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Climate-induced early nesting poses problems for young swallows

early nesting
An adult Tree Swallow feeds a chick in a nest box. Photo by Stubblefield Photography/Shutterstock

Over the years, we have reported on studies that show certain bird species are adapting to the warming climate by shifting their spring breeding season earlier by a few days. Now a new study of Tree Swallows, from the Max Planck Institute of Animal Behavior and Cornell University, shows the drastic consequences of the shift: Chicks hatching earlier face increased risk of poor weather conditions, food shortages, and mortality.

The researchers, who examined decades of data on weather, food availability, and breeding in Tree Swallows, say that the timing of when to breed and when food is available is becoming decoupled for some animals — highlighting the complexity behind how wildlife respond to climate change.

“Simply moving dates earlier to track climate change isn’t necessarily risk free,” says Ryan Shipley, postdoctoral fellow at the Max Planck Institute of Animal Behavior and first author on the paper published in PNAS. “Riskier conditions earlier in the year can expose animals to unintended consequences when responding to bouts of unusually warm spring weather.”

Weather shifts affect insect prey

Shipley and his coauthors found that Tree Swallows had been advancing breeding by 3 days every decade for the last 30 years, but earlier-hatching offspring were at greater risk of exposure to inclement weather, which in turn reduced the availability of the flying insects they rely on for food. 


“Our results raise the possibility that animals relying on food resources that can rapidly change in abundance due to the weather may be particularly at risk to climate change,” says Shipley.

The study has exposed strong fitness consequences for birds that are advancing breeding at a similar rate to climate change. But it may also provide clues to the mystery of why aerial insectivorous birds, such as swallows, swifts, flycatchers, and nightjars, are declining faster than other groups in much of North America and Europe.

Weather in spring can change quickly and tends to be more unpredictable earlier in the year. The activity of flying insects is determined by weather, which means they are stochastically available.

“For birds that feed on flying insects, one day is feast, the next is famine. This means during unusually warm springs, parents are betting current conditions that prompt earlier egg laying are indicative of similar good conditions for rearing hatched young 3 weeks in the future,” says Shipley.


The results lay bare a previously unobserved threat to aerial insectivorous birds. “Considerable attention has been given to potential widespread decline of insect populations, and this might be hitting insectivorous birds particularly hard. But we show a mechanism that doesn’t require change in insect abundance — just availability over short time, like a few days.”

Thanks to Cornell University for providing this news.

Birds and climate change in the last decade


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