Barn owls are doing more than making themselves at home in Napa Valley vineyards. New research from Cal Poly Humboldt found the birds can substantially reduce rodent populations and make the rodents that remain more cautious.
A recent study suggests barn owls could offer wine grape growers a natural way to protect vines from damaging pests while reducing their reliance on chemical rodenticides.
Matthew Johnson, a professor of wildlife habitat ecology at Cal Poly Humboldt, co-authored the study examining how barn owls affect rodent populations and behavior.
“We found that they were doing both in the vineyards,” Johnson said.
His team documented a 38% to 52% reduction in rodent abundance and a 16% to 38% increase in how much danger rodents perceived.
Rodents such as deer mice, voles and pocket gophers can cause costly vineyard damage by chewing grapevines and damaging root systems. To combat the pests, some growers install wooden nest boxes on tall poles to attract barn owls to otherwise open vineyard habitat.
A single owl family, consisting of two parents and three to four chicks, can consume 3,000 to 4,000 rodents a year.
But the owls’ influence goes beyond what they eat. Using camera traps, chew blocks and trays filled with sand and sunflower seeds, researchers measured how quickly rodents abandoned food when they sensed danger.
Researchers also found that vegetation can influence how rodents respond to barn owls. Because barn owls primarily locate prey by sound, they may have an advantage over more visually oriented rodents when vegetation is thicker.
The study found barn owl hunting was associated with lower deer mouse activity in thick thatch after mowing. Researchers said growers could time mowing to reduce mouse activity when grapevines are particularly vulnerable.
"The owls are supremely adapted to hunt with their ears," Johnson said, noting that scurrying sounds in the grass easily betray a rodent's location to the bird's highly sensitive, asymmetric ears.
Johnson cautioned growers against combining barn owl nest boxes with chemical rodenticides, which can threaten owls and other wildlife through secondary poisoning when they eat poisoned rodents.
“This is really a great opportunity for wine grape growers to shift away from using those chemical rodenticides and instead work with nature,” Johnson said.
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- Matthew Johnson, professor of wildlife habitat ecology, Cal Poly Humboldt
