Global warming may be causing birds to alter their nesting habits, according to a new study that found that some species in Chicago are laying their eggs, on average, nearly a month earlier than usual.
The findings, published Friday in the Journal of Animal Ecology, add to a growing body of research on how birds are affected by shifts in their environment and the potential struggles they may face in coping with climate change.
The research on birds' nesting practices combined extensive collections of eggs preserved in museums with recent observations to compare how egg-laying has changed over time. The scientists studied 72 bird species for which historical and modern data were available in the Chicago area and found that around one-third are nesting and laying eggs an average of 25 days earlier than they were 100 years ago.
To examine what may be causing the shift, the researchers studied concentrations of carbon dioxide in the atmosphere over the past 150 years, together with long-term temperature trends, and found that overall increases in both correlated with fluctuations in egg-laying dates for many — though not all — bird species.
"This really highlights the complexity of what climate change means to the biology of birds," said John Bates, curator of birds at the Field Museum in Chicago and the study's lead author.
Bates said it's not yet clear which bird species might be most vulnerable — or even what that vulnerability entails. It's possible, for instance, that one of the consequences of earlier-than-usual nesting may be that some birds are forced to compete for food and other resources in new ways, he said.
"If you've got these birds nesting earlier and earlier," Bates said, "then one of the questions you have to ask is: Does that put them in a situation where there's not enough insects around for them to eat?"
Brooke Bateman, director of climate science at the National Audubon Society, who was not involved with the study, said shifts in birds' nesting habits are likely just one part of a cascade of ecosystem changes as a result of global warming.
"Birds have to cope with climate change by shifting things in time and also space," Bateman said. "They're having to adjust the timing of things but also where they are across the landscape. And that's a lot to cope with."
As such, climate change may be upsetting various stages of a bird's life cycle, from their migration patterns to where and when they reproduce and build nests in the spring.
If birds begin nesting earlier than normal, they run the risk of laying eggs during what climate scientists refer to as "false spring," or a short period of unusually warm weather in late winter that can trick vegetation and hibernating animals into thinking it's the start of a new season. Birds and other plants and animals are then vulnerable to subsequent cold snaps that can occur before the actual onset of spring, Bateman said, adding that insects are also affected by false springs.
"Ninety-six percent of land birds feed insects to their young to survive, so cold snaps in these early springs can be really detrimental," she said.
Robyn Bailey, a wildlife biologist at the Cornell Lab of Ornithology, who was not involved with the study, said the new findings are in line with other research on shifts in birds' nesting habits but emphasized the need to conduct a diverse range of studies to determine how birds are responding to changes in their environment.
"When you think about all the cues that birds are getting — from climate, temperature, pollution, competition with other birds — all of these are acting on birds at the same time, and it's really hard to tease apart which specific thing is responsible for the biological outcomes that we're seeing," said Bailey, a project leader for Cornell's NestWatch, a nationwide nest-monitoring program that has been collecting data since the 1960s.
While there are differences across bird species, Bailey said studying birds in their individual habitats can provide clues about bigger, often concerning changes in their ecosystem.
In 2019, a study published in the journal Ecology Letters found that the body sizes of birds have been shrinking as global temperatures rise and the climate warms, while the length of their wingspans grew, possibly indicating ways that birds are adapting to climate change. That same year, an alarming study published in the journal Science found that nearly 3 billion birds had been lost in the U.S. and Canada since 1970.
This expanding field of research will be even more important as the effects of climate change play out in habitats around the world, Bates said.
"Birds are absolutely a harbinger of what's going on," he said. "They're much more tied to what's going on seasonally than we are. So I think closing the loop on the life histories of birds is something that we need to spend more time getting the basic natural history data to do."