Earlier Nesting in Chicago-Area Birds Linked to Warming


irds in the Upper Midwest are nesting and laying eggs nearly a month earlier than they did 100 years ago, and scientists say warming temperatures may be to blame. 

In a new study published in the Journal of Animal Ecology on March 24, scientists found that the egg-laying dates for 72 species of birds in the Chicago area have shifted by an average of 25.1 days since the early 1900s and late 1800s. The shift was smaller for resident species but larger for short- and long-range migratory birds. 

The researchers analyzed eggs in Chicago’s Field Museum, which hosts a vast collection of more than 50,000 hollowed-out eggs dating back from the 1870s to the 1920s, according to the Associated Press. The Field also holds information about which species laid the eggs and when they were collected. Like most eggshell collections, the number of specimens drops off after the 1920s when egg-collecting went out of fashion for hobbyists and scientists. In the new study, scientists used the preserved eggs as well as modern field observations and had to make do with fractured data: one dataset from 1880 through 1920 and another from 1990 through 2015, reports The Washington Post.

The researchers saw that temperature—approximated by atmospheric carbon dioxide levels for the early years of data collection, when temperature data was not available—correlated with earlier nesting times. To fill in the gap of the missing years, Mason Fidino, a coauthor of the paper and quantitative ecologist at Chicago’s Lincoln Park Zoo, developed a model to incorporate the approximate change in nesting time during the missing era and overlaid those results with the changes in atmospheric carbon dioxide and temperature, CNN reports.

Previous studies have found that climate change has also shifted the seasonal rhythms of plants, pollinators, and animals. Earlier springs can cause ecological mismatch, a phenomenon where species that are reliant on each other fall out of sync. For birds, early hatchers might get out of sync with their traditional food sources, according to the Post. 

“These stresses haven’t necessarily doomed anything to extinction, but they’re definitely changing the conditions that all of these organisms are dealing with,” John Bates, curator of birds at the Chicago Field Museum and a study coauthor, tells CNN. “And that may have really important ramifications—and anything like that has potentially big implications for humans, too.”        

The study also highlights the value of historical archival data. “There are 5 million eggs out there in collections worldwide, and yet, there are very few publications using museum collections of eggs,” Bates tells the AP. “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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