A new model analyzing how birds in western North America will respond to climate change suggests that for most species, regional warming is not as likely to influence population trends as will precipitation changes.
Several past studies have found that temperature increases can push some animal species – including birds – into higher latitudes or higher elevations. Few studies, however, have tackled the role that changes in precipitation may cause, according to Matthew Betts, an Oregon State University ecologist and a principal investigator on the study.
"When we think of climate change, we automatically think warmer temperatures," said Betts, an associate professor in Oregon State's College of Forestry. "But our analysis found that for many species, it is precipitation that most affects the long-term survival of many bird species.
"It makes sense when you think about it," Betts added. "Changes in precipitation can affect plant growth, soil moisture, water storage and insect abundance and distributions."
Results of the study, which was funded by the National Science Foundation with support from the U.S. Geological Survey and others, are being published in the journal Global Change Biology.
The researchers examined long-term data on bird distributions and abundance covering five states in the western United States, and in the Canadian province of British Columbia, testing statistical models to predict temporal changes in population of 132 bird species over a 32-year period. They analyzed the impacts of temperature and precipitation on bird distributions at the beginning of the study period (the 1970s) and then tested how well the predictions performed against actual population trends over the ensuing 30 years.
The scientists keyed in on several variables, including possible changes during the wettest month in each region, the breeding season of different species, and the driest month by area. Their model found that models including precipitation were most successful at predicting bird population trends.
"For some species, the model can predict about 80 percent of variation," Betts said, "and for some species, it's just a flip of the coin. But the strongest message is that precipitation is an important factor and we should pay more attention to the implications of this moving forward."
The study incorporated a lot of complex variables into the model, including micro-climatic changes that are present in mountainous environments. The research area encompassed California to northern British Columbia and the mountain systems drive much of the changes in both temperature and precipitation.
The researchers chose December precipitation as one variable and found it to be influential in affecting bird populations.
"Someone might ask why December, since half of the bird species usually present in the Pacific Northwest, for instance, might not even be here since they're migratory," Betts noted. "But much of the critical precipitation is snow that falls in the winter and has a carryover effect for months later – and the runoff is what affects stream flows, plant growth and insect abundance well down the road."
The rufous hummingbird is one species that appeared affected by changes in December precipitation, the researchers say. The species is declining across western North America at a rate of about 3 percent a year, and the model suggest it is linked to an overall drying trend in the Northwest. The evening grosbeak is similarly affected the authors say.
On the other hand, the California towhee shows a negative association with December precipitation, appears to be drought-tolerant – and its populations remain stable.
"We cannot say for certain that a change in December precipitation caused declines in evening grosbeaks or rufous hummingbirds," said Javier Gutiérrez Illán, a former postdoctoral researcher at Oregon State and lead author on the study. "Our model shows, however, a strong association between the birds' decline and precipitation changes and the fact that this variable pointed to actual past changes in populations gives it validity."
"The study shows that models can predict the direction and magnitude of population changes," he added. "This is of fundamental importance considering predictions were successful even in new locations."
The next phase of the research is to use the model to determine if there are patterns in the sorts of species affected – for instance, birds that are migratory or non-migratory, or short- or long-lived. They also hope to test additional variables, including land use changes, wildfire impacts, competition between species and other factors.
"In general, our study suggests that if climate change results in winters with less precipitation, we likely will see a spring drying effect," Betts said. "This means that populations of drought-tolerant species will expand and birds that rely heavily on moisture should decline."
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