Can Barn Owls be an Effective Gopher Control Method

THOMAS MOORE, and DIRK VAN VUREN, Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, California 95616.

CHUCK INGELS, University of California Cooperative Extension, 4154 Branch Center Road, Sacramento, California 95827.

INTRODUCTION pocket gophers are a serious rodent pest in many California orchards and vineyards. Active throughout the year, they can increase to high numbers if not controlled, causing damage and loss of vines and trees and interfering with irrigation and other cultural operations (Salmon et al. 1992). Pocket gophers are often managed by trapping, fumitoxin and use of toxic bait, usually strychnine, delivered manually by a probe or by a mechanical bait applicator (Marsh 1992). As a result of the desire to continue gopher control and reduce rodenticide use, hundreds of farmers in California and nationwide have installed artificial nest boxes to attract barn owls to farms as part of a rodent management strategy. The prey species most often taken by barn owls are meadow voles, pocket gophers, and mice. Barn owls readily adapt to artificial nest boxes). Availability of nest sites appears to be the factor limiting barn owl population growth in habitats disturbed by humans, including agricultural areas. Barn owls rarely display any territorial behavior, except in the vicinity of the nest site which enables farmers to attract many breeding pairs to relatively small areas. Despite the widespread interest in attracting barn owls to farms, the efficacy of artificial nest boxes in promoting owl numbers and controlling rodents in vineyards and orchards are unknown. The purpose of this study was to use grower surveys and diet analysis to assess the potential of installing barn owl nest boxes in gopher management. The results of the survey will provide information on nest box occupancy characteristics and on the perceived effectiveness of this rodent control. Pellets regurgitated by barn owls contain intact skeletal remains of rodent prey consumed and can be used to ascertain composition of diet.

METHODS Surveys In 1995, questionnaires were mailed to 207 farmers which had previously received information packets on barn owls from the Sustainable Agricultural Research and Education Program, at University of California, Davis, from 1993 to 1995. Farmers were asked about: 1) reasons the growers wanted to install barn owl nest boxes; 2) the severity of pest problems, particularly in regards to voles and gophers; 3) the proportion of nest boxes occupied within the first year; and 4) the perceived effectiveness of barn owl presence in controlling vertebrate pests.

Diet Analysis Ten growers in the Lodi Grape Growing District of the northern San Joaquin Valley were randomly selected to participate in the field research studying the composition of barn owl diets. Crop types within 100 meters of nest boxes consisted of: 1) vineyards; 2) orchards; 3) vineyards and orchards; 4) vineyards and fallow land; 5) vineyards and alfalfa; and 6) orchards, vineyards and alfalfa. The authors selected 38 nest boxes installed by ten growers and collected pellets deposited by 394

barn owls from within and beneath each nest box at regular intervals throughout the nesting season in 1996. Individual pellets were gently broken up by hand and all skeletal parts were analyzed for prey identity. Skeletal remains were identified by comparison with mammal and bird specimens in the Museum of Wildlife and Fisheries Biology, University of California, Davis. Percent frequency of occurrence of prey items among pellets for each collection interval were calculated. To determine the average number of gophers eaten by one pair of owls during the nesting season, the number of right mandibles of gophers found in pellets were counted. The mandible length was measured to estimate occurrence of juvenile gophers in barn owl diets.

RESULTS A total of 88 people responded to the survey for a 43% response rate. Of the respondents, 55 had installed a total of 241 artificial nest boxes. Only survey results from 55 respondents that had installed nest boxes are reported. Forty-eight percent of farmers reported that the most important reason they installed owl nest boxes was to control vertebrate pests. Thirty-six percent of the individuals installed boxes for a hobby, 2% of respondents hoped to increase wildlife on their farm, and the remaining 12% installed boxes for other reasons. Pocket gophers were considered a moderate to severe pest problem by 77% of respondents prior to nest box installation; 18% of people replied that gophers were only a slight or non-existent problem and the remaining individuals were unsure. Only 9% of farmers answered that meadow voles were a moderate or severe pest. Thirty percent of people felt meadow voles were a slight or non-existent pest problem and the remaining 61 % were not sure or had no answer. Respondents reported that 40% of the nest boxes were occupied by barn owls in 1995. 7% of individuals felt that installation of nest boxes to attract barn owls was very effective in controlling gophers. Another 16% of respondents considered nest box installation somewhat effective, and 11 % thought they had no effect at all. The remaining 66% were not sure or had no answer. Installation of owl boxes to control meadow voles was considered very effective by 2% of respondents and somewhat effective by 7%. Twelve percent of individuals thought the approach was not effective and the remaining 79% were not sure, or had no answer. Preliminary diet analysis indicated that gophers and voles were the two most abundant prey in barn owl pellets, each occurring in over one-third of pellets. Occurrence of gophers increased in spring and summer, probably because barn owls were preying upon abundant juvenile gophers. On average, a pair of nesting barn owls consumed a minimum of almost one gopher per day during the nesting season. 

DISCUSSION Results of the survey indicate that controlling vertebrate pests was the most common reason why respondents had installed nest boxes. Most respondents had previously received literature on barn owl nest boxes as alternatives to vertebrate pest management; this literature included estimates of nesting pairs of owls and young consuming over a thousand rodents per nesting season (Colvin 1986). Installation of nest boxes for barn owls has been shown to double the number of breeding pairs in a given area and also produce significantly larger clutches. The recent increase in installation of barn owl nest boxes may reflect a concern with the risks posed to non-target wildlife and domesticated animals. The survey showed that installation of nest boxes is successful in attracting barn owls. Farmers replied that about 40% of boxes had been occupied. Many respondents indicated that they had installed the boxes during or after the nesting season, after which owls were unlikely to occupy nests. The reported occupancy may be underestimated. Although literature had been previously distributed to farmers to help in assessing owl occupancy, determination of occupancy often requires physical inspection inside the box. Most individuals considered pocket gophers a moderate to severe pest problem prior to nest box installation. Although almost one-quarter of farmers felt that the installation of nest boxes that attracted barn owls had an effect on their gopher problems, over two-thirds were not sure of any effect. Even though sample sizes were small in the survey, the results suggest that there is little substantive evidence for growers to ascertain positive effects of nest box installation for gopher control.