Great horned owls are native to a large geographic range that covers most of North America and extends south into Central and South America. Their latitudinal range is from 68 degrees north latitude (around the northern tip of Alaska) to 54 degrees south latitude (near the southern tip of Brazil). (McGillivray, 1989; Meiri and Dayan, 2003; Morrell, 1993)
Great horned owls are well suited for many habitats and environments. They live at a variety of elevations, from sea level up to 3352.8 meters. Great horned owls are most commonly found in interspersed areas of woodland and open fields. Their habitats include grasslands, deserts, swamps, marshes, mangroves, and both rural and urban human settlements. (Dickerman and Johnson, 2008; Dickerman, 1991; Morrell, 1993; Rohner, 2001)
Like similar owl species, great horned owls have a rounded facial structure and forward-facing eyes which allow for binocular vision. They have distinctive horn-like feather tufts on the tops of their heads. The tufts are usually darker than the rest of the head, improving camouflage. They have a distinctive white spot on their throat. Their underbellies are white with brown and black 'bars' distributed throughout. They are white or tan colored around a black bill. Their back is darker in color, featuring a mottled blacks and browns. Their eyes are different shades of yellow in color. (Dickerman and Johnson, 2008; Dickerman, 1991; McGillivray, 1989; Morrell, 1993; Rohner, 2001)
The size and of great horned owls varies by their geographic location and their sex. They exhibit reverse sexual dimorphism, where females are slightly larger than males. Females average about 1.7 kg, while males average 1.3 kg. Evidence suggests this dimorphism is not influenced by environmental factors. This conclusion comes from the observation that great horned owls generally do not migrate sufficient distances to cross-breed with subspecies which may differ in size. In northern latitudes, they tend to have larger core bodies and a longer wingspan. Their overall length is 45.7 to 63.5 cm and their wingspan is 127 to 152.4 cm. This is consistent with Bergmann's rule, which states that in broadly-distributed genuses, larger individuals of species are found in northern latitudes, while smaller individuals are found in southern latitudes. Variations in color also exist depending on geographic location. For example, Bubo virginianus saturatus, a woodland-inhabiting subspecies of great horned owl, may have darker, browner coloration. Bubo virginianus elachistus, which lives in desert habitats in Baja California, may have a lighter, grayer coloration. (Dickerman and Johnson, 2008; Dickerman, 1991; McGillivray, 1989; Morrell, 1993; Rohner, 2001)
Great horned owls are monogamous, forming a mating pair that raise the young. Breeding pairs are territorial, excluding other breeding pairs from their territory to ensure access to prey. However, they may only display territorial behavior in areas close to their nest and do not completely protect their territory. Mates find one another through 'hooting' rituals, which increase in intensity as the mating season approaches. Males hoot throughout the year, but females only hoot during mating season. (Baumgartner, 1938; Rohner, 2001; Walsh, 1989)
Great horned owls inhabit nests abandoned by squirrels or other birds, including other great horned owls. Their brood sizes depend on food availability and geographic location. Smaller broods are more common in years with lower prey abundance. On the eastern seaboard, broods than 2 are considered rare. In central and western North America, however, a brood size of 3 to 4 is not uncommon. They have 1 to 6 eggs per season which hatch in 30 to 37 days. Great horned owl chicks fledge in 6 to 9 days and achieve independence at 5 to 10 weeks. They achieve sexual maturity in 1 to 3 years. Like other birds with a large geographic distribution, great horned owls tend to nest later in the year relative to the increase in latitude of their location. (Baumgartner, 1938; "Great Horned Owl", 2012a; Rohner and Krebs, 1996; Rohner and Smith, 1996)
The mating ritual of great horned owls is described 'violent nodding and bowing', followed by a quieter 'billing and cooing' stage that signifies the completion of copulation. (Baumgartner, 1938)
Males and females alternate roosting and hunting during the time they are nesting. Males do the majority of the hunting while females spend their time protecting the brood. When prey is scarce, females are more likely to leave the nest unattended to search for additional food, especially if the clutch size is larger than average. The location of their nest varies. In late successional areas, they are more likely to roost in elevated areas (up to 100 feet). In prairies and early successional areas, they set up nests in bushes, cliff nooks, and even on the ground. (Baumgartner, 1938; Rohner and Smith, 1996)
The average life expectancy of great horned owls is 13 years. The record for the longest life in the wild is 28 years old. In captivity, the average lifespan is 20 years and the longest recorded lifespan is 35 years. Human activities such as habitat degradation are also expected to affect their lifespan. (Rohner, 2001; Rudolph, 1970; Sullivan, 1995)
Great horned owls are not considered to be a social species, and are solitary except during nesting. They do not migrate but stay in the same general area. Great horned owls communicate with one another by hooting, which is primarily for establishing territory limits. Disputes over territory can result in fatalities. Some great horned owls become 'territorial floaters.' They do not have a set territory, and may travel between home ranges of other great horned owls. This may be because social behavior prevents breeding. It may improve reproductive success for those that do breed. It also prevents young owls from reproducing earlier, when the odds of a successful clutch are less favorable. (Baumgartner, 1938; Dickerman and Johnson, 2008; Houston, et al., 1998; Kinstler, 2009; Rohner, 2001)
