exoskeletons, and bones is the indigestible parts of its meal. The owl then regurgitates or "upchucks" this pellet. Owls may have a favorite roost or perch spot where they both eat and cast out these pellets.

Scientists collect the pellets and gently pull them apart in their laboratories to see what the owl has been eating. Pellets of the GHO are very large, about the size of a man’s thumb; 3- 4" long 1.5" thick. Pellets are dark grayish-black and compact; tightly wrapped. Skulls as wide as 3 cm (1.2") are regurgitated whole.

An interesting gift one can offer budding naturalists, unable to find their own pellets to dissect, are various owl pellets: found here.

Sound

Great Horned Owls have a large repertoire of sounds, ranging from deep booming hoots to shrill shrieks. The male's resonant territorial call "hoo-hoo hoooooo hoo-hoo" can be heard over several miles during a still night. Both sexes hoot, but males have a lower-pitched voice than females. They give a growling "krrooo-oo" or screaming note when attacking intruders. Other sounds include a "whaaa whaaaaaa-a-a-aarrk" from disturbed birds, a catlike "MEEE-OWww", barks, hair-raising shrieks, coos, purrs, and a loud beak-snapping when agitated or trying to make an impression. Some calls are ventriloquial; softer or louder as if ‘throwing their voices’, making it difficult to find just from where the bird is calling. Most calling occurs from dusk to about midnight and then again just before dawn.

This is a wonderful site with several recordings of the Great Horned Owl sounds.

Their call is somewhat similar to Band-tailed Pigeon or Mourning Dove, but don't confuse it with the "Who cooks for you? Who cooks for you all?" hoot of the barred owl.

Breeding / Life

Throughout the winter, courting great horned owls will determine their nesting territory with nighttime hooting. They do not build a nest, but use an abandoned nest of a red-tailed hawk, crow, bald eagle, or heron. Sometimes nest boxes, squirrel nests, rocky outcroppings, cliff ledges, caves, hollow trees, man-made ledges and even the top shelf of a big-box garden center are used. A nest is rarely used more than once and any damage done to the nest is not repaired.

[Addendum…my gentle readers, with personal experience, have corrected that last statement to tell us (in the comments section, below) that owls often DO use the same nests; sometimes several years running. Thank you for the correction!]

Breeding season varies from December to July because of this owl's wide range; it tends to breed earlier than other owls in the same locality. Two to five eggs are laid on successive days with incubation beginning with the laying of the first egg. The eggs are incubated 34-36 days, on average. They raise one brood of young per year. Young remain in the nest for 35–45 days and are cared for by their parents for up to five months.

While both birds may incubate, it is the male who feeds the female during this time. Brooding is almost continuous until the offspring are about 2 weeks old, after which it decreases. Young hatch helpless and don’t open eyes and are quite helpless for about the first month. Young owls move onto nearby branches at 6 weeks (a behavior called ‘branching’) and start to fly about a week later. Sometimes, during this time, they may find themselves on the ground under trees, but they are not abandoned and are fed for another several weeks. By this time, the owlets are fully feathered and capable of short flights, but have not developed the hunting skills they need to survive. Fledglings do not fly well until 9 to 10 weeks old. The young are fed by both parents, and the parents fiercely defend their nest site against intruders.

The offspring have still been seen begging for food in late October (5 months after leaving the nest) and most do not separate from their parents until right before the adults start to reproduce for the next clutch (usually December). Birds may not breed for another year or two, and are often vagrants ("floaters") until they establish their own territories. All adult Great Horned Owls are permanent residents of their territories. Eggs, nestlings and fledgings may be preyed on by other raptors and larger birds, foxes, coyotes, and feral cats.

Fifty to eighty percent of young owls die in their first year. If an owl survives the first two years of life in the wild, it can live up to 14 years. In captivity, the owl may live as long as 30 years or more. The great horned owl reaches maturity at two years.

