March 7, 2024
With the climate warming, leaves and blooms are popping out ahead of schedule. A wide-ranging new study shows why this trend is troubling for a variety of bird species.
For migrating birds, timing is key. Their journeys require massive amounts of energy, so they need plenty of fuel on their way, and after they get to their breeding grounds, they’ll have hungry chicks to feed, too. “Every day during migration, they’re just on this trade-off between starving to death and being able to continue forward,” says Morgan Tingley, an ornithologist at UCLA. “When they’re not flying, they’re mostly voraciously eating.”
These travelers rely on the newly-available resources brought by spring, such as leaves, flowers, and the insects that come out to munch on them. But that abundance of resources dies down later in the season—and if birds arrive at a stopover or breeding site after this peak period of “spring green-up,” they might miss out on the feeding frenzy.
Climate change is raising the risk of this kind of timing mismatch. As temperature and precipitation patterns shift, and spring’s “green-up” arrives earlier and earlier, a major question for scientists has been: Can birds keep up by changing their migrations? According to a sweeping study published this week in the journal PNAS, a wide range of species may already be falling behind.
“We’re used to thinking about warming with climate change,” says study author Scott Loss, an ecologist at Oklahoma State University. “But we’re changing the seasons, the seasonality, all across Earth.” Just this year, following a mild winter and record-warm February, leaves and blooms are already popping out, in some cases weeks ahead of their usual schedules; parts of the West Coast are seeing some of their earliest spring leaf-outs on record.
The new study shows this isn’t an anomaly. Loss and his team analyzed the migratory routes of 150 bird species, from hawks to hummingbirds, that breed in North America. They found that spring green-up was indeed moving earlier across birds’ flight paths, according to satellite observations between 2002 and 2021.
They then stacked those spring shifts against birders’ observations compiled from eBird, and found that migrators generally weren’t keeping pace: “Most of these species were more in sync with past long-term averages of green-up than with current green-up,” says author Ellen Robertson, who worked on the study as a postdoctoral researcher at Oklahoma State University. It’s a concerning mismatch, she says, since it suggests certain birds may not be flexible enough to adapt to a rapidly changing climate. Rather than deciding when to travel based on current conditions, some species may have migratory behavior that is hard-wired into their genes or learned from other birds—factors that could take generations to shift.
These findings add to a growing body of evidence suggesting spring migration is falling out of sync with food sources, says Stephen Mayor, an ecologist at the Ontario Forest Research Institute who was not involved with the study. “This paper expands on previous work to show that the phenomenon is not unique to songbirds, but is common across bird groups,” Mayor says in an email. The analysis covered everything from ducks and geese to kites and woodpeckers.
While the pattern of mismatch showed up across the board, longer-distance migrants—such as vireos and warblers that winter in Central or South America—seemed to have extra trouble adjusting to year-to-year changes. Their schedules appeared to be more tied to the calendar, possibly relying on cues like changing daylight to tell them when to set off, Loss says.
Tingley, who was not involved with this new study, has seen similar patterns in his research: “Most birds can’t keep up well, but there’s a real range,” he says. Short-distance migrants like Eastern Phoebes can more closely track conditions on the ground, which could help them adapt when those conditions change. But “if you’re a bird that’s wintering in South America, you have no understanding, no ability to know whether or not it’s an early spring or late spring here in North America,” Tingley says. “Those are the birds that are really falling behind.”
If migrants can’t find enough sources of food, they may not be able to survive their journeys, or could produce fewer offspring when they arrive, Loss says. And these earlier springs are part of a broader set of challenges for birds and other migratory animals, Robertson points out, ranging from sea turtles to wildebeest. A recent United Nations report found that one out of every five migratory species they tracked was at risk of extinction, battered by threats like habitat loss and overhunting, as well as other risks brought by climate change.
Still, more research is needed to understand exactly how shifting seasonal schedules are affecting bird survival. “The consequences for bird populations are potentially catastrophic, but also not yet entirely clear,” Mayor adds.
There is hope, for example, that even if they can’t shift their migrations, birds can adapt in other ways, like by shortening the window of preparation before they lay eggs—which some species are already doing, Tingley points out. Chicks in particular need to eat lots of insects, so it’s important that their hatches line up with periods of bug abundance. “They’re advancing their breeding, even when they cannot advance their migration,” he says, but it’s not known to what extent these kinds of changes can make up for lost time.
“It could be that even by trying in all these different ways to adapt to climate change, it’s still not enough,” Tingley says. “And at what point that becomes really, really bad for populations is a really big remaining question.”