Great horned owls are territorial with variable home range sizes. Although an individual or pair may inhabit a home range, they may only protect or defend smaller areas in the range. Territories are generally 5.26 to 5.56 sq km in area. (Rohner, 2001; Walsh, 1989)
Great horned owls primarily communicate by hooting. This establishes territorial dominance and is also used to search for mates. Their distinctive territorial call, "hoo-hoo hoooo hoo-hoo," can be heard from miles away. They screech loudly when attacking prey. Sometimes recordings of hooting sounds are used to determine population densities. Great horned owls use their vision to locate prey and navigate in the darkness, even through dense forests. They have binocular vision, which allows excellent frontal vision but weaker peripheral vision. They can lose their vision from trauma, pathogenic infection, and possibly from inherited genetic disorders. (Fite, 1973; Kinstler, 2009; Maclaren, et al., 1995; Rohner, 2001; Walsh, 1989)
Great horned owls are carnivores that primarily eat terrestrial vertebrates. They have a variable diet that depends on the availability of prey. In late successional areas, they feed on lagomorphs and voles. In the southwestern U.S. where great horned owls are smaller, they often feed on smaller prey like juvenile rabbits and small rodents or insects. In fields and deserts, their primary diet is likely to consist of rodents and insects. In a habitat surrounded by or adjacent to water, they are capable of hunting fish, amphibians, crustaceans, and reptiles. When hunting, they perch and search for prey, then swoop down and catch it while airborne if necessary. (Donázar, et al., 1989; Link, 2012; "Great Horned Owl", 2012a)
Great horned owls are at the top of many food chains and are not preyed upon by other species. However, territorial disputes between members of the same species can be fatal. Sometimes crows (Corvus brachyrhynchos) or raccoons (Procyon lotor) eat their eggs, however. (Hunter, et al., 1997; "Great Horned Owl", 2012a; Rohner, et al., 2000; Rohner, 2001)
Reproduction of great horned owls is heavily dependent upon prey availability. For example, populations increase when numbers of its primary prey the snowshoe hare, Lepus americanus, were highest. When snowshoe hare abundance lowered, so did the number of great horned owls. (Rohner, et al., 2000; Rusch, et al., 1972)
Great horned owls are at risk for parasitism, though it is not always lethal. They can be afflicted with avian malaria if bitten by an infected blackfly. They change nest locations depending on blackfly activity; in the summer when blackflies are active, they roost on or near the ground. In the winter, when fly activity is lower, they will return to the canopy areas. (Rohner, et al., 2000; Rusch, et al., 1972)
Great horned owls feed on species at many different trophic levels, including many rodent and insect species that are considered pests by humans. They can adapt to ecosystems inhabited by humans and can act as a control to an overabundance of these pests. (Donázar, et al., 1989; Rusch, et al., 1972)
Great horned owls may prey upon poultry raised as livestock. ("Great Horned Owl", 2012b)
Great horned owls are one of the most widespread and successful bird species in the United States. They are not threatened or endangered. (Houston, et al., 1998)
Drew Dietrich (author), Radford University, Karen Francl (editor), Radford University, Kiersten Newtoff (editor), Radford University, Melissa Whistleman (editor), Radford University, Catherine Kent (editor), Special Projects.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
uses sound to communicate
living in landscapes dominated by human agriculture.
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
Referring to an animal that lives in trees; tree-climbing.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.
an animal that mainly eats meat
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.
parental care is carried out by females
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
parental care is carried out by males
marshes are wetland areas often dominated by grasses and reeds.
Having one mate at a time.
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
scrub forests develop in areas that experience dry seasons.
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
living in residential areas on the outskirts of large cities or towns.
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
uses touch to communicate
Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.
living in cities and large towns, landscapes dominated by human structures and activity.
uses sight to communicate
Minnesota Department of Natural Resources. 2012. "Great Horned Owl" (On-line). Minnesota Department of Natural Resources. Accessed March 18, 2012 at http://www.dnr.state.mn.us/birds/greathornedowl.html.
National Wildlife Federation. 2012. "Great Horned Owl" (On-line). Wildlife Library. Accessed March 31, 2012 at http://www.nwf.org/Wildlife/Wildlife-Library/Birds/Great-Horned-Owl.aspx.
Baumgartner, F. 1938. Courtship and nesting of the great horned owls. The Wilson Bulletin, 50/4: 274-285.
Braekevelt, C. 1993. Fine structure of the pecten oculi in the great horned owl. Histology and Histopathology, 8/1: 9-15.
Brown, R., J. Baumel, R. Klemm. 1994. Anatomy of the propatagium: The great horned owl. Journal of Morphology, 219/2: 205-224.
