NEW YORK- A new study by the National Audubon Society reveals previously unknown population trends in 551 North American wintering bird species. A paper published today in Ecosphere, shows wintering bird populations in Florida, Hawaii, and the Gulf Coast of Texas and Louisiana are rapidly declining, while population increases are strongest in the Northeast, boreal forest and tundra regions.

 “Bayesian Hierarchical Modeling is a relatively new technique that allows us to estimate population trends from citizen science data with more certainty than ever before.  This enables us to confidently identify those bird species most in need of conservation efforts, and understand why some species’ populations are declining while others are stable or increasing,” said Dr. Nicole Michel, Quantitative Ecologist at Audubon. “Identifying wintering bird population trends is particularly important because climate warming is happening more rapidly in winter than during the breeding season.”

For over 100 years, hundreds of thousands of dedicated volunteer citizen scientists have ventured out through rain, snow and sleet to contribute to Audubon’s annual Christmas Bird Count, helping compile one of the largest and longest-running datasets of wintering birds in North America. Until recently, reliable estimates of long-term population trends were not developed because of the variation in the number and location of count circles and observers over time. Thanks to recent developments of new and powerful modelling techniques, Audubon researchers are now able to figure out which species need immediate aid and precisely where conservation efforts may be needed most.

Population trend estimates were generated for 551 species of birds across the United States and southern Canada from 1966-2013. In order to identify regions where wintering birds are doing well or poorly, data combined from 228 species produced cross-species trend estimates for each state, province and larger regions. Two-thirds (68%) of species were found to have positive trends, while populations of species that overwinter along the Gulf Coast and in Hawaii declined rapidly.

“By linking population trends with traits like diet and habitat preferences, we are beginning to understand what exactly causes population declines or increases,” said Dr. Gary Langham, Chief Scientist at Audubon. “Birds that eat seeds, live in grasslands and shrubby areas, and overwinter in warmer locations within their breeding range had declining trends, while birds that eat fruit, have long lifespans, are willing to live close to humans, prefer forested habitats and migrate short distances or not at all, had increasing trends.”  

At first glance it seems that the news is good; wintering birds such as Allen’s Hummingbirds and American White Pelicans are doing well. It’s possible that warmer weather may have contributed to winter bird population increases so far, either by reducing mortality from cold weather or by shifting wintering ranges to the north so more birds overwinter in the U.S. and Canada. But even so, nearly one-third of wintering bird populations – especially grassland and shrubland birds-- declined. Worse, temperatures are predicted to rise even more and more rapidly than they have changed so far, and severe weather may become more common.

Bird-related citizen science efforts are significant to understanding how birds are responding to a changing climate, and the wintering season offers critical information. This documentation is what enabled Audubon scientists to discover that 314 species of North American birds are threatened by global warming as reported in Audubon’s groundbreaking 2014 Birds and Climate Change Study. Audubon will continue to mobilize its network to take action to fight climate change, including efforts that will be aided by this study’s identification of species and regions facing the greatest threats.

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Citation: Soykan, C. U., J. Sauer, J. G. Schuetz, G. S. LeBaron, K. Dale, and G. M. Langham. 2016. Population trends for North American winter birds based on hierarchical models. Ecosphere 7(5):e01351. Doi:10.1002/ecs2.1351

The National Audubon Society saves birds and their habitats throughout the Americas using science, advocacy, education and on-the-ground conservation. Audubon's state programs, nature centers, chapters and partners have an unparalleled wingspan that reaches millions of people each year to inform, inspire and unite diverse communities in conservation action. Since 1905, Audubon's vision has been a world in which people and wildlife thrive. Audubon is a nonprofit conservation organization. Learn more at and @audubonsociety.


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