Birds are laying their eggs earlier to adapt to climate change

The Field Museum’s collection of birds’ eggs fills an entire room that is crammed from floor to ceiling with cabinets. There are thousands of specimens packed in small boxes, each one with its contents blown out, and each one marked meticulously with a label, often hand-written, recording the species of bird, where the egg was collected and when – down to the precise day. 

According to John Bates, curator of birds at the Field Museum, this collection of eggs is a veritable treasure trove of data, even though the specimens were mostly collected more than 100 years ago. “Egg collections are such a fascinating tool for us to learn about bird ecology over time,” he said.

“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.”

After editing a book about birds’ eggs, Bates got interested in studying the museum’s egg collection and wondered whether the information could be used to assess changing breeding patterns over time. 

“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,” said Bates. He decided that, by comparing data from the century-old eggs preserved in museum collections with recent observations of bird breeding times, one would be able to quantify any changes in the start of breeding activity. 

The Field Museum’s egg collection stopped expanding after the 1920s, when egg-collecting ceased to be popular with both amateur hobbyists and scientists. So Bates needed to find some recent data sets on the timing of bird breeding to use for comparison with the museum data.

Bates’s colleague Bill Strausberger, a research associate at the Field Museum, had suitable data from his years of studies on cowbird parasitism at the Morton Arboretum in the Chicago suburbs. To collect his data, he had climbed ladders to examine nests where brown-headed cowbirds may have laid 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,” explained Bates. 

In addition, Chris Whelan, an evolutionary ecologist at the University of Illinois at Chicago, also contributed to the modern data set with songbird nesting data collected in Chicagoland from 1989, when he began work 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,” said 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 team used mirrors mounted on long poles to peer into high-up nests and kept close track of the dates when eggs were laid and hatched.

These field studies provided the modern data set, from about 1990 to 2015, while the museum data was collected between 1820 and 1920. “There’s a gap in the middle, and that’s where Mason Fidino came in,” said Bates. Fidino, a quantitative ecologist at Chicago’s Lincoln Park Zoo and a co-author of the study, built models for analyzing the data that allowed them to address the gap in the middle of the 20th century

The results of the analyses, published today in the Journal of Animal Ecology, show a clear trend in the timing of spring breeding in birds. Of the 72 species for which historical and modern data were available in the Chicagoland region, about a third have nested earlier and earlier as time has gone by. Among those, the first eggs are now laid an average of 25.1 days earlier than they would have been 100 years ago. 

When the researchers considered the reason behind this very clear trend, they turned to rising temperatures as a possible explanation. But when they couldn’t find temperature records for the area that went back 100 years, they had to find an alternative. Instead, they used a proxy for temperature, namely 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,” said Bates. The carbon dioxide data comes from a variety of sources, including the chemical composition of ice cores from glaciers.

Although the temperature changes for the past 100 years have been relatively small in the study area, these subtle changes may have significant consequences for the timing of plant flowering or the emergence of insects from their pupae or eggs. These changes would affect the abundance of food for birds, and therefore their breeding success. 

“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,” said Bates.

It may seem that laying eggs a few weeks early is unimportant in the larger scheme of life, but Bates notes that it’s 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 said. “There are all kinds of really important nuances that we need to know about in terms of how animals are responding to climate change.”

In addition to giving a stark warning about climate change, the study also highlights the importance of museum collections, particularly egg collections, which are often under-utilized. “There are 5 million eggs out there in collections worldwide, and yet, there are very few publications using museum collections of eggs,” said Bates. “They’re a treasure trove of data about the past, and they can help us answer important questions about our world today.”

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By Alison Bosman, Earth.com Staff Writer