For a migratory bird, springtime in the northern hemisphere means bugs, and babies. In the first few warm days of the year North American insects emerge by the millions, to eat young leaves before trees begin to infuse them with an insect deterrent. And songbirds, who were wintering in South and Central America, fly up north to breed—and eat those bugs. The birds time their many-thousand-mile migration to coincide with this temporary insect buffet, and missing the mark could be fatal.
But for some common songbird species, this long-distance choreography is being thrown off by climate change, a team of researchers at the Florida Museum of Natural History found, in a study published in the journal Scientific Reports on Monday (May 15).
Birds manage their incredible timing by sensing the seasonal shift in light, not temperature. When the sun begins to set later and rise earlier just enough to signal the impending spring, birds begin their migration. But insects time their hatching according to the unfurling of tree leaves—which is triggered by the shift towards warmer temperature. So when spring comes earlier to the trees and bugs, the birds don’t always know it. “If you think your local weather person trying to predict the weather for this week is hard, how do you think a bird is going to do that from thousands of miles away?” asks Stephen Mayor, a postdoctoral researcher with the Florida Museum of Natural History and lead author of the study.
Mayor and his team looked at migration data (some of which was drawn from online birder communities, where birders share extremely detailed observations) between 2001 and 2012 for 48 species of common songbirds who fly to the US to breed, and compared it to the arrival of springtime in those same years as measured by leaves unfurling on the trees—what scientists call the “green-up.” Average temperatures across the country increased from 2001 to 2012, causing spring to arrive earlier in the eastern US and—to the researchers’ surprise—later in the West. That in itself is an unexpected discovery, says Mayor, and his team plans to take a closer look as to why.
But back to the birds: Overall, the gap between the green-up and the birds’ arrival increased an average of a half-day every year.
Most bird species were able to compensate a little by adjusting the timing of their migration (though how they could possibly predict the weather from thousands of miles away remains an unsolved mystery). For 39 of the 48 species, Mayor says that explains why the mismatch wasn’t larger. But in nine cases, the birds weren’t able to adjust nearly enough to keep up with how quickly climate change was changing spring.
For example, in an attempt to compensate, the yellow-billed cuckoo was late to its breeding grounds in the southeastern US forests each year, but spring was much, much later, leading to a 12-day mismatch after 12 years.
The Townsend’s warbler, a common species found all over the woods of the Pacific Northwest, also struggled: The gap between their arrival and the earlier arrival of spring in the West increased by a rate of two and a half days every year, growing the gap to a full month by the end of the 12 years.
For now, the team’s data sets are just data sets—they don’t yet know whether, or how, the birds themselves were affected by the changes. But if these “mismatches” continue apace, Mayor expects it won’t be good.
“We expect it to be a growing problem, but we don’t know how bad it’ll get,” Mayor says. “We predict bird populations may decline with this rate of mismatch.”
“The timing of spring really sets up the rest of their biological cycle for that year,” Mayor explains. “They have to arrive late enough so conditions aren’t too cold, and food is available to them, but they have to arrive earlier enough that there are plenty of available nest sites and they can find territory to produce young. And they need plenty of food at that time to increase their body mass and lay eggs.”
Plus, if the birds aren’t around to eat the bugs, the US could have a bug outbreak on its hands. “If there are fewer birds, we could see more insects and greater defoliation of trees.”
For now, Mayor says, enjoy your backyard birds while you can. “They’re part of the American landscape, part of our psyche. To imagine a future where they’re much less common would be a real loss.”
