Climate change is causing birds to lay their eggs earlier than usual.

Spring is here. Birds are singing and building their nests. It happens year after year like a clockwork. A new study has shown that the aforementioned phenomenon is not common. Journal of Animal EcologyIt has been shown that many bird species are now nesting and laying eggs a month earlier then they were 100 years ago. Scientists were able to compare recent observations with museum-preserved eggs from the past 100 years to find that around a third of Chicago’s bird species have increased their egg-laying by an average of 25 days. According to researchers, climate change is the reason for this shift.

John Bates, curator at Egg Collections, says that egg collections are a fascinating way for us to learn more about bird ecology over time. BirdsThe study’s lead author is at the Field Museum. “I love that this paper combines these older datasets with modern ones to examine these trends over 120 years. It helps answer critical questions about climate change’s impact on birds.”

After having edited a book about eggs, Bates became intrigued in studying the museum’s egg collections. He said, “Once we got to know each other, I began to think about how valuable the data in that collection are and how they aren’t replicated in modern collections.”

The egg collection occupies a small room with many cabinets that span the floor. Each cabinet contains hundreds of eggs. Most of them were collected more than a century ago. The eggs (or just their clean, dry shells, with their contents blown out a hundred year ago) are kept in small boxes. Sometimes, labels are hand-written to indicate what bird they belong to, from where they came, and when they were collected.

“The first egg people were remarkable natural historians. Bates states that it is essential to be able to identify the birds before you can go out and collect the eggs. “They were extremely attuned to the time when the birds were beginning to lay, which leads to, in my view, very precise dates for when the eggs laid.”

Like most egg collections, the Field’s dropped off after the 1920s, when egg-collecting became obsolete for both scientists and amateur hobbyists. Bill Strausberger, Bates’s research associate, had been working for years on cowbird parasitism in the Morton Arboretum, Chicago suburbs. He had climbed ladders and examined nests to find out where Brown-headed Cowbirds had laid eggs for other birds. Bates says that Strausberger had to go out every spring to check for parasites. He also had modern nesting data. Chris Whelan (an evolutionary ecologist at University of Illinois at Chicago) also contributed to the modern dataset. This includes songbird nesting data from Chicagoland that he collected starting in 1989, when he started work at Morton Arboretum. Bates said that Strausberger and Whelan’s contributions were crucial to the study because “finding nests can be a lot more difficult than most people realize.”

Whelan states, “Finding nests and following their fate to success is extremely time-consuming. “We learned to recognize what I called “nesty” behavior. This means gathering nest material, such as twigs or roots, bark, and grass depending on the situation. 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 used mirrors mounted on long poles to view high-up nests. He also kept track of hatch dates with his team.

The researchers had two sets of nesting data, one for roughly 1880-1920 and one for 1990 to 2015. Bates states that “there’s a gap between the two sets of nesting data” and that Mason Fidino helped to fill it. Fidino, Chicago’s quantitative ecologist, was also a coauthor of the study. He built models to analyze the data, which allowed them to address the gap at the mid-century, as well the differences in sampling among early egg collectors and Strausberger’s research.

“Because this uneven sampling, it was necessary to share a bit of information between species within our Statistical modelFidino says that this can help to improve estimates for rare species. “We all realized quickly that there might be outliers in our data. These outliers, if not addressed, could have a large impact on the results. We built our model to minimize the impact of outliers if they were found in the data.

The analyses revealed a surprising trend: Among the 72 species for whom historical and modern data were available in Chicagoland, around a third of them had been nesting earlier and later. They were laying their first eggs 25.1 more days earlier than they were 100 years ago, among the birds whose nesting habits had changed.

Researchers looked for the reason why birds lay eggs earlier than usual. The researchers considered rising temperatures as a possible explanation for the earlier nesting, given how the climate crisis has profoundly affected biology in many ways. However, the scientists ran into a problem: there isn’t consistent temperature data from the region that goes back that far. They turned to carbon dioxide as a proxy for temperature.

“We couldn’t find any long-term temperature data for Midwest, which was surprising,” says Bates. The carbon dioxide data comes primarily from the chemical composition of ice-cores from glaciers.

The changes in temperature over time are well-mapped to the amount of carbon dioxide in our atmosphere. Researchers also found that this correlation was evident with the changes in egg-laying times.  “Global climate change has been non-linear over the past 150 years, so species may not have advanced their laying dates in non-linear ways.” Fidino explains that our model included both linear and nonlinear trends. “We found that the simulation data was very similar in appearance to the observed data. This indicated that our model did a good job.”

The changes in temperature are seemingly small, just a few degrees, but these little changes translate to different plants blooming and insects emerging— things that could affect the food available for birds.

“The majority (of the birds we studied) eat insects. Climate also affects insects’ seasonal behavior. Bates states that the birds need to adjust their egg-laying times to adapt.

While it might seem small in the grand scheme, Bates points out that birds can lay their eggs a few days early. “The 150 species of birds that live in our area have different evolutionary histories and different breeding habits so it’s all about the details. He says that these changes in nesting dates could lead to them competing for food or resources in a way they don’t use to. “There are many important nuances to understand about how animals respond to climate change.

This serves as a warning to others about Climate changeBates stated that the study highlights the importance museum collections, especially egg collections. “There are 5,000,000 eggs in collections around the world, but there are very few publications using them. museum collectionsBates said that eggs are the most important part of life. “They are a treasure-trove of data about past events, and they can help answer important questions about the world today.”