BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Otus asio | Eastern Screech-Owl 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Diurnal Activity:  Eastern screech-owls are rarely active before dusk.
In Ithaca, New York, the earliest recorded time of initiation of feeding
activity was 8:25 p.m.; the latest time feeding ended was 4:15 a.m. [39].

Migration Status:  Eastern screech-owls are resident throughout their
range; suggestions that some migration occurs at the northern edge of
the range have not been substantiated [18].  The large majority of adult
birds (87%) were recovered within 10 miles (16 km) of the banding site.
None had moved more than 40 miles (64 km) [37].

Pair Bond:  Craighead and Craighead [7] stated that they had no data on
year-round pairing, but they observed birds that appeared to remain
loosely paired over the winter, using the same hunting areas and
roosting sites as during the breeding season.  They delimited the
beginning of the breeding season by the selection of a nesting
territory; in Michigan this usually occurs around the end of February
[7].

Nesting:  Peak nesting season, when young are in the nest, appears to
coincide with most passerine migration (a rich source of prey) [37].  In
the Southeast, nesting season is from mid-March to late May, peaking in
April [15].  In Michigan earliest egg date was April 18 (1942) [7].  In
New York and New England, egg dates ranged from April 12 to May 18.  In
Pennsylvania and New Jersey eggs were present in nests from March 23 to
May 19, and in Florida, eggs were present from March 11 to May 18 [18].
The shortest time between eggs is 48 hours; the time between eggs is
often 72 hours and occasionally longer [19].  Typical clutch sizes are 3
to 5 eggs; clutch sizes range from 1 to 10 [18].

Incubation:  Incubation is almost exclusively by the female; if she
leaves the nest to drink or bathe, the male will incubate until she
returns and pushes him away.  The male brings food but the female eats
very little during incubation [19].  The incubation period is variable,
lasting from 26 to 32 days [37].

Development of Young:  The altricial young are downy, with eyes closed
[19].  The young have been observed shivering almost constantly for the
first 2 weeks of life [5].  The eyes are usually open by the 20th day.
Flight feathers begin coming in around the 25th day.  The average time
in the nest is 4 to 5 weeks [7,19].  Fledglings stay on branches near
the nest until full flight is achieved.  Even after leaving the nest
tree, fledglings continue to be fed by the parents for up to 5 weeks
after fledging.  By the ninth week, fledglings usually begin molting to
the first-winter plumage.  In Ohio the overall nesting success rate
(the proportion of active nests that fledged at least one young) was
86.1 percent, and the overall breeding success rate (the proportion of
eggs that resulted in young fledged) was 73.8 percent [37].

Juvenile Dispersal:  Family groups begin to split up by late August.
Dispersal, in an apparently random direction, occurs in early fall
[18,19].  In a Connecticut banding study, juvenile dispersal fell into
two groups; juveniles either travelled fewer than 20 miles (32 km) or
much greater distances (a minority).  The longest distance recorded for
juvenile dispersal was 185 miles (300 km) [37].

Breeding Age and Longevity:  One banding study indicated that at least
25 percent of eastern screech-owls bred as yearlings [37].  In the wild,
the average lifespan for eastern screech-owl in central Texas was
estimated as 3.6 years, although some individuals survived to at least 8
years of age [14].  Captive birds have lived more than 20 years [19].


PREFERRED HABITAT : 
Suitable eastern screech-owl habitat seems to be wherever nesting and
roosting cavities are near or adjacent to open areas with available
rodent prey [29].  Eastern screech-owls usually inhabit open woodlands
adjacent to meadows, marshes, or fields; forest clearings; old orchards;
and parks and other open areas in urban or suburban areas [10,9].  They
are more tolerant of human habitation and human activities than other
owls, and are found in or near barns, outbuildings, birdhouses,
hedgerows, and fencerows [19,29].  They are rarely found in swamps or
deep forest habitats [28].

