Study Reveals Birds May Be Laying Eggs Weeks Earlier Than Normal: Here’s Why

by TK Sanders
(Photo by Jonathan Daniel/Getty Images)

Some scientists believe that climate change is causing a few species of birds to lay their eggs roughly a month earlier than normal. The Journal of Animal Ecology recently published a study reporting that about a third of all birds in the Chicago area nested about 25 days earlier than normal.

According to the study, both warmer and earlier springs contribute to the birds’ new nesting and eggs cycles. Spring is the beginning of breeding season for most North American bird species.

“The [birds] have evolved to try to mate at a certain period of time because that is when food is most abundant to feed their babies,” said Jeremy Kirchman, curator of birds at the New York State Museum. “Now, they’re nesting earlier. The cues that they are following in order to know when to breed are climate related.”

Some researchers believe that reproduction during “false springs” could lead to confusion for the birds, mismatched lay dates, and food shortages. They also believe unforeseen springtime cold snaps — another byproduct of climate change — may pose a threat to birds looking for food.

Overall, bird populations in the U.S. are declining. Scientists estimate that around 3 billion less birds now live in the country than in the 1970s. Climate change, they say, will make the drop even more severe.

“If we don’t do anything about climate change it’s not only that we are going to see these cold snaps, but there are other things that we are going to have to deal with, like heat waves, in the spring that could be detrimental to breeding,” said Brooke Bateman, director of climate science at the Audubon Society.

To determine why American birds lay their eggs earlier, researchers turned to some of the country’s best archives

Publishers of the study made their conclusion after observing the nesting habits of 72 Midwestern bird species. They then compared their data with archived records in Chicago’s Field Museum, the Western Foundation of Vertebrate Zoology, and the Chicago Academy of Sciences.

The Field Museum, in particular, boasts one of the best egg collections in the world — a fantastic resource for bird enthusiasts. There, egg lay collectors from decades ago began donating specimens they found in the wild. The hobby of egg lay collecting was extremely popular over 100 years ago.

“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,” said John Bates, curator of birds at the Field Museum. Bates was the lead author of the study, as well. “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.” 

Bates and his team analyzed two separate collections for the study on why birds lay eggs earlier as of late. The first included data from 1880 to 1920; and the second from 1990 to 2015.