The Great Horned Owl is primarily a nocturnal hunter, however when prey is scarce during the winter months the owls may begin hunting earlier at night and hunt into the morning to meet their food requirements (Houston, 1998).   The Great Horned Owl is a voracious predator that hunts its prey mostly from perches where they survey open spaces .  The owls typically perch on dead trees, snags, tall shrubs, telephone and electric line poles, cliff ledges, fence posts, barns, and buildings to track and observe their prey (Houston, 1998).  They also hunt by flying slowly above the ground and scanning the area for food sources. They detect their prey by vision and hearing. Once the owls find a target, they swoop down to the ground with their wings folded and grab their prey with large talons.  Their large talons are so strong that they often kill their prey instantly (Houston, 1998).  The Great Horned Owl is also extremely strong for its size and may take prey that is 2 to 3 times heavier than itself (The Owl Pages, 2003). These owls have also been known to hunt by walking on the ground to catch small prey and by standing in shallow water to snatch small frogs and fish.  In some cases daring owls have been known to walk into chicken coops to snatch unsuspecting domestic birds (The Owl Pages, 2003). This practice does not sit well with farmers and the Great Horned Owl is frequently shot by farmers when caught stealing chickens.

Great Horned Owl, MO © Paul Miller

They swallow smaller prey whole and often dismember larger species while seated on a perch or in their nest before swallowing. After they eat small vertebrates or pieces of larger vertebrates, the owls digest all but the bones fur and feathers.  These parts are formed into hard, feathered pellet which is regurgitated.  The owls usually regurgitate one pellet per victim.  When they feed on insects or amphibians, the regurgitated pellets often contain the exoskeleton and indigestible parts of one or more individuals (Dobkin 1988).

The Great Horned Owl feeds on a large range of animals and has the most diverse prey profile of all North American raptors (Houston, 1998). Over 253 different species have been identified as prey of the owl (The Owl Pages, 2003). Their meal of choice is rabbits or hares, however they also eat a wide variety of mammals, birds, reptiles, amphibians, fish, and large insects.  Some the mammal species they feed on are rodents, squirrels, minks, raccoons, moles, muskrats, skunks, domestic cats and dogs, shrews, armadillos, porcupines, and bats.  They feed on all other owls except the Snowy Owl.  Other bird species they feed on are Red-Tailed Hawks, grouse, crows, turkeys, woodpeckers, pigeons, Great Blue Heron, ducks, swans, seagulls, and many more.  The common reptile species are turtles, lizards, snakes, and even small young alligators.  Some of their amphibian prey species are frogs, toads, and salamanders.  They also eat scorpions, spiders, crayfish, worms, centipedes, and road kill animals (The Owl Pages, 2003). 

The Great Horned Owl is considered to be monogamous in their mating behavior (Houston, 1998).  The mating season for the owl begins in January or February.  During this time the males and females, call to each other with a hooting sound to attract a suitable mate.  According to Weller (1965) most individuals are not sexually active until 2 years of age (Houston, 1998)  Once the birds meet they bow to each other with drooped wings and begin rubbing their bills and preening their feathers and wings.  Once a mate has been found the owls often inhabit the nest of another bird.  They often select the nest of a squirrel, crow, hawk, or a heron.  They also have been known to nest in hollow trees, squirrels nests, broken snags, or man made platforms (Heintzelman,1979) .  After the mates have found a nest they defend it aggressively and will attack intruders until they are killed or chased away (The Owl Pages, 2003). 

Once the birds have mated, the female usually lays 2 to 4 eggs which are incubated for a short period of 26 – 35 days.  After this time the eggs hatch and the birds are cared for and protected by their parents.  The young owls do not leave the nest until the sixth or seventh week, when they begin exploring branches around the nest.  After the birds reach 9 to 10 weeks they begin to fly and leave the nest more regularly.  The parents still feed the birds for another few weeks while the birds are slowly weaned.  The family stays together until the fall when the young leave and go off on their own. When the juveniles are old enough to live on their own, they leave the home territory and spread out over a wide range.  Some juvenile owls have been known to travel over 150 miles in the fall (The Owl Pages, 2003).  The adults stay near their nesting area and maintain the nearby territory year round.  Pairs of owls have been found to stay in the same nesting ground for up to 8 consecutive years, although the owls do not stay with their mate during the year and live a solitary life (The Owl Pages, 2003).