Cawthorn, R., P. Stockdale. 1982. The developmental cycle of Caryospora bubonis Cawthorn and Stockdale 1981 (Protozoa: Eimeriidae) in the great horned owl, Bubo virginianus (Gmelin). Canadian Journal of Zoology, 60/2: 152-157.
Coefield, S., T. Fredericks, R. Seston, M. Nadeau, D. Tazelaar, D. Kay, J. Newsted, J. Giesy, M. Zwiernik. 2010. Ecological risk assessment of great horned owls (Bubo virginianus) exposed to PCDD/DF in the Tittabawassee river floodplain in Midland, Michigan, USA. Environmental Toxicology and Chemistry, 29/10: 2341-2349.
Dickerman, R. 1991. Specimens from the subarctic nesting population of great horned owl from New York, New Jersey, and Connecticut. The Kingbird, Summer: 154-157. Accessed May 04, 2012 at http://www.msb.unm.edu/birds/publications/Great_Horned_Owl_TK_2.pdf.
Dickerman, R., A. Johnson. 2008. Notes on great horned owls nesting in the Rocky Mountains, with a description of a new subspecies. Journal of Raptor Research, 42/1: 20-28.
Donázar, J., F. Hiraldo, H. Delibes, R. Estrella. 1989. Comparative food habits of the eagle owl Bubo bubo and the great horned owl Bubo virginianus in six palearctic and neartic biomes. Ornis Scandinavica, 20/4: 298-306.
Fite, K. 1973. Anatomical and behavioral correlates of visual acuity in the great horned owl. Vision Research, 13/2: 219-230.
Houston, C. 1975. Reproductive performance of great horned owls in Saskatchewan. Bird Banding, 46/4: 302-304.
Houston, C., D. Smith, C. Rohner. 1998. "Great Horned Owl" (On-line). Birds of North America Online, Cornell Lab of Ornithology. Accessed September 21, 2012 at http://bna.birds.cornell.edu.proxy.lib.umich.edu/bna/species/372/articles/migration.
Hunter, D., C. Rohner, D. Currie. 1997. Mortality in fledgling great horned owls from black fly hematophaga and leucocytozoonosis. Journal of Wildlife Diseases, 33/3: 486-491.
Kinstler, K. 2009. Great horned owl Bubo virginianus vocalizations and associated behaviours. Ardea, 97/4: 413-420.
Link, R. 2012. "Living With Wildlife" (On-line). Accessed April 12, 2012 at http://wdfw.wa.gov/living/owls.html.
Maclaren, N., S. Krohne, R. Porter Jr., M. Ringle, D. Lindley. 1995. Corynenbacterium endophthalmitis, glaucoma, and sceral ossicle osteomyelitis in a great horned owl (Bubo virgininanus). Journal of Zoo and Wildlife Medicine, 26/3: 453-459.
McGillivray, B. 1989. Geographic variation in size and reserve size dimorphism of the great horned owl in North America. The Condor, 91: 777-786.
Meiri, S., T. Dayan. 2003. On the validity of Bergmann's rule. Journal of Biogeography, 30/3: 331-351.
Miller, A. 1934. The vocal apparatus of some North American owls. The Condor, 36/5: 204-213.
Morrell, T. 1993. Status and Habitat Characteristics of the Great Horned Owl in South-Central Pennsylvania. Ann Arbor, MI: Proquest Dissertations and Theses.
Rayment, L., D. Williams. 1997. Glaucoma in a captive-bred great horned owl. The Veterinary Record, 140/18: 481-483.
Rohner, C. 2001. Non-territorial floaters in great horned owls (Bubo virginianus). 2nd Owl Symposium: 347-362.
Rohner, C., C. Krebs. 1996. Owl predation on snowshoe hares: Consequences of Antipredator Behaviour. Oecologia, 108/2: 303-310.
Rohner, C., C. Krebs, B. Hunter, D. Currie. 2000. Roost site selection of great horned owls in relation to black fly activity: An anti-parasite behavior?. The Condor, 102/4: 950-955.
Rohner, C., J. Smith. 1996. Brood size manipulations in great horned owls Bubo virgininanus: Are predators food limited at the peak of prey cycles?. The International Journal of Avian Science, 138/2: 236-242.
Rudolph, S. 1970. Predation ecology of coexisting great horned and barn owls. The Wilson Bulletin, 90/1: 134-137.
Rusch, D., E. Meslow, P. Doerr, L. Keith. 1972. Response of great horned owl populations to changing prey densities. The Journal of Wildlife Management, 36/2: 202-296.
Smith, D. 2002. Great Horned Owl. Mechanicsburg, PA: Stackpole Books.
Sullivan, J. 1995. "Index of Species Information" (On-line). Accessed March 31, 2012 at http://www.fs.fed.us/database/feis/animals/bird/buvi/all.html.
Walsh, D. 1989. Habitat Use, Population Densities, and Vocal Behavior of the Great Horned Owl in Central Utah. Ann Arbor, MI: Proquest Dissertations and Theses.