Eastern screech-owls generally prefer areas with widely spaced trees
interspersed with grassy openings [10].  In Connecticut, red maple
woodlands, upland woodland, evergreen hedgerows, and edge habitats were
used by eastern screech-owl more often than expected if habitat choice
were random.  Lawns, mixed woods, and evergreen woods were used less
often than would be expected if habitat choice were random.  Although
lawns were not a selected habitat, they were a major component of all
monthly home ranges (a monthly home range is the amount of territory
used in the span of a month) [30].  In central Kentucky radio-tagged
eastern screech-owls used woodlots and edges more than expected based on
availability, and used pastures, old fields, and croplands less than
expected [32].  Lynch and Smith [38] reported that eastern screech-owl
abundance in Connecticut was positively related to the amount of natural
area within urban open space (examples include shrubs, old fields, and
marshes).  There was also a positive relationship among eastern
screech-owl abundance, total habitat diversity, and the amount of linear
edge.  Areas with mixtures of habitats including relatively high amounts
of undisturbed successional communities and edges tended to support many
owls [38].  In the southeastern states, eastern screech-owls are found
up to 4,500 feet (1,371 m) elevation (but are rare at the higher
elevations) [15].

Hunting:  Eastern screech-owls usually hunt in grassy openings, fields,
meadows, and along wooded field margins and streams [10].

Home Range:  In suburban areas most food gathering trips are less than
330 feet (100 m) round trip.  In areas with lower prey density food
trips may be longer [14].  Home range size is related to prey
availability.  Smith and Gilbert [30] estimated that the area traversed
in one night (the nightly range) for female eastern screech-owls was the
largest (38 acres [15.5 ha]) from November to February (when prey was
most scarce), was smallest during egg-laying and incubation (13 acres
[5.4 ha]) in March and April, and expanded again in June to 22 acres
(8.9 ha).  The larger nightly ranges in winter were thought to reflect a
need to cover greater areas to obtain adequate food.  The total home
range of one female was 323 acres (130.9 ha), but she typically hunted
only a small area each night and a larger cumulative portion each month.
Home range was largest while a female was selecting a nest site and
smallest during egg laying, incubation, and care of young.  The actual
defended territory included only the area immediately around the nest
cavity [30].  Gehlbach [14] estimated Connecticut home ranges as 9.9 to
14.8 acres (4-6 ha) and Texas home ranges exceeding 74.1 acres (30 ha).

Estimated Population Density:  In the Southeast, sampled breeding
density was listed as 2.8 pairs per 100 acres (2.8 pairs/40 ha) [15].
Lynch and Smith [38] observed a range of 1 to 18 eastern screech-owls
per square mile (1-7/sq km); an average of 6 birds per square mile
(2.3/sq km) occurred in 4 suburban areas in Connecticut.


COVER REQUIREMENTS : 
Nesting:  Eastern screech-owls nest in natural cavities, woodpecker
holes, nest boxes, and other artificial structures [9].  Within the
oak-hickory forest region, maples, apples (Malus spp.), sycamores
(Platanus occidentalis) with natural cavities, and pines with woodpecker
holes are preferred [16].  In Louisiana eastern screech-owls chose nest
cavities in trees with lianas more often than expected according to
availability [24].  Mean minimum diameter of cavity trees is 12 inches
(30 cm) [10].  Eastern screech-owls usually choose cavities with
openings 3 to 5 inches (7.6-12.7 cm) in diameter.  Typically, cavity
height is between 5 and 30 feet (1.5-9 m) above ground [5].  Nest
cavities are usually reused in successive years [11].  Nest boxes are
often reused by the same pair; one female was recorded using the same
box 8 years in a row [37].

Roosting:  Eastern screech-owls prefer to roost in natural hollows
protected from rain and snow, and out of direct sunlight.  They avoid
hollows used by fox squirrels (Sciurus niger) [7,19].  Roosting hollows
are typically 15 to 20 feet (4.5-6 m) and usually less than 40 feet (12
m) above the ground.  Roosting cavity openings are about the same size
as nest cavity openings [7].


