More info for the terms: cover, density, presence, tree
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].
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