Families remain loosely associated during summer before young disperse in the autumn. Adults tend to remain near their breeding areas year-round while juveniles disperse widely, as much as 150 miles, in the autumn. Territories are maintained by the same pair for as many as 8 consecutive years, however, these owls are solitary in nature, only staying with their mate during the nesting season. Average home ranges in various studies have been shown to be approximately one square mile.

Protection / Aggression

When an owl is threatened, it increases its size threefold by puffing its chin feathers, flaring its wings, and expanding its chest.

Great Horned Owls are often said to be the most dangerous owl, and it is reportedly the only bird of prey that has been known to kill a human being, but it should be noted that these attacks are never predatory, and that the only known fatal attack was triggered by the victim, who was trying to steal eggs or chicks from the owl's nest.

The great horned owl is a fairly shy bird and normally stays out of sight and avoids people. However, if a person climbs to a horned owl nest that contains small young, the parents may become quite aggressive and dive at the visitor and strike him with their talons as they fly by.

This from Bent’s Life Histories of North American Birds:
"Donald J. Nicholson (in Oologist, vol.43, p.14) received …[rough] treatment when he climbed to within 6 feet of a nest containing eggs; he writes: “Swiftly the old bird came straight as an arrow from behind and drove her sharp claws into my side, causing a deep dull pain and unnerving me, and no sooner had she done this than the other attacked from the front and sank his talons deep in my right arm causing blood to flow freely, and a third attack and my shirt sleeve was torn to shreds for they had struck me a third terrible blow on the right arm tearing three long, deep gashes, four inches long; also one claw went through the sinew of my arm, which about paralyzed the entire arm.”

Nope, angering a Great Horned Owl is not recommended. But unless climbing a nest tree, people have little to fear from this magnificent predator. Unattended cats and small dogs may be a different story …and yet another reason to keep cats inside. ;)

Conservation Status

Despite the reputation that the great horned owl has gotten from angry poultry raisers, they are not as harmful as thought in the past. That they control pest populations has been recognized. Now, the great horned owl and other birds of prey are given complete protection in most states throughout the United States.

Dwight G. Smith states in his book The Great Horned Owl that these predators are susceptible to secondary poisonings from applications aimed at reducing pests; anticoagulants for killing poisons and strychnine-laced grain for killing pigeons. Mercury contamination and other pollutions from waste repositories are also a huge issue. Like other predators on the top of the food chain, (Tuna is another good example), these big creatures tend to acquire greater toxic levels of these pesticides and heavy metals. As the predator lives, it accumulates more and more such poison in its tissues; eventually affecting basic physiology, altering behavior and finally proving lethal. Humans, being even further up said food chain, are advised to eat such creatures, like Tuna, on a limited basis; as we too, accumulate the toxic substances to our detriment.

Fun Fact

A group of owls has many collective nouns, including a "bazaar", "glaring", "parliament", "stooping", and "wisdom" of owls.

In addition, I have to say I throughly enjoyed this piece regarding 'what owls mean'.

Research:

Photos on this post are from Wikipedia

15 comments:

Kitt said...

Awesome post! Such fascinating birds. And who knew you could buy pellets online?

Beverly said...

LOL yeah, who knew? I found that little nugget (pellet?) for that ol’ codger who commented on my previous post. Not that he’d need to buy ‘em …but
I’d read the possibilities while doing my research. I like to write a piece about some bird I’ve seen and need to learn more about!

Thanks for visiting, neighbor! (sorta) Gawd, visiting your blog (which is stunning), I got so homesick. Not just for Denver, where I lived for about 30 years…but also just for civilization! Your food-photos are awesome!

Susan Gets Native said...

Awesome birds, aren't they? Our GHOW, Sylvester, is one of my favorites to work with. They have this look....like they know a really awful secret about you.
: )
One correction: GHOW will use the same nest year after year...but they may take a break in breeding and only nest every other year if the habitat/food supply is less than perfect.

Cool fact: It has been observed that a GHOW is capable of tackling and killing a Great Blue Heron. Now THAT'S a hard core bird.

