Scientists have long focused on rising temperatures to understand how climate change is reshaping the natural world. But there's a critical blind spot in that picture: rain. A new global study reveals precipitation has been largely overlooked in studies of how climate change impacts birds, even though it can be just as influential as temperature.
The findings, published in the high-impact journal PNAS, show rainfall patterns play a powerful role in shaping bird populations, but also reveal how those effects unfold and which species are most vulnerable as precipitation becomes more erratic in a warming world.
"In generally dry places, rain is often a good thing because more rain makes more food and more food is essential for birds to be able to reproduce. But in very wet places, more rain can be bad for multiple reasons," said Western biology professor Alice Boyle, senior author of the study.
For the study, Boyle and her former postdoctoral scholar at Kansas State University Katy Silber undertook an ambitious global review, combing through decades of published research to identify studies that documented links between rainfall and bird reproduction. The result was a sweeping analysis that brought together data on species from every corner of the world, across deserts, grasslands, forests and wetlands, capturing birds of many kinds to reveal how precipitation shapes breeding success on a global scale.
As a key differentiator, the study distinguished between the effects of long-term rainfall patterns and short-term storms. As predicted, Boyle and Silber found storms occurring during the breeding season reduce reproductive success, while rainfall over longer periods can reshape ecosystems in ways that support reproduction.
"Over longer time periods, rain is likely to affect the environment in ways that increase food supply or even change the physical properties of a habitat," said Boyle, a core faculty member of Western's Centre for Animals on the Move. "Those are all things that make it good for reproduction."
Timing, however, is critical.
"Bad weather during the reproductive season can be a stressor for birds," said Boyle. "Making babies already takes a lot of energy, so if the rain falls while they're already stressed, changes are there's going to be a negative response."
And why does rain during nesting reduce reproduction rates?
"We don't know for sure. While nests can be washed away or waterlogged, the more general explanation is that in endotherms, or warm-blooded animals like birds, the cost of staying warm increases when you're wet," said Boyle. "Think about us. We get cold much faster when we're wet. The exact same thing happens with birds."
Physiologically, birds possess significant adaptations to prevent water from reaching their skin. But while their feathers are generally water-resistant, they are not completely waterproof in all conditions.
"Birds have to use a substantial amount of energy to maintain internal temperature when they're wet. And for birds incubating eggs, options are limited. They have to sit there and just endure it. If that means getting wet, the cost of keeping those eggs warm just got much higher. And they can't go out and get more food to produce fuel because they're sitting on the nest," said Boyle.
Underexplored axiom of climate change
The study also shows the importance of geography. At high elevations in high-latitude regions, birds had lower reproductive success after rainfall, whereas at high elevations in low latitudes, such as the tropics, elevation didn't matter so much.
In Canada and other high-latitude countries specifically, any increases in rain at high elevations during breeding is likely to have especially detrimental effects.
"Again, it's an interaction between temperature and rainfall. In colder places, those costs are bigger," said Boyle, who warns climate change adds urgency to these findings.
"There's an underexplored axiom of climate change. We think about animal distributions changing with temperature, but not with changing rainfall," said Boyle. "In many places, climate change means more variability: heavier and bigger storms.
A key challenge, Boyle explains, is defining what 'rain' actually means in ecological studies.
"When you talk about rain, the question is: what about rain? Do you measure total annual rainfall? Number of storms? Intensity? There are so many ways to categorize climate."
The answer depends on how those factors impact survival or how many babies are produced," said Boyle.
Talk to the animals
Research and public perception of climate change often focus on our warming planet. And if people think about rain at all, they tend to think of droughts. But Boyle says that's only part of the story.
"When we talk about changes to precipitation under climate change, people picture dryness: desertification, post-apocalyptic landscapes. But it's not just about drying out. Some places are getting wetter. Even where total rainfall doesn't change much, the timing and intensity of events are changing," said Boyle. "It's not simply wetter or drier. It's how the rain falls that really matters."
This new study is built on a landmark paper Boyle and her collaborators published in 2020 in the journal Trends in Ecology & Evolution. The new study is a major test of those ideas.
And if Boyle could go full Doctor Dolittle and speak to birds, what would she say?
"I would say, 'Look out for the storms. Make sure your nest isn't susceptible to flooding. Try to stay dry. And find an umbrella,'" said Boyle.
