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Results of Barn Owl Prey Study in the Lodi Grape District
by Chuck Ingels, Sacramento County Farm Advisor
(Editor's note: This article is a summary
of work conducted in 1996 and reported in a 1998 article by Dirk
Van Vuren, Tom Moore, and Chuck Ingels, titled, "Prey
Selection by Barn Owls Using Artificial Nest Boxes." The
article was published in California Fish and Game, vol. 84, no.
3. Contact the Lodi Woodbridge Winegrape Commission at (209)
367-4727 if you would like a copy of the article.)
Introduction:
Barn owls have received considerable attention because of their
potential to control rodent pests. Barn owls are limited by
availability of nesting cavities but readily use artificial nest
boxes; installation of nest boxes may therefore increase numbers
of breeding barn owls. Also, barn owls exhibit a low degree of
territoriality, so several breeding pairs might be concentrated
in a relatively small area. In this 1996 barn owl prey study, we
sought to assess the diets of barn owls attracted to nest boxes
in order to evaluate their potential as a means of controlling
gophers.
Methods: We randomly selected 10 growers in the
Lodi area who had installed 38 nest boxes. Nineteen of these
boxes were used for nesting, as evidenced by the presence of
nestling owls or broken egg shells, and 19 were used as nest
sites. Crops within 100 yards of each nest box consisted of
vineyards, orchards, alfalfa or fallow land. In November 1995,
before nesting in January, we removed and discarded all
regurgitated pellets found in or near our nest boxes. At 5 week
intervals thereafter, we collected all pellets found beneath the
nest boxes and adjacent roost sites. At the end of the study in
August 1996, we collected all pellets found in each nest box,
which represented additional prey eaten from January to July that
had not been recorded during the 5-week interval collections.
Results:
We collected 621 pellets from 38 boxes. Pocket gophers and voles
were the most frequent prey in barn owl diets. Deer mice and
house mice were also common in diets. Birds and rats were
occasionally eaten, and rabbits, insects, crayfish, bats and
moles were eaten rarely. Gophers were eaten most often during the
spring and summer, whereas voles were eaten most often during the
winter.
Body mass is strongly related to mandible length. In our study,
mandible length averaged 2.0 cm., corresponding to an estimated
body mass of 61 grams. Because the median body mass of adult
gophers exceeds 90 grams for females and 120 grams for males,
barn owls were evidently eating mostly juvenile gophers. However,
the size of gophers eaten changed seasonally; in general, smaller
gophers were eaten during spring and summer. Among the 6
collection intervals, gopher size was negatively correlated with
percent occurrence of gophers in the diet. The number of gophers
eaten per nesting pair of owls varied greatly and averaged 152
gophers for the nesting season, about 22 gophers per month per
pair. This represents a minimum figure, since barn owls may
regurgitate pellets away from nest boxes where we did not find
them.
Discussion: At
a minimum, a pair of nesting owls ate an average of almost one
gopher per day, in addition to other prey. Whether this rate is
sufficient to limit gopher numbers depends on gopher densities
and rate of reproduction, as well as barn owl densities. Gopher
numbers of 8 per acre in non-irrigated fields in the Central
Valley are typical, and densities in irrigated fields can be as
high as 48 per acre. One female gopher produces 6 to 20 young per
year, depending on whether the field is irrigated. Assuming a
female gopher density of 6 per acre, a reproductive rate of 6
young per female (all produced during the barm owl nesting
season), and no other predators, barn owl densities of one pair
per 5 acres in non-irrigated fields would be necessary to remove
the annual reproductive output of gophers.
Whether barn owls can achieve densities sufficient to limit
gopher numbers through predation alone remains uncertain.
Nevertheless, barn owls may play an important role in an
integrated approach to gopher management by reducing the
frequency of rodenticide applications. Although vertebrate
predators cannot control numbers of an abundant prey, if prey
numbers are first removed by other means, predators may keep
numbers low for a time. Thus, once numbers have been reduced by
rodenticides, barn owl predation may slow the recovery of the
gopher population, thereby lengthening the interval until
rodenticides are needed again.
Predation by barn owls may be particularly effective at delaying
population recovery of gophers through reproduction, because of
their apparent preference for juveniles. Further, depleted rodent
populations can be restored rapidly through immigration; thus,
barn owl predation on dispersing juveniles may be especially
important in reducing the rate of re-invasion of a depopulated
field from surrounding areas.
Frequency of occurrence of prey items and body mass of pocket gophers in diets of barn owls in San Joaquin County, California.
Collection Date | 1 Feb. | 2 Mar. | 1 Apr. | 1 May | 20 Jun | 3 Aug. | Overall |
Number of pellets | 254 | 111 | 42 | 89 | 94 | 31 | 621 |
Average of pellets w/ bones from... | Average | ||||||
Gopher | 32 | 30 | 33 | 60 | 55 | 83 | 42 |
Meadow vole | 51 | 42 | 40 | 30 | 35 | 13 | 42 |
Deer mouse | 22 | 32 | 26 | 17 | 10 | 6 | 18 |
House mouse | 17 | 30 | 21 | 10 | 7 | 3 | 15 |
Bird | 4 | 2 | 8 | 1 | 10 | 13 | 5 |
Rat | 4 | 3 | 6 | 1 | 1 | 0 | 3 |
Gophers | |||||||
Number | 55 | 24 | 13 | 82 | 66 | 37 | 277 |
Avg. Weight (g) | 69 | 90 | 79 | 52 | 67 | 53 | 61 |
Range (g) | 28-212 | 39-231 | 42-182 | 20-199 | 25-194 | 22-162 | 20-231 |
(Please note: Some pellets contained bones from more than one prey, resulting in percentage totals greater than 100%.)
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