A new study led by the Field Museum shows that climate change is scrambling birds’ egg-laying timing.
This latest research, published in the Journal of Animal Ecology, made use of historic nesting information available through the Field and compared it with data gathered between 1990 and 2015 by ecologists working for Morton Arboretum.
The resulting analysis revealed that approximately one-third of the 72 species studied are laying their first eggs an average of 25.1 days earlier than they were 100 years ago.
The team obtained long-term data on carbon dioxide levels in the atmosphere, which corresponds to temperature trends, and found that it correlated with changes in egg-laying dates.
On the one hand, the study’s conclusions could be viewed as positive, in that they demonstrate birds’ adaptability to climate change, said John Bates, curator of birds at the Field Museum and the study’s lead author.
But the complexity of bird biology and behavior, and their relationship to other organisms within a broader ecosystem, raises all sorts of questions about whether earlier nesting leaves them more or less vulnerable to climate change, he said.
Are the insects these birds rely on for food or the plants they require for habitat adapting at the same rate? How will this affect competition for resources? Bates anticipates that the egg-laying data will spawn idea after idea for other researchers to investigate.
“There are all kinds of really important nuances that we need to know about in terms of how animals are responding to climate change,” he said.
Given the study’s focus on northeast Illinois, he’s also keen to have colleagues across the country replicate the process to see if they observe similar changes.
This latest revelation about the impact of climate change was made possible by a hobby that’s long gone out of fashion: egg collecting.
Much of the Field’s cache of eggs — tens of thousands, tucked away in drawers, resting on pillows of cotton wool — dates to the golden era of collecting in the late 19th and early 20th century, when it was all the rage. Eventually, legal restrictions, namely the Migratory Bird Treaty Act of 1918, all but put an end to the practice.
The ethics of egg collecting poses a “yes, but” dilemma for scientists, said Bates.
“Yes, they were viable birds. Collecting anything obviously has lethal implications. But we can do a lot with this data,” he said.
The thing is, for a long time, nobody was paying any attention to egg collections, including Bates.
A lot of museums and private collectors shed their samples, and much of what remains is either gathering dust or is locked away out of sight.
Bates became interested in the Field’s collection, housed in a space roughly the size of a bungalow belt kitchen, while editing “The Book of Eggs,” a coffee-table reference tome showcasing the Field’s eggs, written by biologist Mark Hauber.
It struck Bates that the 5 million or so eggs still found in collections at places like the Field were an overlooked resource.
“We have this massive data set, every single specimen is a data point. And people are ignoring this data,” he said.
For the purpose of the egg-laying study, the value of the Field’s collection lay as much in the detailed notes accompanying each specimen as the eggs themselves. The labels specify the type of bird, the location of the nest and the date collected. Because eggshells are easiest to preserve just after they’ve been laid, before any incubation occurs, the collection date is a fairly accurate stand-in for the date laid, according to Bates.
The trickier part was finding modern nesting data for comparison, given the dearth of collecting that’s taken place since the 1920s.
Bates turned to his Field colleague Bill Strausberger, and Chris Whelan, an evolutionary ecologist at UIC, for assistance. Both men had worked on separate projects at the Morton Arboretum that involved locating nests, monitoring them and recording the laying and hatching of eggs. (All eggs remained in their nests.)
It’s impossible for Bates to overstate the good fortune of having access to that sort of information.
“What Chris and Bill did was so unusual,” he said.
Their datasets, which covered the past 25 to 30 years, bookended the Field’s own information. That left a mid-20th century gap, which Mason Fidino, a quantitative ecologist at Lincoln Park Zoo, filled in with a data model.
So what other secrets could the Field’s eggs unlock?
Bates said researchers are only just beginning to realize the potential of such collections. Are the physical characteristics of eggs, such as shape and size, changing? And if so, to what purpose? What about their chemical composition? Has it evolved over time or remained static?
“That’s why these collections matter,” he said, but only if they’re used. “If they’re just sitting here, we aren’t doing our job.”