The arrival of spring brings seemingly unchanging rituals – longer days, flowering plants and increased bee activity. But the start of spring is now being distorted by the climate crisis, with new research revealing that many bird species are nesting and laying their eggs nearly a month earlier than a century ago.
US scientists who analyzed bird nesting patterns from egg samples collected from the Chicago area found that of the 72 species for which historical and modern records exist, about a third now nest much earlier in the year than before.
These species, including blue jays, yellow warblers and field sparrows, are now laying their first eggs 25 days earlier, on average, than they did 100 years ago, according to research. The warming of the atmosphere, due to the combustion of fossil fuels, seems to upset a process that seemed unshakable for a long time.
“It was shocking to find this out,” said John Bates, curator of birds at the Field Museum and lead author of the study. “What we can see clearly indicates that climate change is having a significant effect on bird behavior. This is another piece of the puzzle that we are trying to understand in terms of impacts.
The study, published in the Journal of Animal Ecology, relied on records of bird eggs collected during a period, from around 1880 to 1920, when people could rampantly collect them from nests without penalty.
These records, consisting largely of egg boxes with handwritten labels describing the type of bird and when the eggs were collected, were compared to modern nesting data compiled by Bill Strausberger, a colleague of Bates at 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 observe the nests from high up.
A model built by the researchers found that birds advancing their nesting dates have been closely correlated with rising carbon dioxide levels in the atmosphere, one of the main drivers of rising global temperatures.
Birds choose when to nest based on other developments in the spring, such as plant budding and increased insect numbers. These nature interactions are being discouraged by climate change – in the United States bears are coming out of hibernation earlier and cherry, peach, pear, apple and plum trees are blooming weeks earlier than before. In the UK, plants flowered a full month earlier between 1987 and 2019 than before 1986, according to recent research.
“If you’re a bird and you nest earlier, you expose yourself to those cold snaps that can still come in the spring, which then affects plants and insects,” Bates said. “This then has an impact on the reproductive success of the birds. Springs are becoming more volatile and this is taking its toll.
Bates said more research needed to be done, but the blurring of seasons may well be a big factor in the decline of many bird species, along with other factors such as habitat loss and indiscriminate use. of pesticides.
In 2020, a study found that nearly 3 billion birds had disappeared from the United States and Canada since 1970, a loss of almost a third of the total number of birds. The researchers said the losses have been “staggering”, with declines being greatest among sparrows, blackbirds, warblers and finches.