Birds lay their eggs earlier, and climate change is to blame

(ORDO NEWS) — Spring is in the air. Birds sing and start building their nests. This happens every year like clockwork. But a new study published in the Journal of Animal Ecology shows that many bird species are nesting and laying eggs almost a month earlier than they did a hundred years ago.

By comparing the latest sightings to century-old eggs held in museum collections, the scientists were able to determine that about a third of bird species nesting in Chicago have delayed oviposition by an average of 25 days. And as far as researchers can tell, climate change is the culprit for this shift.

“Egg collections are such a fascinating tool to allow us to study bird ecology over time,” says John Bates, bird curator at the Field Museum and lead author of the study. “I love that this work brings together old and modern datasets to look at these trends over 120 years and help answer the really big questions about how climate change is affecting birds.”

Bates became interested in studying the museum’s egg collection after editing a book on eggs. “Once I got to know our egg collection, I started thinking about how valuable the data from this collection is and how that data is not replicated in modern collections,” he says.

The egg collection itself occupies a small room lined with floor-to-ceiling cabinets, each containing hundreds of eggs, most of which were collected a hundred years ago.

The eggs themselves (or rather, only their clean, dry shells, the contents of which were blown out a hundred years ago) are stored in small boxes and are accompanied by labels, often handwritten, which indicate which bird they belong to, where they come from, and exactly when they were collected, up to the day.

“These early egg people were incredible naturalists to do what they did. You have to know birds to find nests and collect things,” says Bates. “They were very precise about when the birds started laying, and that allows, in my opinion, very accurate dates when the eggs were laid.”

Field’s egg collection, like most others, dwindled after the 1920s, when egg collecting fell out of fashion for hobbyists and scientists alike. But Bates’s colleague Bill Strausberger, a Field research assistant, spent years working on ladybug parasitism at the Morton Arboretum in suburban Chicago, climbing ladders and inspecting nests to see where brown-headed cows laid their eggs for other birds to hatch.

“He had to go out there every spring and find as many nests as possible and see if they were parasitic or not, so it occurred to me that he had up-to-date nesting data,” says Bates.

Chris Whelan, an evolutionary ecologist at the University of Illinois at Chicago, also contributed to the modern dataset by collecting songbird nesting data in Chicagoland beginning in 1989, when he started working at the Morton Arboretum.

According to Bates, the contributions of Whelan and Strausberger to the study were very important because “finding nests is much more difficult than almost everyone thinks.”

“Finding nests and following their fate to success or failure is an extremely time-consuming and difficult task,” says Whelan. We have learned to recognize what I call “nesting” behavior.

It involves collecting nest material, such as twigs, grass, roots, or bark, depending on the species of bird, or capturing food, such as caterpillars, but not eating this probably indicates that the parents are foraging. for chicks.” Whelan and his team used mirrors mounted on long poles to look into high-lying nests, and carefully tracked the dates of laying and hatching of eggs.

The researchers then obtained two large sets of nesting data, one from around 1880-1920 and another from 1990 to 2015.

“There was a gap in the middle, and that’s where Mason Fidino came in,” says Bates. Fidino, a quantitative ecologist at Chicago’s Lincoln Park Zoo and co-author of the study, built models to analyze the data that allowed them to bridge the gap in the mid-20th century, as well as sampling differences between early egg pickers and Whelan and Strausberger’s studies.

“Because of this non-uniform sampling, we had to split a bit of information between species within our statistical model, which could help improve the estimates for rare species a bit,” says Fidino. “We all realized pretty quickly that there might be some outliers in the data, which, if not accounted for, could have a pretty big impact on the results. So we had to build our model in such a way as to reduce the overall impact of any outliers if present in data”.

The analysis showed a surprising trend: of the 72 species for which historical and current data were available in the Chicagoland region, about a third were nesting earlier and earlier. Among 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 were looking for a reason for this. Given that the climate crisis has drastically affected many aspects of biology, the researchers looked at rising temperatures as a possible explanation for earlier nesting.

But scientists are faced with another problem: there is no consistent temperature data for such a long period in the region. So they turned to a proxy for temperature: the amount of carbon dioxide in the atmosphere.

“We couldn’t find any source of long-term temperature data for the Midwest, which was surprising, but you can approximate the temperature from carbon dioxide levels, which is 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 is clearly reflected in larger temperature trends, and the researchers found that it also correlates with changes in the timing of egg laying.

“Global climate change has not been linear over this nearly 150-year period, and therefore species may also have non-linearly shifted egg laying dates. So we included both linear and non-linear trends in our model,” says Fidino. “We found that the simulated data was very similar to the observed data, suggesting that our model performed well.”

Changes in temperature seem small, just a few degrees, but these small changes lead to flowering of various plants and the appearance of insects, which can affect the food available to birds.

“Most of the birds we’ve studied feed on insects, and the seasonal behavior of insects also depends on the climate. Birds have to postpone egg laying to adjust,” says Bates.

And while birds that lay their eggs a few weeks early may seem insignificant in the grand scheme of things, Bates notes that this is part of a larger story. “The birds in our study area, which are over 150 species, have different evolutionary histories and different breeding biology, so it’s all about the details.”

These changes in nesting timing could cause them to compete for food and resources in a way that they never did before,” he says. “There are a lot of really important nuances that we need to know about to understand how animals react to changing of the climate”.

Bates says the study not only serves as a warning about climate change, but also highlights the importance of museum collections, especially egg collections, which are often underused.

“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.”


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