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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
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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