North American Bird Populations More Affected By Precipitation Changes Than Warming Temperatures
July 11, 2014

North American Bird Populations More Affected By Precipitation Changes Than Warming Temperatures

Brett Smith for - Your Universe Online

While many previous studies have focused on how rising temperatures associated with climate change might affect North American birds, a new study is looking into how projected changes in precipitation might affect these birds.

Published in the journal Global Change Biology, the new study found that “changes in precipitation and winter temperature appear to have already driven shifts in the geographic patterns of abundance of bird populations in western North America.”

“When we think of climate change, we automatically think warmer temperatures,” said study author Matthew Betts, an associate professor of forestry in Oregon State. “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.”

In the study, the team of North American scientists analyzed long-term information on bird distributions and densities across the western United States and Canada, assessing statistical models to calculate temporal shifts in population of more than 130 bird species over a 32-year period. They also reviewed the effects of temperature and precipitation on bird distributions during the 1970s and then tried to see how well the predictions held up against actual population trends over the following 30 years.

The study tracked numerous factors, including possible variations during the wettest month in each region, the mating season of distinct species, and the driest month by region. The team discovered that models including precipitation were best 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 integrated numerous complex variables into the model, including micro-climatic shifts that are seen in mountainous environments. The study location spanned from California to northern British Columbia and the mountain systems of this region drive much of the shifts in both temperature and precipitation, the researchers noted.

The scientists selected December precipitation as one potential variable and saw it was 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.”

While the model suggested the recent decline of the rufous hummingbird is linked to an overall drying trend in the Northwest, it also showed the California towhee to be drought-tolerant – with its populations remaining stable.

“We cannot say for certain that a change in December precipitation caused declines in evening grosbeaks or rufous hummingbirds,” said study leader Javier Gutiérrez Illán, a former postdoctoral researcher at Oregon State. “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 researchers said future work will involve looking for patterns in the sorts of species affected and testing additional variables, such as land use and wildfire impacts.

“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.”