        The Great Horned Owl lives for a relatively long time.  In the wild owls live up to 18 years, however in captivity they have been found to live 29 to 38 years (The Owl Pages, 2003).  The greatest threat to the mortality of the owls is humans.  The owls are often killed by shooting, traps, electrocutions, and road kill.  The Great Horned Owl does not have any real enemies or predators except other Great Horned Owls, although sometimes the owls are killed by Northern Goshawks during disputes over nesting territory. 

                                         

Distribution:

Distribution of the Great Horned Owl     North, Central and South America

The Great Horned owl resides and breeds throughout North America from Western and central Alaska and central Yukon to east Labrador and Newfoundland. (Sullivan, 1995)   The Northern limit of their distribution is just below treeline and their southern range reaches down into Central and South America to Tierra del Fuego. (Sullivan, 1995)  In the winter, their distribution remains mostly the same except for some far northern populations that migrate southeast during harsh winters. (The Owl Pages, 2003) As demonstrated by its wide distribution, the Great Horned Owl can live in a wide range of habitats.   They live in desert, grassland, suburban, and forest habitats, North up to tree line.  This is the largest range of habitats for any American owl species (Houston, 1998).  The owls generally avoid the high mountain areas of the United States and are found only in sparse numbers in the Rocky Mountains and the Appalachian Mountains. The Great Horned Owl population is also limited in the Sonoran and Mojave deserts (Houston, 1998). The Great Horned Owl is found throughout Mexico except in the humid areas of the southeast part of the country.  However, the owl is found in the area of the central and north Yucatan peninsula.  Additionally,  the owl is considered rare in Costa Rica and only 2 sightings have ever been recorded in Panama (Houston, 1998).  The owl is also absent from the tropical mangrove areas of the northern coast of South America and is considered very rare in the Amazonia.

(Sullivan, 1998)

            The Great Horned Owls' distribution in Central America  is centered mainly in the range from 3000m - 4500m from Bolivia to Columbia and in the puna zone of Peru, west Bolivia, and northwest Argentina.  It is also found in higher elevations in the treeless barranca zone of Ecuadorian Andes where it is found at elevations higher than 4,300m (Houston, 1998).

Evolution:                                                                               

        The earliest known relative of modern birds is archaeopteryx, an ancient reptile that is postulated to have had the ability to fly.  In the 19th century, the first fossil evidence of archaeopteryx was discovered in eastern Europe.  This discovery gave rise to the theory that birds evolved from a common ancestor related to modern reptiles and was also the first indication that birds had any connection to the extinct dinosaurs.  The most important aspects of the fossilized evidence of archaeopteryx that indicated a relationship to modern birds was the amount of shared similarities and the fossilized evidence of one feather that was found with archaeopteryx (Gill,1990).  It is believed that archaeopteryx was a bipedal crow sized reptile that had a snout and small reptilian teeth and used its out stretched wings to glide from high perches.  It is hypothesized that archaeopteryx could not fly from the ground due to the lack of specific small muscles necessary to initiate flight (Gill,1990) .

 

The fossil record of owls is limited, however it does provide interesting insight into certain groups.  The oldest reliable evidence of owls is a fossilized tarsometatarsus classified as Ogygoptynx wetmorei  which is believed to be from the Paleocene of Colorado (Feduccia, 1996) .  This fossil leaves little information on the origin of owls, but shows many similar characteristics to the owl families Tytonidae and Strigidae.  It is believed that during the Eocene period owls under went their major adaptive radiation.  During this time, the diversification of owls produced four families - three which are now extinct and the extant Tytonidae (Feduccia, 1996).

         The fossil record found at Phosphorites du Quercy in France indicates that the family Tytonidae was highly diversified in the Paleogene of Europe (Feduccia, 1996).  The family Strigidae first appears in the Lower Miocene of North America and Europe.  The evidence found for this time period indicates that the Tytonidae were superseded by the typical Strigidae owls during this time period (Feduccia, 1996).  

