Heat waves threatening bird reproduction on US farmlands: study

Climate-driven temperature extremes are taking a toll on bird populations that reproduce on agricultural land, a new study has found.

Birds that bred near farms were half as likely to see at least one fledgling successfully leave the nest during heat waves, the study authors determined, publishing their observations on Thursday in Science.

Forests, on the other hand, seemed to offer a protective buffer against such temperature spikes, as shaded areas helped bolster nesting success, according to the research.

“Canopy cover probably constitutes an important climate refuge for birds that can thrive in various habitats,” co-lead author Katherine Lauck, a Ph.D. candidate in ecology at the University of California, Davis, said in a statement.

When Lauck and her colleagues investigated how heat waves influenced nesting outcomes in urban areas, they found less of a negative effect than in agricultural zones.

They attributed these differences to the fact that the nests were often in city parks and residential areas with high tree cover.

“This suggests that places like backyards and parks may provide important bird habitat that is somewhat more buffered from climate extremes in the future,” Lauck said.

To draw their conclusions, the researchers analyzed more than 152,000 nesting records from 1998 through 2020, including about 60 bird species that bred in farms, forests, grasslands and urban areas nationwide.

The data came from a repository called NestWatch, created by Cornell University’s Laboratory of Ornithology to monitor bird nesting habits around the country.

At the helm of the current project was senior author Daniel Karp, an associate professor at UC Davis’s Department of Wildlife, Fish and Conservation Biology.

“What is really unique about this dataset is that we could look at bird reproduction at a very broad spatial scale,” he said in a statement.

“With these data, we could begin to unravel how climate change and habitat loss are together affecting many North American birds,” added Karp, who launched the project during the height of the pandemic, to keep in touch with students.

While the scientists found that all bird species studied endured the negative impacts of heat waves, they saw significant declines in bluebirds and tree swallows when temperatures spiked in agricultural areas.

Threatened birds and those that build nests without any coverings, meanwhile, were even more vulnerable to the dangers posed by heatwaves, according to the study.

For these animals, the approximately 50 percent decline in nesting success jumped to about 70 percent, according to Karp.

“This suggests that species already in decline may have an even greater difficulty rearing young in the future as heat waves become more common and more land is converted to agriculture,” he said.

In the future, the scientists surmised that maintaining forest patches in human-inhabited areas could help birds withstand certain extreme climate events. Building sun-shielded or insulated nest boxes in shaded locations could also be beneficial, the study concluded.

“Thinking about some of those interventions might matter a lot for birds looking forward,” Karp said.