BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Bubo virginianus | Great Horned Owl 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Migration:  Great horned owls are resident in most parts of their range
in North America [32].  Most (93%) banded birds from a wide range of
study sites have been recovered within 48 miles (80 km) of the banding
location.  Great horned owls banded in the South had traveled shorter
distances than great horned owls banded in the North [24].  Great horned
owls migrate away from conifer bogs and forests in the northern parts of
their range in severe winters.  It is also common for great horned owls
to move from Canada to the northern Great Lakes States in winter [32].

Pair Formation:  Great horned owls are usually the earliest nesting
raptors.  Pair formation occurs in early winter; the male chooses a nest
site and attempts to attract a female by copious vocalizations [24].

Nesting:  In Maryland and the District of Columbia great horned owls
nest from late January to late May; extreme egg dates are January 27 and
April 12 [38].  The typical great horned owl clutch is two or three
eggs; clutch sizes range from one to six eggs.  The male feeds the
female while she incubates the eggs.  Incubation time is 25 to 30 days
[12].

Development of Young:  Hatching dates usually occurs in mid-February.
The altricial young are downy, with eyes closed.  Their eyes usually
open by 7 days.  Hatchlings are brooded almost constantly by the female
for up to 3 weeks.  The male parent guards the nest closely.  Nestlings
often edge out of the nest by about 32 days but remain near the nest and
continue to be cared for by the parents until after full flight is
achieved.  At about 6 weeks of age nestlings begin flapping and learning
to fly; first flight may occur by 9 weeks but sustained flight is
usually not achieved until about 12 weeks.  Fledglings may spend up to
14 days on the ground prior to achieving full flight capability [12,25].

Age at First Breeding:  The great horned owl usually first breeds at
2 years of age [25].

Diurnal Activity:  In Manitoba juvenile great horned owls were observed
hunting between 8 p.m. and 12 a.m. in July and August [7].

Longevity:  The oldest banded great horned owl recovered in the wild was
13 years old.  Captive birds may live more than 20 years [25].


PREFERRED HABITAT : 
Great horned owls occupy more diverse habitats than any other owl;
habitats harboring great horned owls include deep forests (both
coniferous and deciduous), open woodlands, chaparral, desert cliffs,
woodlots, and wooded urban environments [12,32].  Great horned owls
prefer mature successional stages with openings [32].  Great horned owl
habitat usually includes fields and/or wetlands.  Recent findings
suggest a lower density of great horned owls in heavily forested tracts
than in more open areas; for example, there is a lower frequency of
great horned owls in the heavily wooded northern third of Wisconsin than
in the more open southern two-thirds [32].  In Utah great horned owls
were found most frequently in thinned stands.  They often forage in
slash piles [43].  In Maryland great horned owls are found in forests
and woodlots, adjacent fields, and marshes [38].  In New Jersey great
horned owls avoid close contact with human habitation and prefer to be
near water courses and in upland and lowland hardwoods.  In southeastern
Wisconsin, an average of 27 percent of the entire annual home range was
wooded or in marshlands and wet shrub communities; the remainder varied
among open areas and agricultural developments.  Actual usage patterns
within home ranges were concentrated in areas with appropriate hunting
and nesting sites (i. e., perches near open areas) [31,32].  The great
horned owl is found from sea level to timberline [13,37].

Home Range:  In southeastern Wisconsin the average size of the great
horned owl's annual home range was about 813 acres (329 ha).  Home
ranges of successful breeders were larger.  Home ranges decreased for
both successful and unsuccessful breeders in spring as prey availability
increased.  During summer, home ranges gradually expanded again [31].
Petersen [31] reported a density of one pair per 3 square miles (7.5 sq
km) in southeastern Wisconsin, but an active breeding density of one
pair per 4 square miles (9.3 sq km).  Nesting densities of great horned
owls in Michigan ranged from 0.15 to 0.26 bird per square kilometer.
The average home range was 524 acres (212 ha) [9].  In eastern South
Dakota the daily range of great horned owls exceeded observation plot
size of 100 acres (40 ha) [16].

Roosting:  Great horned owls usually roost in dense foliage, tree
cavities, old nests, and crevices in rocks [12].
 
Nesting:  Nest sites are often chosen adjacent to open areas suitable
for hunting lagomorphs (Leporidae) and rodents (Rodentia) [13].  In
western grasslands, great horned owls were observed in four habitats:
unbroken grassland, creek bottoms, cliffs, and cultivated land.  The
majority of great horned owl observations (80.5%) occurred in creek
bottoms [30].  Riparian areas are preferred for nest sites in the
northeastern states [28].  Supporting substrates for great horned owl
nests included trees (85%), cliffs (9.8%), and creekbanks (4.9%) [30].
In southeastern Wisconsin the number of active breeding pairs of great
horned owls was apparently related to cottontail (Silvilagus spp.)
density; owl productivity (number of young fledged) was positively
related to abundance of staple prey [31].

