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Estimating the Number of Rodents Removed by Barn Owls Nesting in Boxes on Winegrape Vineyards

  • Author(s): Johnson, Matthew D.
  • St. George, Dane
  • et al.
Abstract

To mitigate the economic and environmental costs of rodent pests, winegrape producers in Napa Valley, California, have installed nest boxes to attract barn owls to their properties, but their effectiveness to control rodent pests in vineyards has not yet been thoroughly tested. A rigorous estimate of the number of rodents that barn owls remove from the landscape is a necessary step, and this study aimed to produce estimates of rodent removal and prey species composition by using remote nest box cameras. Results indicate that each barn owl chick received 170.2 ± 8.92 rodents before dispersing from the nest box. Combined with the average number of chicks fledged (3.62 ± 1.40), this finding indicates adults deliver on average 616 rodents per nest box, with low and high estimates ranging from 358 to 899 rodents. With conservative assumptions of owl survival and consumption during the non-breeding season, we estimate a barn family could remove 3,466 rodents in a full year (estimates ranged from 1,821 to 7,563). An analysis linking videography to owl telemetry data suggested that 43% of rodents killed were taken from vineyard habitat, which nearly matches the availability of vineyard habitat around the monitored nest boxes (46%). In contrast, more prey were captured from riparian habitat and fewer from grassland habitat than expected given their availabilities. Our results suggest barn owl nest boxes could contribute meaningfully to integrated pest management. Future research should involve rodent surveys in vineyards without and without barn owl nest boxes.

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