̽����ѡ

My head hurts. I’ve spent a few weeks trying to understand what we know, what we don’t know, and what we need to know about one of the most significant challenges facing birds: how climate change is altering the timing of the seasons and, in turn, affecting their migration.

Even for someone like me, an ornithologist who has tracked owls, hawks, songbirds, and other migrants across the globe for decades, the science is confusing and concerning. The awe I feel when I follow a one-ounce Swainson’s Thrush tagged in Alaska all the way to its wintering site in Argentina is tempered by the knowledge that systems birds depend on—predictable weather patterns and favorable winds, food at the right time and in the right quantities, and cues provided by temperature, all varying across thousands of miles of travel—will inevitably change as the world warms.

After talking with experts, all I can say is: It’s complicated. Whether you’re an optimist (who believes migrants will adapt) or a pessimist (who thinks they won’t), there’s evidence to confirm your inclination.

One thing is clear: All around the Northern Hemisphere, almost wherever scientists have enough data to��track it, the timing of the seasons is changing. Autumn, when climate signals are naturally more complex, is something of a muddle. But , gathering speed at a gallop by the year, by the decade. As spring’s warmth advances, the average emergence of tree leaves and other vegetation—the northbound “green wave”—is also accelerating. That means that scores of other effects are likewise happening sooner, such as the seasonal flush of caterpillars and other insects that sustain baby birds.

Some as the seasons move. Migrants that travel short and medium distances, including Eastern Phoebes and American Robins, are arriving on their U.S. and Canadian breeding grounds earlier in spring than they did a few decades ago. Because those species mostly winter within the United States, they likely can pick up on the signals of an early spring, such as warming air, and time their migration accordingly. Even so, they’re not keeping pace. Long-distance migrants, like many of the warblers, tanagers, and orioles that winter in the Neotropics, are struggling even more. They rely on fixed cues like their internal clock and day length to time their migrations, and so are falling much further behind schedule.

The fear is, if migrants drop far enough out of sync, disaster may follow. The biggest concern is what’s known as a , “phenology” being a $10 word for the timing of the seasons. A phenological mismatch occurs when previously synchronized seasonal events, like migrants arriving in time for the insect emergence, fall out of lockstep. Already, with only 1 degree Celsius (1.8 degrees Fahrenheit) of average global temperature rise since the Industrial Revolution, some birds have lapsed so far behind spring and so deeply out of step with their nestlings’ food supply that populations have crashed.

This early evidence that climate change can decouple globe-spanning ecological relationships has alarmed scientists. But it’s also provided an unprecedented opportunity: to study birds’ ability to adapt their migration to a rapidly changing world.

��

Birds are flexible animals. While they have no control over earlier insect hatches, flower blooms, and fruit production, they can adjust their behaviors to try to close the gap. For instance, we know that birds are pushing themselves to extremes to keep up with the racing spring season. Colorado State University’s Kyle Horton is an expert in using weather radar to study the movements of billions of birds. He and his colleagues have shown that long-distance migrants that come north across the Gulf of Mexico they kept decades ago. Once they make landfall, however, they rapidly increase their pace as they dash to catch up with spring.

Horton suspects they are shaving time off their stopovers, the essential periods when migrants rest and refuel. They’re forced to sacrifice their recovery time, and risk arriving in poorer physical condition, to try to match their migration to the season.��

Birds are creatively rearranging their schedules in other unexpected ways. Morgan Tingley, an ecologist at the University of California, Los Angeles, was part of a team that laboriously retraced bird surveys that had been conducted across California’s Sierra Nevada a century earlier. He found that , which was expected in a warming climate: Many of the species in the survey moved up in elevation to cooler altitudes, while others, responding to changing rainfall patterns, actually moved downward. Surprisingly, Tingley told me, in certain regions of the Sierra Nevada, about 40 percent of the bird species hadn’t shifted in elevation at all. That puzzled him, until he looked at their timing: Many of the birds bred, on average, nine days earlier than they once did. That means their eggs were exposed to an average temperature 1 degree Celsius cooler than if they’d stayed the course—and that, perhaps not coincidentally, matches exactly how much the climate there has warmed over the years.

If birds are nesting earlier, then they have no choice but to lose time elsewhere. In southwestern Pennsylvania, researchers at Carnegie Museum of Natural History’s Powdermill Nature Reserve, led by Molly McDermott and including Luke DeGroote, with whom I collaborate on a wildlife tracking project,��. They found that a number of species, including Wood Thrushes, are returning about five days earlier in spring than they did in the 1960s. But based on when juvenile thrushes appear in their mist nets after fledging in midsummer, the birds now breed a full 22 days earlier than they did back then. Somehow, like those radar-tracked migrants powering north faster and faster from the Gulf of Mexico, Wood Thrushes in Pennsylvania have managed to compress what should take weeks into mere days, perhaps by abandoning a period of rest and recovery after they arrive and before they mate.

