Spring has arrived; Birds have begun to sing and build their nests. Like clockwork, it happens every year. However, according to a new study published in the Journal of Animal Ecology, several bird species are nesting and laying eggs nearly a month sooner than they were a century ago.
Scientists were able to estimate that nearly a third of the bird species nesting in Chicago have shifted their egg-laying up by an average of 25 days by comparing recent observations with century-old eggs preserved in museum collections. Climate change, according to the experts, is the cause of this transformation.
“Egg collections are such a fascinating tool for us to learn about bird ecology over time,” says John Bates, curator of birds at the Field Museum and the study’s lead author. “I love the fact that this paper combines these older and modern datasets to look at these trends over about 120 years and help answer really critical questions about how climate change is affecting birds.”
Bates got interested in studying the museum’s egg collections after editing a book about eggs. “Once I got to know our egg collection, I got to thinking about how valuable that collection’s data are, and how those data aren’t replicated in modern collections,” he says.
The egg collection itself is housed in a small room with floor-to-ceiling cupboards loaded with hundreds of eggs, the most of which were acquired over a century ago.
The eggs themselves (or rather, their clean, dry shells with the contents blasted out a century ago) are kept in little boxes with handwritten labels describing what sort of bird they belong to, where they came from, and when they were gathered down to the day.
“These early egg people were incredible natural historians, in order to do what they did. You really have to know the birds in order to go out and find the nests and do the collecting,” says Bates. “They were very attuned to when the birds were starting to lay, and that leads to, in my opinion, very accurate dates for when the eggs were laid.”
The birds in our study area, upwards of 150 species, all have different evolutionary histories and different breeding biology so it’s all about the details. These changes in nesting dates might result in them competing for food and resources in a way that they didn’t used to. There are all kinds of really important nuances that we need to know about in terms of how animals are responding to climate change.John Bates
After the 1920s, when egg-collecting became unfashionable for both amateur hobbyists and scientists, the Field’s egg collection, like most others, fades away. Bill Strausberger, a research associate at the Field, had spent years at the Morton Arboretum in the Chicago suburbs studying cowbird parasitism, climbing ladders and inspecting nests to identify where Brown-headed Cowbirds had left their eggs for other birds to raise.
“He had to get out there every spring and find as many nests as he could and see whether or not they were parasitized, 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, contributed to the contemporary dataset by collecting songbird nesting data in Chicagoland beginning in 1989, when he started working at the Morton Arboretum. Whelan and Strausberger’s contributions to the study were critical, Bates says, because “finding nests is a lot harder than almost anybody realizes.”
“Finding nests and following their fate to success or failure is extremely time-consuming and challenging,” says Whelan. “We learned to recognize what I called ‘nesty’ behavior. This includes gathering nest material, like twigs, grass, roots, or bark, depending upon bird species, or capturing food like caterpillars but not consuming the food item this likely indicates a parent is foraging to gather food for nestlings.”
Whelan and his crew peered into high-up nests with mirrors attached on long poles, keeping meticulous records on when eggs were laid and when they hatched. The researchers had two large sets of nesting data at the time: one from around 1880 to 1920, and the other from roughly 1990 to 2015.
“There’s a gap in the middle, and that’s where Mason Fidino came in,” says Bates.
Fidino, a co-author of the study and a quantitative ecologist at Chicago’s Lincoln Park Zoo, built models for analyzing the data that allowed them to close the gap in the middle of the twentieth century, as well as the sampling differences between early egg collectors and Whelan and Strausberger’s research.
“Because of this uneven sampling, we had to share a little bit of information among species within our statistical model, which can help improve estimates a little bit for the rare species,” says Fidino.
“We all realized rather quickly that there may be some outliers present in the data, and if not accounted for, could have a rather large influence on the results. Because of this, we had to build our model to reduce the overall influence of any outliers, if they were present in the data.”
The findings revealed an unexpected trend: nearly a third of the 72 species for whom historical and present data were available in the Chicagoland region were nesting sooner and earlier. They were depositing their first eggs 25.1 days earlier than they were a century ago among the birds whose nesting patterns had changed.
The researchers investigated why birds are producing eggs sooner in addition to demonstrating that they are. The researchers looked to rising temperatures as a possible explanation for the earlier nesting because the climate crisis has had such a significant impact on so many elements of life.
However, the researchers ran into a problem: there isn’t any consistent temperature data for the region going back that far. As a result, they used carbon dioxide levels in the atmosphere as a proxy for temperature.
“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 is derived from a variety of sources, including the chemical composition of glacier ice cores.
Carbon dioxide levels in the atmosphere have been observed to correspond 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 lay date in non-linearly as well. Therefore, we included both linear and non-linear trends within our model,” says Fidino. “We found that the simulated data was very similar to the observed data, which indicated that our model did a decent job.”
The temperature variations appear to be little, only a few degrees, but these minor shifts result in various plants flowering and insects appearing, which could impact the food available to birds.
“The majority of the birds we looked at eat insects, and insects’ seasonal behavior is also affected by climate. The birds have to move their egg-laying dates to adapt,” says Bates.
While depositing eggs a few weeks early may appear insignificant in the broad scheme of things, Bates points out that it is part of a larger story.
“The birds in our study area, upwards of 150 species, all have different evolutionary histories and different breeding biology so it’s all about the details. These changes in nesting dates might result in them competing for food and resources in a way that they didn’t used to,” he says. “There are all kinds of really important nuances that we need to know about in terms of how animals are responding to climate change.”
The study, according to Bates, underscores the value of museum collections, particularly egg collections, which are typically under-utilized, in addition to serving as a warning about climate change.
“There are 5 million eggs out there in collections worldwide, and yet, they’re very few publications using museum collections of eggs,” says Bates. “They’re a treasure trove of data about the past, and they can help us answer important questions about our world today.”