Many bird species nesting and laying eggs nearly a month early, study says | birds

The arrival of spring has seemingly immutable rituals – lengthening days, blossoming plants and a surge in bees’ activity. But the onset of spring is now being warped by the climate crisis, with new research finding that many species of birds are nesting and laying eggs nearly a month earlier than they did a century ago.

US scientists who analyzed the nesting trends of birds from egg samples collected in the Chicago area found that of the 72 species for which historical and modern data exists, around a third are now nesting much earlier in the year than before.

These species, including bluejays, yellow warblers and field sparrows, are now laying their first eggs 25 days earlier, on average, than they were 100 years ago, the research found. The heating of the atmosphere, due to the burning of fossil fuels, is seemingly upending a process that long appeared unshakeable.

“It was shocking to find this,” said John Bates, curator of birds at the Field Museum and the study’s lead author. “What we can see is clearly pointing in the direction that climate change is having a significant effect on the behavior of birds. It’s another piece of the puzzle we are trying to figure out in terms of impacts.”

The study, published in the Journal of Animal Ecology, drew upon records of birds’ eggs gathered during a period, from about 1880 to 1920, when people could rampantly collect them from nests without penalty.

These records, largely consisting of boxes of eggs with hand-written labels describing the type of bird and when the eggs were collected, were compared with modern nesting data collated by Bill Strausberger, a colleague of Bates’ at the Field Museum, and Chris Whelan , an evolutionary ecologist at the University of Illinois at Chicago. Whelan and his team used mirrors mounted on long poles to peer into high-up nests.

A model built by the researchers revealed that birds moving their nesting dates forward has been closely correlated with rising carbon dioxide levels in the atmosphere, a leading driver of rising global temperatures.

Birds choose when to nest in line with other developments in spring, such as the budding of plants and the increase in insect numbers. These interactions of nature are being unpicked by climate change – in the US bears are emerging from hibernation earlier and cherry, peach, pear, apple and plum trees are blossoming weeks earlier than they once did. In the UK, plants flowered a full month earlier between 1987 and 2019 than they did before 1986, recent research found.

“If you’re a bird and you nest earlier, you put yourself at risk of these cold snaps that can still arrive in spring, which then affects the plants and insects,” said Bates. “That then impacts the reproductive success of the birds. Springs are becoming more volatile and that is taking its toll.”

Bates said more research needed to be done but the scrambling of the seasons may well be a significant factor in the decline of many bird species, along with other factors such as habitat loss and the indiscriminate use of pesticides.

In 2020, a study revealed that nearly 3 billion birds had disappeared from the US and Canada since 1970, a loss of nearly a third of all bird numbers. Researchers said the losses have been “staggering”, with the declines heaviest among sparrows, blackbirds, warblers and finches.

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