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UW Researcher Contributes to Paper About Impacts of Changing Rainfall on Tropical Birds

January 9, 2017
blue-green bird sitting on a nest
A female blue-crowned manakin incubates a nest in Panama. Twenty species of tropical birds in Panama are affected -- in terms of survival, recruitment and, ultimately, population growth -- depending on the amount and timing of rainfall the tropical area receives, according to a recent paper in Nature Climate Change to which Corey Tarwater, a UW assistant professor of zoology and physiology, contributed. (Patrick Kelley Photo)

Twenty species of tropical birds in Panama are affected -- in terms of survival, recruitment and, ultimately, population growth -- depending on the amount and timing of rainfall the tropical area receives. And long-term climate change may well have an adverse impact on many of these bird species in the next 50 years, according to a University of Wyoming researcher involved with the study.

“We captured over 250 different species in our mist nets but, because many tropical birds have low densities, we were only able to use the 20 most common species in the understory to estimate demographic rates,” says Corey Tarwater, a UW assistant professor in the Department of Zoology and Physiology. “This makes the results of the paper even more troubling because the species we used are ones that are common now, and no one is concerned about, but they could be in trouble in the future.”

Tarwater was a contributing author to a paper, titled “Impacts of Changing Rainfall Regime on the Demography of Tropical Birds,” which appeared in Nature Climate Change Dec. 19. Nature Climate Change is a monthly journal dedicated to publishing the most significant and cutting-edge research on the science of climate change, its impacts and wider implications for the economy, society and policy.

Jeffrey Brawn, professor and head of the Department of Natural Resources and Environmental Sciences at the University of Illinois at Urbana-Champaign, is the paper’s lead author. Brawn was Tarwater’s adviser while she was a master’s degree and Ph.D. student at the University of Illinois.

“I helped capture the birds from 2003-2012, and I now have students or field assistants help net birds,” Tarwater says. “I provided intellectual input in the development of the paper, in terms of how rainfall might impact birds and why; and I helped write the manuscript.” 

Thomas Benson, an adjunct assistant professor in the Department of Natural Resources and Environmental Sciences, and the Prairie Research Institute at the University of Illinois at Urbana-Champaign; and Maria Stager and Nicholas Sly, both graduate students from the University of Montana’s Division of Biological Services, also contributed. 

The research looked at the population biology of 20 bird species sampled in central Panama over a 33-year period. Tarwater says the study took place in Central Panama, right along the Panama Canal, and where a lot of intact forest still exists. Annual rainfall fluctuates greatly, but, on average, rainfall is 2,500 mm/year. To put this in perspective, Laramie gets, on average, 350 mm of rain per year, she says.

Longer dry seasons decreased the population growth rates of 19 of the 20 species sampled, the paper concludes. Simulations with modest increases in dry season length suggest that consistently longer dry seasons will alter the structure of tropical bird communities. Such change may occur even without direct loss of habitat, a finding that has fundamental implications for conservation planning.

Systematic changes in rainfall may threaten some populations and communities of tropical birds, even in large tracts of protected habitat. The findings suggest the need for collaboration between climate scientists and conservation biologists to identify areas where rainfall will be able to plausibly maintain wildlife populations.

The study shows that adults were generally buffered against changes in rainfall, Tarwater says. Instead, it was recruitment of offspring to the breeding population that was strongly impacted by changes in rainfall. This highlights the importance of understanding what influences the annual number of young that adults produce and the juvenile period (from dispersal to survival to obtaining mates/territories).

blue-green bird sitting on a nest
This male red-capped manakin derives the majority of its diet from fruit, and primarily breeds in Panama’s dry season. Results of a paper in Nature Climate Change indicate that this species, as well as the blue-crowned manakin, respond negatively to lower rainfall. Both species, although abundant now, are predicted to decline with climate change. (Patrick Kelley Photo)

The paper concludes that the resilience of wildlife populations to climate change will be determined by their ability to cope with novel physical and ecological conditions. Species with wide physiological tolerances or behavioral plasticity will likely persist and maybe even thrive in altered environments. Other species will undergo changes in their distribution, adapt in their original place, or experience widespread eradication and possible extinctions.

“There isn’t really one group of species that I think has a great chance to adapt in response to climate change,” Tarwater says. “Our work showed that the fruit- and seed-eating species will do the worst, potentially because of changes in fruit availability and the shorter breeding period compared to some of the other species.

“Many of the insectivores also don’t do well, and other studies have shown that they have narrow physiological tolerances, meaning that they can’t handle a variety of environmental conditions. But, some insectivores are very flexible. They can eat a variety of insects. They can forage throughout the forest (from ground to canopy), and they have long breeding seasons. Being more flexible will likely help them adapt to climate change.”

What is still not known, says Tarwater, is what exactly is causing the changes in survival and recruitment. Do changes in rainfall alter the predators of these birds or the food resources of the birds and, if so, in what ways?

“This is where this research needs to go. We have identified the patterns, but not the mechanisms,” she says. “My lab is now focused on examining these mechanisms because this is necessary to make better predictions about what will happen in the future.”

The research that ended up in Nature Climate Change was funded by the National Science Foundation, the U.S. Department of Defense Legacy Resource Program, the U.S. Department of Agriculture’s National Institute of Food and Agriculture, the University of Illinois and the Smithsonian Tropical Research Institute’s Environmental Science Program.

“The birds in this study are currently abundant birds and doing well,” Tarwater says. “This study shows that even common birds may be in trouble with climate change, and we need to figure out why.”

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