For the humans, breakfast meant waking up an hour before dawn in a tent and grabbing something quick—generally a granola bar that had frozen overnight, recalls Morgan Tingley, an ornithologist at the University of Connecticut. Then he and the other researchers would hike through forests in California’s Sierra Nevadas, listening for the peeps and trills of birds stirring in the crepuscular light. As a graduate student in the mid-2000s, Tingley helped lead the Grinnell Resurvey Project, an effort to repeat the remarkably thorough biological inventories that Joseph Grinnell, the founding director of Berkeley’s Museum of Vertebrate Taxonomy, had orchestrated across California 100 years previously. “You’d write down every single bird you could detect, by sight or mostly by sound,” Tingley recalls.
For the songbirds, breakfast meant leaving a nest or tree hollow at dawn to seek insects, nectar, fruit. The birds’ meals depend on local phenology, the seasonal timing of the cycles of plant and animal life. But as the climate has changed in the Sierra Nevada mountains, that age-old rhythm has slipped. By the time they are ready to nest, migratory songbirds often find that their food sources are available either higher up mountains or further north than in the past. For the most part, the birds follow suit. But a surprising number keep nesting in their old ranges, and until recently, researchers haven’t had a plausible explanation as to why. Now, a new analysis of the Grinnell Resurvey data suggests that many California songbird species are compensating for the effects of climate change, not by altering their range, but by nesting and raising their young much earlier in the year.
Socolar used data from NestWatch, a citizen science program that records nesting success of wild songbirds across the country, to seek a connection between nest temperature and nestling survival. “It was a massive data set,” he says. “We restricted ourselves, and we had 47,000 nests.” As the researchers predicted, nestlings in cold areas fared poorly in colder years and did much better in warmer years. Nests in warm areas had the opposite fate. To Socolar, this demonstrated that nest temperatures matter for baby bird survival.
Which is not to say that scientists know what is motivating these birds to nest earlier. “Is the main reason finding a good temperature to nest under? Or tracking insect emergence and the temperature story is a happy side effect? We don’t know the answer to that question,” Socolar says. Either way, the birds get both benefits. If some birds can cope with climate change by shifting their nesting time, then this research challenges a fundamental understanding of how species respond to climate change. “One core prediction that is canonical of climate change biology … is that as it warms up, they should shift to where it’s cooler,” he says. But these birds are breeding so much earlier, they may not need to.
Socolar is quick to point out that this research is only one possible explanation for why some birds are not moving their nesting locations in response to climate change. Other possibilities haven’t been ruled out yet, he says—maybe some California birds can tolerate wider temperature ranges than others, for example. As temperatures continue to rise, the future of many of California’s songbirds may depend on that answer.