Fall, 1992 (v5n1)
by Chuck Ingels, SAREP
Farmers seeking to reduce or eliminate are often frustrated inability
to control vertebrate pests. Preventive strategies, such controlling
vegetation around orchard tree trunks and field borders, can help
with meadow mice and gopher control. Common non-chemical methods
include shooting, trapping, and flooding. While these methods
can be very effective, they are not without limitations; flooding
is not always possible, and trapping and shooting can be very
time-consuming and impractical where large areas are infested.
Birds of prey can contribute to vertebrate pest management, especially
in fields located near riparian areas. While raptors are seldom
relied upon as the primary means of vertebrate control, they can,
with a little help, be more effective than many people think.
This article explores the effectiveness of owls and hawks in
vertebrate pest management and techniques for enhancing their
populations.
Barn Owls
There are many different species of owls, but the barn owl (Tyto
alba) is the most helpful to farmers. It is often
called "the most beneficial bird in the world" because
of its hearty appetite for gophers, ground squirrels, and meadow
mice. Farmers who have learned of the barn owl's virtues strive
to keep this "cat with wings" in close proximity to
crops. One nest of six young barn owls and two adults may consume
more than 1,000 small mammals during the nesting season.
Because of their high first-year mortality, short life-span (four
years maximum), and dependence upon the fluctuating nature of
rodent populations, barn owls have developed a tremendous reproductive
capability in order to survive. They are often referred to as
"reproductive machines." This capability functions
in response to availability of prey; they can quickly colonize
an area if suitable habitat (prey and nest sites) is available.
Barn owls are strictly nocturnal hunters, having the remarkable
ability to see their prey in complete darkness. They hunt from
perched or flying positions, and have been known to spot prey
from a distance of several hundred feet. Barn owls routinely
fly one mile from their nests to hunt, and may venture up to three
miles or more. Their preferred hunting sites are grassland and
wet meadow habitats, either with a few trees or in wooded areas.
Barn owls do not build nests. They lay eggs in hollow trees,
crevices in cliffs, and holes in sandbanks, and also find home
sites in abandoned buildings, granaries, or barns. According
to many researchers and farmers, it is fairly easy to attract
barn owls to fields, orchards, or vineyards by constructing nest
boxes.
Grower Success
Merced County farm advisor Lonnie Hendricks reported that
several almond growers have drastically reduced gopher populations
in orchards by installing barn owl nest boxes. One such grower,
Bill Genn of Hilmar, had orchards so badly infested with
gophers that his flood irrigation water often spilled onto neighbor's
land from gopher holes at the edge of the orchard. Genn was advised
to install nesting boxes for owls in trees and on poles near the
orchard. Owls now live in the boxes and Genn's gopher problems
have disappeared; rodent bones litter the ground under the boxes.
Hawks
Hawks can also aid in vertebrate pest management. Important species
include the red-tailed hawk (Buteo jamaicensis), and the
American Kestrel (Falco sparverius), also known as a small
falcon, sparrow hawk or kitty hawk. Hawks eat meadow mice, small
birds, grasshoppers and other insects. To encourage hawks, whose
presence also frightens starlings and other pest bird species,
some farmers install perches and nest boxes near their crops.
Perches may be especially important in winter and early spring
to aid hawks in spotting food sources before the rodents' breeding
season, and when many crops are either absent or provide little
cover.
Kestrel Houses
Kestrels prefer to nest in dead trees and other crevices, but
also use secluded buildings and wood raptor houses. Open fields,
meadows and fence rows are good locations for kestrel houses.
Houses can be mounted on utility poles, buildings, lone trees
or posts. According to a Soil Conservation Service (SCS) bulletin,
the house can be made of long-lasting redwood or cedar, and should
be mounted 10 to 15 feet from the ground with the entrance clear
of branches. Because the house needs to be checked and cleaned
periodically, it should be erected where it can be reached. Complete
kestrel house plans are available from SCS. November through
January is the best time to build a kestrel house. Houses should
be checked weekly in the spring to make sure starlings and other
pest birds are not using the box. Kestrels bring no nesting material
into the house, so any material found in the box is from pest
birds and should be removed. Kestrel eggs are white/cinnamon
colored with spots of brown and hatch in about 28 days. Starling
eggs are pale blue. Screech owls, which may also use the boxes
and are desirable birds, have white eggs. Kestrel houses should
face south or east, and should be located within 200 yards of
a tall tree or pole because the raptors like high perches nearby.