        The most fascinating fossil owls are found in the Ice Age or Pleistocene era.  These fossil owls were found on islands were without mammalian predators, different owl lineages all over the world developed gigantism.  On Cuba, Hispanola, and Puerto Rico fossil material recovered from Pleistocene cave deposits show that a large number of rodents, ground sloths, and insectivores existed on the islands during this time period (Feduccia, 1996).  Huge numbers of fossilized rodents and ground sloths were recovered from caves on the islands, and most of the fossilized records were recovered from owl pellets.  Further investigations have found fossils of a giant barn owls in Cuba, which probably stood more than 3 ft. tall!  This fossil owl had a tarsometatarsus that was over two times the length of the Great Horned owl, the largest modern owl.  This giant barn owl from Cuba is the largest owl known to have existed (Feduccia, 1996).

        The modern classification of owls distinguishes two families within the order Strigiformes.  These families are Tytonidae, barn owls, and Strigidae, typical owls.  The Great Horned Owl is classified in the latter genus Strigidae, which includes 156 different species of typical owls.    The Strigids differ from the Tytonids in their distinctly different osteological character or bone structure, an unforked and rounded tail, and a round head that frequently has distinctive ear tufts (Feduccia,1996)

 

Other Interesting Facts:     The eyes of the Great Horned Owl do not move, however the owl has a wide range of flexibility in the atlanto-occipital joint.  This flexibility allows the owls head to move more than 180o and enables it to look in any direction.  The Great Horned Owls' eyes are also specially equipped with numerous rods that allow the pupils to open widely in the dark giving the owls night vision (Houston, 1998).          The hearing of the Great Horned owl is aided  by its moderate facial disc which acts as a parabola to direct sound waves toward the ears. This feature helps aid the acute hearing of the owl (Houston, 1998)         The Great Horned Owl female has developed an amazing ability to keep her eggs warm during extremely cold temperatures.  Females can maintain their eggs at 37oC, the incubating temperature even if the ambient temperature drops more than 70o colder (Houston, 1998).           Great Horned Owls in the northern hemisphere irrupt periodically into more southern latitudes.  This movement is due to fluctuations in the population of small mammals of the boreal region.  There is a four year cycle among tundra and grassland rodents and ten year cycle that is observed in snowshoe hares.  When declines in these populations occurs, there is corresponding southward irruption of the Great Horned Owl. (Dobkin, 1988)

Great-horned Owl, Fields Oasis, OR          Photo Copyright © Don Baccus

Bibliography
 

Banks, R. C., R. W. McDiarmid, and A. L. Gardner. 1987. Checklist of Vertebrates of the United States, the U.S. Territories,

        and Canada Resource Publication. p.79. United States Department of the Interior Fish and Wildlife Service.

        Washington, D.C., USA

Dobkin, David S., Ehrlich, Paul R., Wheye, Darryl; (1988) The Birders handbook: The Natural History of  North American

        Birds.   Published by Simon/Fireside Books

Feduccia, Alan. 1996.  Theo Origin and Evolution of Birds. p. 291-295. Yale University Press. New Haven and London.

Gill, Frank B., 1990.  Ornithology. New York, NY. W.H. Freeman and Company.

Heintzelman, Donald S.  1979.  Hawks and Owls of North America.  New York, NY. Universe Books.

Houston, C. S., Smith, D.G., Rohner, C. Great Horned Owl. The Birds of North America, The American Orinthologists Union,

        No. 372, 1998

The Owl Pages: Information about Owls. (No Date Available). Great Horned Owls: Bubo Virginianus. [Online]. 

        Available:  http://www.owlpages.com/species/bubo/virginianus/Default.htm  [10/8/03]

Peterson, Alan P. 1998. Zoonomen - Zoological Nomenclature Resource. [Online] Available: http://www.zoonomen.net/   [10/8/03]

Sibley, Charles G. and Ahlquist, John E.. 1990. Phylogeny and Classification of Birds. New Haven and London. Yale University Press.

Sibley, Charles G. and Monroe, Burt L. Jr. 1990.  Distribution and taxonomy of birds of the world. New Haven.  Yale University Press.