Two small tree swallows peek out of a nest box hanging in a tree.
Two tree swallow fledglings peek out of a nestbox, part of a highway of safe nesting sites. Credit: Evelien de Greef, image courtesy Museum of Wildlife and Fish Biology

On a hot day in California’s Central Valley, Melanie Truan opened a handmade cedar birdbox. Inside sat a clutch of young ash-throated flycatchers, still fluffy with developing feathers. She tagged the birds’ little legs, weighed them, returned them, and moved on. There were plenty more stops to make that day on a 50-kilometer (30-mile) stretch of Putah Creek known as the nestbox highway.

Really warm years and really wet springs are hammering the birds.

The highway is a collection of hundreds of nesting boxes placed by researchers as part of a long-term study of songbirds’ adaptation to climate and land use changes and is operated by the Museum of Wildlife and Fish Biology at the University of California, Davis. Truan, a research ecologist, started the project in 2000 as part of her doctoral research. The time- and labor-intensive study requires an army of student interns to visit every box, every week of the nesting season, every year.

Their efforts are helping to paint a picture of songbirds’ struggles against the region’s changing climate, according to a recently published study in Biological Conservation. What they’ve found is clear: “Really warm years and really wet springs are hammering the birds,” said Jason Riggio, a postdoctoral ecologist at the museum and coauthor of the study.

Three young tree swallows rest in a person’s hands.
Student researchers weigh every baby bird they find, including these three tree swallow nestlings. Credit: Evelien de Greef, image courtesy Museum of Wildlife and Fish Biology

Changes in the Central Valley

Residents of California’s Central Valley are used to wet winters and hot summers, but those conditions are getting more extreme. The past decade alone included the wettest winter on record, as well as the worst drought in 1,000 years. Climate forecasts show this extreme oscillation will continue.

The Central Valley’s recent variability let researchers explore how climate affects the nestlings of four species: ash-throated flycatchers, house wrens, tree swallows, and western bluebirds. Their results showed how heavy rains and high temperatures during the nesting season affect reproduction, though it’s too soon to say whether these birds will adapt or move on.

“It takes a really long time for a species to totally disappear from an area,” Riggio said. Counts of newborn birds give researchers a quicker picture of the birds’ health. “By looking at reproduction, we know every single year if that population is producing young or not,” he added.

Wet Months Make Weak Birds

The researchers collected hatchling data from 2004 to 2008 and again from 2015 to 2020. In those 11 years, they noted 2,305 nesting attempts and weighed 7,174 baby birds.

During the hottest years recorded in the data set, all four species generated smaller clutch sizes. Those eggs were also less likely to result in chicks that successfully left the nest. And any bird that did fly away probably weighed less, too. For example, in the coolest year of the study, 85% of tree swallow eggs resulted in a chick that left the nest. In the hottest year, only 46% of tree swallow eggs produced a successful fledging. Baby tree swallows were also 19% lighter in the hottest year compared to the coolest year.

Wet springs and summers also took a toll on the birds. Again, for tree swallows, nestlings weighed 18% less in the wettest year than the driest year. Fledging success dropped from 73% to 28%. Heat and rain may push prey out of sight, force adult birds to postpone foraging, and make nestlings expend extra energy regulating their temperature, according to Riggio. Those conditions result in reduced fitness when they leave the nest.

Though the study area is just a 50-kilometer-long creek, the authors said their results have larger implications.

“The changes that we’re seeing here in the Central Valley—increasing summer temperatures disproportionate to the rest of the globe, the greater year-to-year variability in wild wet years and extreme drought years—that’s happening also in other Mediterranean climates,” said Riggio. “And if those things are negatively impacting wildlife reproduction here, that could be true on a broader scale.”

Eleven Years of Bird Banding

“Getting that fledging survival is the holy grail because once they leave, you often don’t see them again.”

“This study highlights the importance of long-term [wildlife] monitoring,” said Jeanne Fair, an ornithologist at Los Alamos National Laboratory who was not involved in the study. Gathering 11 years of data is an achievement, she noted, and the study focused on important parameters. “Getting that fledging survival is the holy grail [of health data] because once they leave, you often don’t see them again,” Fair said. “It’s a phenomenal study.”

Though Truan said it’s gratifying to see the research finally published, the project will keep trucking along.

Nesting season will ramp up in March. Another army of students will fan out across Putah Creek, and the nestbox highway will produce another year of valuable climate data, with an eye to Central Valley songbirds.

—J. Besl (@J_Besl), Science Writer

Citation: Besl, J. (2023), Climate extremes threaten California’s Central Valley songbirds, Eos, 104, Published on 16 February 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.