Davis Survey
How effective are perches and nest boxes, and how effective are
hawks in vertebrate pest management? The data is mixed; many
growers report success, yet others contend that avian predators
alone cannot keep populations of meadow mice low for extended
periods of time because predators leave the area when prey abundance
is low. Surveys show mixed results of perch and box effectiveness.
Shawn Smallwood, a researcher in the agronomy and range
science department at UC Davis, recently completed a two-year
survey of the use of perches by hawks. His survey covered 200
miles in the Sacramento Valley, and included farms of all major
crops grown in the region. He studied artificial perches, trees,
telephone poles, and fenceposts.
Smallwood found that most hawks avoid the smaller perches installed
by farmers (horizontal dowels or boards supported by posts or
metal pipes); only occasionally did he find a small hawk using
one of them. Most hawks were found using telephone poles or vertically-oriented
"snags" on trees. Hawks prefer large perches which
can comfortably hold their whole body, according to Smallwood.
He also found that the height of a perch was not as important
to the raptors as the fact that it provided a broad view of the
surrounding land. Large trees are ideal roosts, Smallwood reported,
but they function best as perches if the canopy is opened so raptors
can get a clear view. Dead limbs sticking up above leaves are
used more than branches within the canopy.
Washington Study
Researchers in Washington state conducted a study examining the
use and effectiveness of artificial perches and nest boxes. Three
orchards in the Wenatchee area were used in the study. Researchers
made direct observations and examined predator pellets.
In this study, none of the barn owl boxes and only 13 percent
of the kestrel boxes were inhabited. However, more birds were
attracted to the orchards where perches were placed than those
without perches. The biomass and height of the understory vegetation
had no bearing on the use of perches in this study. The effect
of raptors on meadow mice populations was unclear; populations
were reduced in one orchard only. However, the level of human
activity may have played a major role. Where houses and roads
were most heavily used, few birds visited the perches. Human activity
was minimal near the orchard in which raptor use of perches was
high and the mice population was reduced.
Other Studies
In an Oregon study, American kestrels and great-horned owls showed
a preference for 5-meter perches over 2.5-meter perches, but the
raptors accepted the shorter perches in the absence of taller
ones. Barn owls did not show a height preference.
Pest bird activity in vineyards was not affected by the presence of artificial perches, according to a Napa study. Although four hawk species were observed in the area, none was seen using the artificial perches. Pest birds were not deterred by hawk models on some of the perches.
(We gratefully acknowledge the information provided by Paul
Gorenzel, Cooperative Extension Wildlife Unit, University
of California, Davis.)
FOR MORE INFORMATION:
Askham, L.R. 1990. Effect of artificial perches and nests in attracting raptors to orchards. Proc. Vert. Pest Conf. 14:144-48.
Colvin, B.A. 1986. Barn owls: Their secrets and habits. Illinois Audubon, No.216, Spring 1986.
Hall, T.R., W.E. Howard, and R.E. Marsh. 1981. Raptor use of artificial perches. Wildl. Soc. Bull. 9(4):296-298.
Hendricks, L.C. 1992. Gopher control. Tree & Vine Notes, April 1992. UC Coop. Ext., Merced County.
U.S. Dept. of Agric. 1984. Kestrel house plans and instructions. Soil Cons. Serv. Job Sheet CA-499. SCS, 2121 Second St., Davis, CA 95616; (916) 757-8200.
U.S. Dept. of Agric. 1985. Barn owl nest box plans
and instructions. Soil Cons. Serv. Job Sheet CA-50l (Rev). SCS,
2121 Second St., Davis, CA 95616; (916) 757-8200.
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