Hunting:  Prey availability in grasslands is a function of prey density,
vegetation structure, and mode of hunting.  Raptors tend to hunt in
areas with high capture probability [4].  In northeastern Colorado great
horned owls were observed in grazed bottomlands more often than in
ungrazed bottomlands, possibly because the lower vegetation left prey
more vulnerable [11].  Sympatry with common barn owls (Tyto alba) is
made possible by differing hunting strategies; for example, great horned
owls tend to hunt from perches and common barn owls typically capture
prey during flight [23].


COVER REQUIREMENTS : 
Nesting:  The great horned owl usually uses nests that were built by
other bird species, especially hawks (Buteonidae), herons (Ardeidae),
and crows (Corvus spp.) [14], but also common ravens (C. corax),
ferruginous hawks (Buteo regalis), and red-tailed hawks (B.
jamaicensis).  Great horned owls also use nest cavities excavated by
pileated woodpeckers (Drycopus pileatus) [18].  In northeastern Wyoming
great horned owls use nests of golden eagle (Aquila chrysaetos),
ferruginous hawk, red-tailed hawk, and Swainson's hawk (B. swainsonii)
[33].  Great horned owl nests are up to 70 feet (21 m) above ground in
cavities, tree limb crotches, stumps, caves, and rocky crevices, and on
ledges [14,33].  In the Great Basin great horned owls nested in juniper
trees, on cliffs, and in abandoned quarries with steep fronts [18].  The
great horned owl commonly uses the same territory but a different nest
each season.  In Wyoming one pair continuously occupied the same
territory for 7 successive years and another pair held a territory for 8
years.  Great horned owls occasionally reuse the same nest in successive
breeding seasons [7].

Roosting:  Great horned owls usually roost in places that allow maximum
concealment during daylight hours.  They often choose trees with dense
foliage that are separated from other trees in the area.  Conifers are
favored when present; in deciduous forests great horned owls prefer
trees that hold clusters of dead leaves over the winter (i.e., oaks and
American beech) [24].

In urban settings, great horned owls nest in deserted buildings,
powerline towers, haylofts of abandoned barns, and artificial nests
[10].

Hunting:  Great horned owls often hunt from perches adjacent to open
areas.  They usually fly below the treetops but occasionally fly
slightly higher [25].


FOOD HABITS : 
The great horned owl uses a wide variety of prey and takes animals up to
the size of young wild turkey (Meleagris gallopavo) and common porcupine
(Erethizontidae dorsatum).  Small- to medium-sized mammals and birds are
preferred, including hares and rabbits (Leporidae), mice (Muridae),
Virginia opossum (Didelphis virginiana), common muskrat (Ondatra
zibethicus), squirrels (Sciuridae), pocket gophers (Geomyidae), and
voles (Microtus spp.) [12].  Great horned owls also take reptiles
(including snakes), amphibians, large insects, and fish [12,14].  In the
Sierra Nevada 61 percent of great horned owl diet consisted of
cottontails; woodrats (Neotoma spp.) were the second most consumed food
item.  Pocket gophers and snakes were minor dietary components.  In
Oklahoma 25 percent of great horned owl diet was cottontails, 18 percent
pocket mice (Perognathus spp.), 12 percent kangaroo rats (Dipodomys
spp.), and 10 percent grasshopper mice (Onychomys spp.) [40].  Other
prey items include skunks (Mephitis and Spilogale spp.), ducks and geese
(Anatidae), ring-necked pheasant (Phasianus colchicus), grouse
(Phasianidae), domestic chickens, woodpeckers (Picidae), orioles
(Icterus spp.), and jays (Corvidae) [12].  Large prey can include small
dogs, domestic cats, and young foxes (Vulpes and Urocyon spp.) [10,25].
Predation on songbirds is minimal [12].  In northeastern Wyoming
riparian areas and adjacent big sagebrush-grasslands, the prey base for
great horned owls consists largely of white-tailed jackrabbit (Lepus
townsendii), cottontails, and black-tailed prairie dog (Cynomys
ludovicianus).  Other mammals and reptiles are common prey items [33].

Hunting Style:  Great horned owls usually hunt from a perch at the edge
of a clearing, making short flights out to capture prey.  They also
forage on the wing.  On occasion, great horned owls have been observed
walking on the forest floor turning over litter and other materials to
find insects, mice, and shrews (Soricidae).  Great horned owls wade into
shallow water for crayfish, fish, frogs, and turtles [24,25].


PREDATORS : 
Adult great horned owls have no natural routine predators.  Antagonistic
interactions with red-tailed hawks and crows are common [25].  Crows mob
and harass great horned owls during the day; great horned owls attack
roosting groups of crows at night, killing many at a time [12].
  


