How to find where bigger owls threaten little ones

The barred owl—an unrivaled nocturnal predator and procreator—is moving into the Pacific Northwest and encroaching on northern spotted owl territories and outcompeting this smaller, threatened cousin.

A model from Michigan State University shows how it’s happening and gives wildlife conservationists a highly accurate, yet cost-effective tool to help shape management policies.

“Our model estimates population abundance and demographic rates, such as survival and reproduction, from relatively ‘cheap’ data,” says lead author of the study Sam Rossman, postdoctoral researcher at Michigan State and the Hubbs-SeaWorld Research Institute.

“Typically, estimating these quantities requires intensive sampling efforts involving capturing, marking, releasing animals, and then repeatedly tracking and recapturing them at later dates.”

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The new model appears in the journal Ecology and uses data on two simple factors: presence or absence of animals across space and time. While the input may be simple, the model is not. “Dynamic N-occupancy” is capable of providing accurate estimates of local abundance, survival rates, and population gains—including reproduction and immigration—while accounting for the fact that the presence of a species may be detected imperfectly during sampling.

northern spotted owl
Northern spotted owl. (Credit: Kameron Perensovich/Flickr)

“Simply put, the model is telling us the rate at which barred owl numbers are increasing and offering clues as to why that’s happening. This, in turn, can help us understand how endangered spotted owl populations in the same region may respond,” says Sarah Saunders, a postdoctoral researcher and one of the paper’s coauthors.

“Barred owls are bigger, more aggressive, maintain a smaller territory, produce more young, and are even outcompeting spotted owls in old-growth forests, what was once thought to be spotted owl strongholds.”

This paper adds to our understanding of barred owl demographic rates and their growth over space and time. The team’s analysis revealed consistently high survival rates and a strong positive relationship between regional population abundance and local-level reproduction and immigration, which has led to dramatically increasing barred owl numbers within the study area.

Barred owls’ arrival in Oregon has been pronounced. However, the team found that their population growth within the study area has slowed over the last five years. This potentially indicates that population is approaching saturation.

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The team used 26 years worth of data collected by US Forest Service researchers. In a direct comparison of survival estimates, the team’s results were consistent with those from a nearby radio-tracking study, using data that were much more expensive and labor intensive to collect.

Dynamic N-occupancy’s speed, accuracy, and affordability, as well as its capacity to involve citizen scientists in the process, have the research team excited about its potential and myriad applications.

“We are like wildlife detectives. People come to us when they can’t solve their mysteries.”

The model can work from everything from estimating monarch populations to helping better understand bird populations based on Audubon’s Christmas Bird Count. It can work especially well on tracking the increase of invasive species or the decrease of endangered species, says Elise Zipkin, an integrative biologist and one of the paper’s coauthors.

“We’ve developed a crucial tool in helping explain why populations are changing and at what costs,” she says. “We are like wildlife detectives. People come to us when they can’t solve their mysteries. We’re able to quickly sift through extensive data sets, involve citizen scientists in data collection, and help them solve their problems.”

Additional contributors to the study include scientists from US Geological Survey and the US Forest Service.

Source: Michigan State University

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Source: Futurity