Spring is in the air. The birds sing and begin to build their nests. It happens every year, like clockwork. But a new study in the Journal of Animal Ecology shows that many species of birds are nesting and laying their eggs almost a month earlier than a hundred years ago. By comparing recent sightings with century-old eggs held in museum collections, scientists were able to determine that about a third of bird species nesting in Chicago shifted their laying by an average of 25 days. And as far as researchers can tell, the culprit of this change is climate change.

“Egg collections are such a fascinating tool for us to learn more about bird ecology over time,” says John Bates, curator of birds at the Field Museum and lead author of the study. “I love that this paper combines these ancient and modern datasets to look at these trends over about 120 years and help answer some really critical questions about how climate change is affecting birds.”

Bates became interested in studying the museum’s egg collections after editing a book on eggs. “Once I got to know our egg collection, I started to think about the value of the data in that collection and how that data isn’t replicated in modern collections,” he says. .

The egg collection itself occupies a small room filled with floor-to-ceiling cabinets, each containing hundreds of eggs, most of which were collected a century ago. The eggs themselves (or rather just their clean, dry shells, the contents of which were blown out a hundred years ago) are stored in small boxes and accompanied by labels, often handwritten, indicating what type of bird they are for. belong, where they’re from, and precisely when they were collected, until day.

“These early egg peoples were amazing natural historians, so to do what they did. You really have to know the birds to go out and find the nests and do the collecting,” says Bates. “They were very paying attention to when the birds were beginning to lay, which leads, in my opinion, to very precise dates for egg laying.”

The Field’s egg collection, like most, falls after the 1920s, when egg collecting fell out of fashion for both hobbyists and scientists. But Bates’ colleague Bill Strausberger, a Field associate researcher, had worked for years on cowbird parasitism at the Morton Arboretum in suburban Chicago, climbing ladders and examining nests to see where cowbirds brown had laid their eggs for other birds to raise. . “He had to go out there every spring and find as many nests as he could and see if they were parasitized or not, and so it occurred to me that he had modern nesting data,” says Bates. Chris Whelan, an evolutionary ecologist at the University of Illinois at Chicago, also contributed to the modern dataset with songbird nesting data collected in Chicagoland from 1989, when he began working at the Morton Arboretum. Whelan and Strausberger’s contributions to the study were key, says Bates, because “finding nests is a lot harder than almost everyone realizes.”

“Finding nests and following their fate to success or failure is extremely time-consuming and difficult,” Whelan says. “We learned to recognize what I called ‘nesty’ behavior. This includes collecting nesting material, such as twigs, grass, roots or bark, depending on the bird species, or capturing food such as caterpillars but not consuming the food – this probably indicates that a parent is looking for food for the chicks. Whelan and his team used mirrors mounted on long poles to observe nests from above and closely track egg laying and hatch dates.

The researchers then had two large sets of nesting data: one from around 1880 to 1920 and the other from around 1990 to 2015. “There’s a gap in the middle, and that’s where Mason Fidino intervened,” says Bates. Fidino, a quantitative ecologist at Lincoln Park Zoo in Chicago and co-author of the study, built models to analyze the data that allowed them to close the gap in the mid-20th century, as well as sampling differences between early egg collectors and the research of Whelan and Strausberger.

“Because of this uneven sampling, we had to share a bit of information between species in our statistical model, which can help improve estimates a bit for rare species,” says Fidino. “We all realized pretty quickly that there could be outliers present in the data, and if they weren’t accounted for, they could have a pretty big influence on the results. For this reason, we had to build our model to reduce the overall influence of any outliers, if present in the data.

Analyzes showed a surprising trend: Of the 72 species for which historical and modern data were available in the Chicagoland area, about one-third nested earlier and earlier. Among the birds whose nesting habits have changed, they laid their first eggs 25.1 days earlier than a hundred years ago.

In addition to illustrating that birds lay their eggs earlier, the researchers looked for a reason. Since the climate crisis has dramatically affected many aspects of biology, researchers have looked into rising temperatures as a potential explanation for earlier nesting. But scientists have encountered another problem: there is no consistent temperature data for the region dating back that far. So they turned to a temperature indicator: the amount of carbon dioxide in the atmosphere.

“We couldn’t find a single source of long-term temperature data for the Midwest, which was surprising, but you can approximate temperature with carbon dioxide levels, which are very well documented,” says Bates. . Carbon dioxide data comes from a variety of sources, including the chemical composition of ice cores from glaciers.

The amount of carbon dioxide in the atmosphere over time correlates nicely with broader temperature trends, and the researchers found it also correlated with changes in egg-laying dates. “Global climate change has not been linear over this nearly 150-year period, and therefore species may not have advanced their egg-laying date in a non-linear fashion either. Therefore, we included both linear and nonlinear trends in our model,” says Fidino “We found that the simulated data was very similar to the observed data, indicating that our model did a decent job.”

The temperature changes are apparently small, only a few degrees, but these small changes result in the flowering of different plants and the emergence of insects, which could affect the food available to birds.

“The majority of the birds we observed eat insects, and the seasonal behavior of insects is also affected by climate. Birds have to move their laying dates to accommodate,” says Bates.

And while the birds laying their eggs a few weeks earlier might seem like a small matter in the grand scheme of things, Bates notes that it’s part of a larger story. “The birds in our study area, over 150 species, all have different evolutionary histories and different breeding biology, so it’s all in the details. These changes in nesting dates could cause them to compete for food and resources in ways they weren’t used to,” he says. “There are all sorts of really important nuances that we need to be aware of in terms of how animals react to climate change.”

As well as serving as a warning about climate change, Bates says the study highlights the importance of museum collections, especially egg collections, which are often underutilized. “There are 5 million eggs in collections around the world, and yet there are very few publications using museum egg collections,” says Bates. “They are a treasure trove of data about the past, and they can help us answer important questions about our world today.”