Barn owls

“All testimony goes to show that the barn owl is one of the most beneficial of rapacious birds”

-- A.K. Fisher, USDA Report 1893

Here is a linktree with links to a bunch of resources, or click the links below for our latest results and recommendations

Live nest cam in Lodi! [check back next season!]

Predictive map of nest box occupancy in Napa

Predictive map of hunting pressure in Napa

Nest box Recommendations

Nest box plans

Track a hunting owl in Lodi

Owls and farmers

The lab's research on how birds can help control insect pests in tropical coffee farms inspired us to look closer to home, and we found an amazing study system with barn owls in California farms. For decades, farmers have been installing nest boxes to attract these rodent-eating raptors, but little ecological research had been done in California. So in 2015 we started work on 65 winegrape vineyards in Napa Valley, aiming to unravel the ecology of these awe-inspiring birds and how they hunt their prey. Ecologists call this an "ecosystem service" -- a process provided by nature that benefits people. But we prefer to think of this as a reciprocal relationship, and we aim to clarify how owls and farmers can mutually benefit each other. These owls are limited by the availability of nesting sites, so farmers can provide high-quality and safe nest boxes for these birds, and in return the owls will do what owls do -- eat a bunch of rodents!

Nest box selection

We have monitored over 350 nest boxes in Napa Valley since 2015, with some additional work in Lodi, Sonoma, and Fresno in some years. Roughly 30-75% of the boxes are occupied by breeding owls in a given year, and the boxes vary in design (material, size, entrance orientation etc.), local habitat, and surrounding landscape composition, providing a perfect system to ask why the owls choose some boxes over others. Are these preferences consistent across regions? Does the presence of other owls influence the likelihood a new nest box is colonized? These questions relate to foundational topics in animal habitat ecology and are central to our lab's core themes, but answering them has also allowed us to make practical recommendations for farmers. Several former graduate students have propelled this work, especially Carrie Wendt and Jaime Carlino, who discovered that barn owls prefer tall wooden nest boxes (24" tall), mounted on tall poles (>12 feet), with abundant grasslands and not much forest nearby. Here are additional recommendations and a predictive map of nest box occupancy in Napa Valley.

Hunting habitat selection

Habitat selection is a core theme of our lab, and with barn owls we're using the latest technology to investigate where the owls hunt prey. Work by former lab members Allison Huysman and Xeronimo Castaneda revealed how often the owls hunt in vineyards and their resilience to periodic wildfires that occur near Napa Valley. Sam Chavez expanded on that work, creating one of the first maps of an ecosystem service provisioned by a mobile vertebrate predator. The latest work by Jadzia Rodriguez involves small backpacks with GPS trackers equipped with tiny accelerometers to tell us where the owls fly each night, and where they strike the ground with enough force to indicate an attack. This work is providing an unprecedented opportunity to ask how composition of habitats, including native oak woodlands and grasslands, affects the delivery of economically valuable pest control by barn owls in winegrape vineyards.

The quality of habitats and individuals

Some owl families are highly successful, fledging 5 or more chicks, and others fledge only one or two or even fail altogether. Why such variation? Are some birds just better than others, or do the habitats around the nest boxes govern a family's success? This question of habitat quality is another core theme of our lab, and heterogenous landscapes of Napa Valley provide a great setting to test these ideas. Jaime Carlino and Christian Cortez found that habitat composition affects not only the number of young fledged, but also their physiological condition. But just like people, not all barn owls are alike. For one, they are "polymorphic", with some owls being almost pure white on the breast, and others reddish brown. They also have a variable number and size of black spots. Former lab member Laura Echavez explored how habitat associates with polymorphism. In addition, some owls behave aggressively, and others are more docile. Across many taxa, ecologists have recognized a "bold-shy continuum," with trade-offs at each extreme (e.g., bold animals are better at exploring, but often investigate areas less thoroughly than their shy counterparts). The latest work by Lauren Jackson is examining whether bold and shy owls hunt differently, and whether this affects their reproductive success.

Two barn owls differing strongly in plumage color and spotting on the breast (photo courtesy of our colleague A. Roulin from Switzerland).

Pest removal services

In California's winegrape vineyards, barn owls primarily hunt pocket gophers, voles, and deer mice. Based on videography work by Dane St. George, we learned that an average family of barn owls (mom, dad, and 3-4 young) will eat 3,000-4,000 rodents in a year! But there are a lot rodents out there, is that enough to make a difference? Work by Ash Hansen and Katherine Larson indicates that the owls do indeed reduce the abundance and activity of gophers, voles, and mice. We also know that installing and maintaining owl nest boxes is much cheaper than trapping rodents, making nest boxes a viable component of chemical-free integrated pest management for rodents.

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