FOOD HABITS : 
The eastern screech-owl is an opportunistic feeder.  Prey species are
primarily small rodents, but numerous other animals are consumed [10].
Norberg [25] stated that insects are the major portion of the diet of
Otus owls, but since the eastern screech-owl is sedentary it switches to
mammalian and avian prey in winter [37].  The mammalian prey list
includes mice and rats (Muridae), shrews (Soricidae), moles (Talpidae),
flying squirrels (Glaucomys spp.), chipmunks (Tamias spp.), and bats
(Chiroptera).  Avian prey includes songbirds, rock dove (Columbia
livia), northern bobwhite (Colinus virginianus), ruffed grouse (Bonasa
umbellus), American woodcock (Scolopax minor), common snipe (Gallinago
gallinago), American kestrel (Falco sparverius), and even other eastern
screech-owls [18].  Eastern screech-owls have killed domestic pigeons,
hens, and ducks with prandial intent [19].  Other vertebrate prey
includes snakes, lizards, frogs, toads, salamanders, and small fish.
Invertebrate prey includes many insects, snails, crayfish, spiders,
scorpions, millipedes, and earthworms [18].

Even though the prey lists are long, a few species form the main diet.
Favored prey includes meadow vole (Microtus pennsylvanicus),
white-footed mouse (Peromyscus leucopus), house mouse (Mus musculus),
and woodrats (Neotoma spp.) [19].  Snyder and Wiley [31] estimated that
year-round prey consumption by eastern screech-owls is 30.7 percent
invertebrates, 0.6 percent lower vertebrates, 65.6 percent mammals, and
3.3 percent birds.

Seasonal Diet:  In Michigan pellet analysis during spring and summer
indicated a high proportion of meadow vole, white-footed mouse,
crayfish, and small birds in eastern screech-owl diets [7].  Pellet
analysis must be interpreted with caution, since many invertebrate prey
items do not contribute to pellet formation.  In Ohio almost 65 percent
of the food items brought to nest boxes were birds, the majority of
which were migratory songbirds [37].

Craighead and Craighead's [7] analysis of pellets (Michigan study area)
indicated that meadow vole formed the bulk of the eastern screech-owl
winter diet.  In a year when meadow vole numbers were lower than
average, a higher percentage of small birds appeared in pellets.  In
both years, white-footed mouse were consumed in amounts comparable to
those of meadow voles.  Other major pellet components were shrews and
moles [7].  In Ohio fall and winter diets contained 30 percent birds
[37].

Foraging Habits:  Eastern screech-owls tend to follow approximately the
same hunting route within the territory, tending to visit previously
successful sites [5].  They usually forage in a perch-and-pounce manner,
taking short flights to capture already-spotted prey.  Eastern
screech-owls have been observed to take a position near a bat colony and
capture bats as they leave the colony [39].  Insects are often caught in
flight.  Eastern screech-owls also walk on the ground to forage for
insects, particularly night-dormant grasshoppers, and wade into shallow
water after fish and aquatic invertebrates [18,21,37,39].


PREDATORS : 
Larger owls prey on eastern screech-owl; the primary predator is the
great horned owl (Bubo virginianus), but others include barred owl
(Strix varia), great gray owl (S. nebulosa), snowy owl (Nyctea
scandiaca), long-eared owl (Asio otus), short-eared owl (A. flammeus),
and common barn owl (Tyto alba).  Large hawks also take eastern
screech-owl, but only rarely since hawks are largely diurnal.  Other
predators include domestic cat (Felis catus), Mustelids (minks, weasels,
and skunks), northern river otter (Lutra canadensis), bobcat (Lynx
rufus), and domestic dogs (Canis familiaris).  Snakes have killed adult
eastern screech-owls (usually because an eastern screech-owl attacked a
snake too large to capture easily) but are more commonly nest predators
[19].


