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Irruption Takes a Short-Term Toll, But Doesn’t Have Long-Term Consequences

By Elizabeth Serrano
Red-breasted Nuthatch by David Speiser.

From the Winter 2020 issue of Living Bird magazine. Subscribe now.

This article originally appeared in print as a sidebar to How Ron Pittaway Developed His Acclaimed Winter Finch Forecast.

Every two to three years, winter finches, along with Red-breasted Nuthatches (an “honorary” winter finch), are put to the test in an irruption—a forced migration of sorts due to fluctuations in their food supply. For birders, irruptions can be a special treat, because they get to see uncommon birds from the Canadian North flock to backyard bird feeders. But research published last spring in The Auk: Ornithological Advances shows that for some birds, irruptions are a bit more intense.

Irruptions are difficult phenomena to study because the transient bird populations are literally “moving targets,” says Erica Dunn, the author of the Auk research article. Dunn used several decades of citizen-science data from the Cornell Lab of Ornithology’s Project FeederWatch, the Christmas Bird Count, and eBird to analyze these waves of irruptions.

Dunn is a retired scientist today, but back in 1976 she started the Ontario Bird Feeder Survey, and 10 years later she partnered with the Cornell Lab to expand the survey to the whole of North America and rename it Project FeederWatch. Today with more than 20,000 participants each year, Project FeederWatch is an international citizen-science program that monitors the occurrence and abundance of wintering birds throughout North America via weekly counts submitted by birders at backyard bird feeders.

As a continuing independent researcher who studies FeederWatch data, Dunn has focused on the Red-breasted Nuthatch, a species that experiences regular irruptions in a two- to three-year cycle. For this research, she examined whether there was a negative relationship between a winter irruption and the next year’s summer breeding population. Dunn found that after an irruption occurs, the next year’s breeding Red-breasted Nuthatch population can be as much as 50% lower than the previous year’s.

But Dunn also found that the population losses stemming from an irruption don’t typically have long-term effects. Over the past 52 years, she found that the studied Red-breasted Nuthatch population had quadrupled, despite the 23 irruptions that had occurred during her study period.

Dunn says this kind of research is only possible through citizen science.

“I simply had a great data set and wanted to see what I could learn from it,” she says. “Citizen-science data is great for this kind of exploration, because the data sets are so large and are freely available to anyone who wants to work with them.”

Elizabeth Serrano is a student editorial assistant. Her work on this story was made possible by the Cornell Lab of Ornithology Science Communication Fund, with support from Jay Branegan (Cornell ’72) and Stefania Pittaluga.

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American Kestrel by Blair Dudeck / Macaulay Library

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