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Some birds are laying eggs earlier. Scientists are paying attention.

Climate change may be causing birds that migrate to travel earlier to nesting areas.

The Field Museum in Chicago, Illinois, has 20,000 sets of eggs in its collection dating to 1872. Scientists used the data on when the eggs were laid and compared it with when birds today lay eggs. (John Bates)
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During a birdwatching stroll last spring, John Bates spotted a newly hatched killdeer chick. Common in North America, the long-legged birds often dart across the ground hunting for insects.

Bates noted the date: April 30. Killdeer eggs take about 24 days to hatch, so this youngster must have come from an egg laid at the beginning of April. That’s roughly one month earlier than birds were laying eggs a century ago — a big difference, says Bates. He is curator of the Division of Birds at Chicago’s Field Museum. (A curator oversees the collection and handling of historical items.)

Warmer temperatures

Killdeer aren’t the only birds changing their timing. Bates and other scientists recently published a study on the historical nesting habits of 72 bird species in the Midwest. The team found that one-third of birds studied are laying eggs earlier than before — 25 days sooner on average. Researchers think Earth’s rising temperatures are the reason.

One bird in the study, the yellow-billed cuckoo, spends winters in South America. Bates says these birds are returning to the United States earlier to breed, probably because the caterpillars they eat are emerging sooner each spring.

“What this highlights is that there is a level of resilience in these birds in terms of tracking stuff, with the really critical [caution] that we need to keep monitoring,” he says.

Besides nesting earlier, birds might be finding other ways to cope with climate change. American robin populations are increasing, yet the study shows their average first egg date is later than before, Bates says. Robins are good at re-nesting, so it’s possible the study identified nests that were the birds’ third attempt after a couple of failures. As Bates says, “maybe one of the secrets to their success is that they literally have this really broad spectrum of times” to breed.

Amateur egg collectors

In the study, scientists analyzed different egg collections dating to 1872 and compared them with recent observations. The Field Museum’s egg collection contains 20,000 sets of eggs. A set, or clutch, contains eggs from one nest that were usually laid by the same female. Many were gathered by hobbyists from the 1880s to the 1920s. “While they were amateurs, they were really good at collecting data,” says Bates.

To preserve the eggs, collectors drilled small holes, then flushed out the contents. They also made nest cards to record details about their findings.

What’s it like working with these ancient eggs? “It’s nerve-racking. I try to be very careful,” says Bates. “We have them in this incredibly cramped little space in the museum.”

He hopes the study will encourage people to collect their own observations about nature. Such data can help people protect birds and other wildlife. For example, by studying the eggs of peregrine falcons and other birds, scientists were able to prove that a pesticide called DDT was causing egg thinning, leading to nesting failures. This research was a key reason the Environmental Protection Agency banned DDT in 1972.

Bald eagles, ospreys and peregrine falcons have all made spectacular recoveries since DDT was banned,” Bates says. “I like it as a neat example of illustrating that we can do things that make a difference.”

Learn more

Watch a video about peregrine falcons’ recovery at

See a video about the Field Museum’s egg collection at