MANAGEMENT CONSIDERATIONS : 
Population Stability:  The great horned owl is a widespread and common
raptor.  All the midwestern states report great horned owls as common to
abundant, and they are common to abundant in the Great Plains.  Most
great horned owl populations are currently stable or increasing.  Only
on the High Plains Border and the Unglaciated Missouri Plateau
physiographic provinces did Breeding Bird Survey data suggest declining
populations [32].  Nesting densities and territory sizes fluctuate
annually, probably due to changes in food supply.  In Wyoming a
particularly small nesting population was observed in the same year that
cottontail populations were unusually low.  Temporary declines in prey
base are unlikely to cause any long-term great horned owl population
perturbations since great horned owl pairs unsuccessful breeders one
year are often successful in other years [34].  Only the most general
raptor management considerations need to be made for great horned owls,
such as retention of cavity trees, protection of riparian woods within
grasslands, and protection from human harassment [30,33].

Forest Composition:  In the northeastern United States, woods consisting
of hardwoods and pines are good habitat for great horned owl and other
raptors (barred owl [Strix varia], northern goshawk [Accipiter
gentilis], broad-winged hawk [Buteo platypterus], and red-tailed hawk).
Forest unit management guidelines should be set in relationship to
selected home range size; since the great horned owl has a very large
home range, managers should consider forest composition objectives in
groups of units rather than individual units.  All cavity trees and at
least 10 percent of remaining forest cover should be mature and/or
decadent trees.  Silviculture treatments favoring raptors in the White
Mountain National Forest, New Hampshire, included one 0.25- to 0.5-acre
(0.1-0.2 ha) plot left uncut for every 10 acres (4 ha) cut in
regeneration plots.  Uncut areas contained at least one living tree 18
inches (45.7 cm) or more in diameter with at least two major defects.
Wherever possible, uncut areas included existing raptor nests (any
species, since great horned owls use old nests of other raptors) [28].
Similarly, in Utah wildlife management considerations included leaving
large old trees for cavity nesters.  Woyda and Kessler [43] were of the
opinion that harvest patterns that result in even-aged regeneration
would only be appropriate for great horned owl and other species that do
not require a variety of tree age classes.

Young [44] lists the great horned owl as capable of persisting in
agricultural areas; nesting densities are strongly influenced by
intensity of land use, agricultural practices, and human activity.
Winter use of agricultural areas by great horned owls is common [44].

Artificial Nests:  Great horned owls use artificial nests when more
suitable nest sites are unavailable.  However, adding artificial nest
and perch sites in grasslands may encourage the use of grasslands by
woodland and edge species at the expense of raptors that primary use
grasslands [4].

Urban Settings:  Wintering great horned owl density reflects
availability of medium-sized prey such as skunks, domestic cats, etc. [10].

Mortality:  Major causes of death for great horned owls include
collisions with vehicles, shooting, and starvation [24,32].  The highest
mortality rates are among juveniles, largely due to cannibalism and
severe weather; annual mortality rate was 15 percent for nestlings, 58
percent for juveniles, 44 percent for 1- to 2-year-olds, and 28 percent
for adults over 2 years old [24,25].  At least 52 percent of banded and
recovered great horned owls had been shot and 12 percent had been
trapped; perhaps as many as 96 percent had been killed intentionally
[24].  Mortality due to pesticides during 1946 to 1968 was minimal [32];
however, poisoning of great horned owls due to pesticides used in urban
and suburban environments has been increasing [12].

Nuisance:  The majority of respondents to a raptor nuisance survey
(33/54) reported great horned owl nuisance, damage, or safety problems.
Most reported problems involved predation at poultry and game farms, and
at beagle clubs (preying on domestic rabbits kept for club activities).
Other nuisance problems involved predation on pets including exotic
animals and birds [21].

Raptor Reintroduction Projects:  Great horned owl presence at peregrine
falcon (Falco peregrinus) release sites is detrimental to the survival
of nestling and juvenile peregrine falcons.  Great horned owls occupy
suitable peregrine falcon nest sites (which are usually scarce) and prey
on juvenile great horned owls.  Much debate has occurred over the
feasibility and necessity for great horned owl removal during peregrine
falcon reestablishment.  The intent of most great horned owl removal
have been to reduce the threat to young peregrine falcons immediately
after release.  Personnel at the U.S. Fish and Wildlife Service have
stated that great horned owl control is unnecessary once falcons are
established and reproducing (with naturally fledged peregrine falcons
present).  They do, however, support localized great horned owl control
for a few months after peregrine falcon release and/or fledging [22].
Similar problems pertain to prairie falcon (F. mexicanus) management;
most nest sites large enough for prairie falcons are also suitable for
great horned owls, and young prairie lafcons are vulnerable to predation
by great horned owls [30].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Bubo virginianus | Great Horned Owl