Birds are laying eggs earlier as climate change shifts springs

The early bird is getting even earlier. With climate change spurring earlier springs across much of North America, many birds are laying their eggs earlier in the year, according to a new study adding to mounting evidence that global warming is turning wildlife habits upside down.

Of 72 bird species examined around Chicago, roughly a third lay their eggs about 25 days earlier than they did a century ago, researchers report in the paper published on Friday in the Journal of Animal Ecology.

Those affected include the mourning dove, American kestrel and Cooper’s hawk.

The scientists so far haven’t found any clear traits shared by these species, such as size or migratory status, that might explain why they’re changing their schedules.

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But “the majority of the birds we looked at eating insects, and insects’ seasonal behaviour is also affected by climate,” said lead author John Bates, curator of the bird division at the Field Museum in Chicago.

How animal and plant life cycles are affected by climate change and seasonal disruptions is a question that’s “becoming more front and centre in people’s minds,” Bates said.

Just a few temperature degrees off from the long-term average can have a big impact on when insects emerge, when trees sprout leaves, when flowers bloom and, according to the new research, when the eggs hatch.

Scientists believe those changes could be among the many reasons for the steep decline in bird populations since the 1970s, with the United States and Canada losing roughly a third of their birds - or about 3 billion birds - according to a 2019 study in the journal Science.

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Bates and his colleagues studied more than 1,500 eggshell records held at Chicago’s Field Museum, many dating back to the period between 1872 to 1920 when gathering eggs was a popular pastime. These Victorian era ova enthusiasts left detailed, handwritten labels listing information such as the birds species and collection date.

The scientists then compared those records with more than 3,000 modern records, along with data describing carbon dioxide levels on the nesting dates across time, for their analysis.

The findings echo similar results from studies carried out in recent decades in the United Kingdom, which also found that egg-laying was happening earlier along with reported changes in the growing season.