As climate change worsens, spring temperatures come earlier in the year, cueing tree swallows to build their nests sooner. This leaves baby swallows vulnerable as the inevitable cold snap happens.
“It’s getting warmer overall. They’re thinking, OK, it’s a good time to breed, to lay my eggs,” said Lily Twining of the Max Planck Institute for Animal Behavior in Germany.
But late-season cold snaps can still happen. When they do, these sudden cold temperatures can harm hatchlings. Baby birds aren’t able to regulate their body temperatures, making them vulnerable to hypothermia. When it gets too cold, the insects stop flying, too. This means the baby tree swallows find themselves going without meals, potentially leading to starvation.
A study published in 2020 reviewed almost 50 years of breeding records. Researchers concluded that tree swallows had advanced their egg-laying by approximately 3 days each decade. But by laying eggs 2 weeks earlier, they are now exposed to twice the cold snaps that they experienced in the 1970s. The study also confirmed that many of the baby birds died because low ambient temperatures equaled no bugs to eat.
“Even a single inclement weather event can reduce offspring survival by >50%,” according to the study. “The decoupling between cold snap occurrence and generally warming spring temperatures can affect reproductive success and threaten long-term persistence of populations.”
In 2016, a single cold snap in Ithaca, New York, killed more than 70% of baby tree swallows. “And there have been more and more of these severe cold weather die-off events for these tree swallows as they’ve been breeding earlier and earlier over the past 40 or so years,” Twining said.
Swallows are just one of many species of animals trying to survive our warming planet. Climate change affects everything from reproduction to food availability to migration.
Image via Jason Crotty