MANAGEMENT CONSIDERATIONS : 
Population Status:  Census of eastern screech-owl by song playback
provides a quick and relatively easy method to obtain accurate numbers
[29].  Eastern screech-owl is common in the eastern United States, where
it is often the most common raptor [10,29].  Eastern screech-owl
population densities vary with the relative proportion of woodland cover
in an area [29].  In Connecticut long-term survey data indicated a
decline in eastern screech-owl populations in the 1970's and 1980's,
possibly as a result of increased habitat fragmentation and conversion
of open woods to residential areas [38].  In Ohio the eastern
screech-owl population fluctuated during the 30 years prior to 1975, but
no long-term trend was apparent in owls studied in suitable habitat.
The estimated annual recruitment needed to maintain population size is
2.21 fledglings per nesting pair.  Eastern screech-owls have probably
increased with the opening up of dense forest but may be threatened by
increasing conversion of woods to residential areas [37].  The increase
in early successional forest area in the latter half of the twentieth
century, largely due to abandonment of agricultural lands, may benefit
eastern screech-owls.  Management considerations for maintenance of
populations include provision of natural and artificial cavities and
preservation of woodlots in suburban areas [38].  Recommended size for
eastern screech-owl nest boxes:  floor 8 inches by 8 inches (20 cm X 20
cm), cavity depth 8 to 10 inches (20-24 cm), and an entrance hole 3
inches (7.5 cm) in diameter [16].

Pesticide Accumulation:  In the early 1970's eggshell thinning and DDE
and PCB levels in eggs were found to be relatively low for eastern
screech-owls.  It was suggested that the consumption of rodents (lower
trophic levels) prior to egg-laying contributed to the relatively low
pesticide loads in eggs.  Consumption of birds (higher trophic levels)
increased after eggs were incubated, and thus did not contribute to egg
pesticide loads [37].  Other pesticides have not been investigated in
relationship to eastern screech-owl.

Predator-Prey Relationships:  As a general rule, predators that are
specialized on small rodents tend to destabilize rodent population
cycles, whereas generalized predators tend to have a stabilizing effect,
sometimes suppressing cycles altogether.  The eastern screech-owl is
more generalist in its effects on rodent populations [25].  Even though
eastern screech-owls sometimes prey heavily on songbirds, they are more
likely to help keep rodent and insect populations in check [19].
Orchard owners reported that putting up nest boxes for screech-owls
resulted in a decrease in rodent damage to orchard trees [5].  Eastern
screech-owls commonly nest in boxes put up for wood ducks (Aix sponsa),
American kestrels, squirrels, and purple martins (Progne subis) and will
even attempt to nest in mailboxes.  Wood ducks often use nest boxes
after eastern screech-owls have left the nest; VanCamp and Henny [37]
suggested that wood duck nesting success is enhanced by eastern
screech-owl presence because owls discourage the usurpation of nest
boxes by European starlings (Sternus vulgaris).

Mortality Rates and Causes of Mortality:  Mortality rate for the eastern
screech-owl was estimated as 69.5 percent the first year; eastern
screech-owls over 1 year old experience approximately 34 percent annual
mortality [37].  Predation accounts for substantial adult mortality
[37].  Collision with motor vehicles is also a major cause of mortality.
Other causes include shooting (which has become less frequent since
legal protection of raptors was enacted), electrocution, collision with
windows, or entrapment in buildings, and drowning (apparently these owls
take many baths and can become waterlogged and drown in rain barrels)
[18].  There is a small amount of sibling cannibalism, usually after
accidental injury to a nestling [19].  In Michigan nest loss due to fox
squirrels (Sciurus niger), which usually then appropriate the nest
hollow, has been observed [7].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Otus asio | Eastern Screech-Owl