Beverly said...

Susan, yes, I like the big guys, too.

Oddly, nearly every single site, that mentioned nests at all, said the bird doesn’t use the same nest year after year. One went so far as to say that, not only do they not build their own…they do not repair those which are damaged. I dunno…I’m no expert; that’s why I did the research! LOL

I’ll let everybody know if the breeding pair at the golf-course returns to the same nest. They breed so early; they’ll have first pickins!

I mentioned that about the herons, but after watching that clip that circulated awhile back, of an eagle attacking a flying swan…somehow I can’t imagine even a heron putting up much more of a fight…though they are a good sized bird. While I know those big bills are pointed, it can’t be as dangerous as picking off Red-tailed Hawk or an Osprey, for the owl…could it?

By the way…I found the photo you shared of what was found in a Great Horned Owl nest. STUNNING!

Thanks for visiting, again!

Kitt said...

Hi neighbor! (You're closer than many of my visitors, so you qualify.) Glad you like my blog! You live in a beautiful part of the state, so I'm a little jealous of you, too.

Debbie said...

Thanks, Beverly. Quite interesting. What I found particularly interesting was the fact that they hunt more from sound than from vision. I always think of them as being nocturnal so I presumed they had great night vision. Hoo knew?

Beverly said...

Oh my…I’m going to have to go re-read that! What I understood is that while they are very, very adept at hunting using sound, their vision in low light is quite good. Like all owls their night vision is very well-developed, it’s just that this guy also has excellent hearing, too!

Owlman said...

Wow, very detailed and extremely interesting-thanks!

Beverly said...

Hey Owlman, thanks for visiting! Ya know, my original plan to only put in the really odd bits I learned…knowing there are thousands of sites that give the general specifics. Somehow it got away from me and is wayyyyyyyyyyyy longer than I imagined; probably not a good thing for a blog. But…you’re right; this IS an extremely interesting bird! I was tickled to have found the “Bent quote”, too! :) I appreciate your comments, Sir.

HANNIBAL said...

Hi Beverly! Got to your site via Owlman and glad I did. I'm quite an owl fan and loved the fascinating info you provided. I also want to put my 2 cents in about using the same nest: My experience is limited to an owl couple I have been watching for 5 years now, who do use the exact same site. It's an interesting site also, which is in a tree outside of a county courthouse in the heart of downtown, with too much traffic and fire trucks screaming throughout the day. The whole town comes by to check on the babies every year all day long for the entire nesting season. I can't imagine such a disruptive site. Weird but true. Whodathunkit?

Beverly said...

Wow, that is just way cool, Hannibal! Thanks for visiting. Based on what you and Susan have said, I placed an Addendum in the piece so that the mistake I read (over and over) ends with me. Owls DO use the same nests!

I let Owlman know how much I appreciate the comment he put on his site (I’d not seen it till now). This kind of ‘thread’ is exactly what I like about the ‘community’ of bloggers. I’ve met such neat people and found so many awesome websites; like yours.

My my, what wonderful photography, too! Awesome.

By the by…have you seen the incredible clip regarding Mantis I found on YouTube? I embedded it on my site; you can either search ‘Mantis’ or just follow this link.

I don’t really ‘get’ the religious reference (other than the bug sort of looks like it prays…(preys?)) but what I did like was the fascinating ‘costumes’ the creature comes dressed in.

Thanks again,
Beverly

Ecobirder said...

Beverly, lots of great information. What a great post.

Beverly said...

Ahhhhh, you did find it! :)

Thank you...and thank you again for letting me use your photo of Samantha! Such a beauty...and so different from the other pictures of Great Horned Owls I posted (free: from Wikipedia)

Someday I hope to have a decent camera!

Scott said...

Terrific post. Well done. I just posted some photos of a young GHOW on my Posterous site and am linking your post to them.

Beverly said...

Thanks, Scott! The ultimate compliment, I'd say...

You're very kind.