There may be a limit to such temporal gymnastics. “If every spring is going to be earlier and earlier, are they��going to be able to keep up?” DeGroote asked. Exactly when a Wood Thrush departs on its migration is driven by a variety of internal clocks and external cues, from the bird’s innate circadian rhythms, to the seasonally changing ratio of daylight and darkness, to local weather and wind conditions. Many of these triggers are genetically embedded in the bird’s DNA, and natural selection would take time to change them, DeGroote said. “The million-dollar question is: How much can these birds continue to adapt to these changing springs?”

Wood Thrushes aren’t alone in this quandary; scientists in New Hampshire have observed . In fact, they found that those warblers that speed up their nesting during warmer springs often of chicks instead of just one, boosting their reproductive output—a potential benefit of the warming climate. But, as with thrushes, the warblers can only compensate so much. As Tingley wryly noted: “You can’t lay eggs before you arrive.”

��

Chicks are famously hungry. As soon as they hatch, they begin clamoring for food. For most songbirds that means protein-rich insects that will superpower their growth from bald, helpless babies into feathered, flying teenagers in mere weeks. Chicks’ insatiable demand for bugs is cited as a major reason why migration evolved in the first place: If you can wing your way to higher latitudes with their long summer days, you get an insect buffet. But climate change threatens to decouple the bird and insect hatches. The nightmare scenario is if chicks miss the insects too many years in a row, sending populations into an extinction spiral.

The best example of the kind of calamity that can befall a migratory bird in a changing climate is the European Pied Flycatcher, a trans-Saharan migrant that winters in tropical Africa. Its breeding season in western Europe is synchronized with the early-summer peak abundance of caterpillars, which, aligning with the warming of the planet, now emerge earlier. But, as ecologists Christiaan Both at the University of Groningen and Marcel Visser at the Netherlands Institute of Ecology found in the early 2000s, the flycatchers were migrating back north from Africa each spring on their traditional, genetically encoded timetable. The birds had fallen so far out of step with the caterpillar peak that in some parts of the Netherlands their population because they couldn’t feed their chicks.

In North America the closest analogy may involve shorebirds that migrate long distances, such as the that nests on the shores of Canada’s Hudson Bay. In that region, climate change has��produced unusually cold and snowy spring weather—and then a climatic whiplash, with profoundly hot summer temperatures. After winging their way from southern South America, the birds must wait for snow to melt before they can begin nesting. But the sudden switch to summer heat means the chicks’ food—insects that respond rapidly to warmth—are already past their peak when the young birds are hungriest. As a result, chick survival among godwits in Churchill, Manitoba, has recently been as low as 6 percent. Populations of other shorebirds that nest in this region, like Dunlin and Red-necked Phalaropes, have also that scientists link to these same changes.

So far, though, North American songbird migrants appear to be staying in sync with their offsprings’ food supply. That might reflect the continent’s forest health. Its woodlands, especially those in the east, have a caterpillar fauna more diverse than that found in Europe’s oak forests—one that produces a broad, season-long buffet instead of a sharp, compressed peak. In effect, North American songbirds have more wiggle room built into the timing of their migration, while European��migrants like the pied flycatcher can easily miss their brief insect pulse, especially now that its rhythm is becoming less predictable.

Fifteen years ago, when Both and Visser published their findings, the flycatcher’s situation looked alarming. Christiaan Both was the last scientist I contacted for this essay, and what he shared with me of his recent work was a pleasant shock.

“They have caught up,” he said of the flycatchers. “They are now, again, keeping up with the spring phenology.” A national bird census had shown the Dutch population in a steady decline, with 35 percent of the birds having disappeared from 1984 to 2004. Since then, pied flycatchers have been increasing in the Netherlands, Both told me. They’re nesting significantly earlier than they once did, again in synchrony with the caterpillar peak. What’s more, based on the exhaustive pedigrees Both and his colleagues have maintained for generations of banded, nest-box-hatched flycatchers, the change appears to be an evolutionary one. The birds aren’t simply changing their behavior; instead, the adjustment appears to be baked into their genes.

The rebound, Both says, lines up with a pause in the Netherlands’ rapid warming. From the 1980s to about 2005, the birds’ nesting grounds were getting hotter during late April and early May, the springtime period most important to nesting flycatchers. Then the warming trend ceased. “It’s not that climate change has stopped,” Both cautioned. “It’s that at this particular place and moment of the year, we don’t see so much temperature change.” Not all migrants are liable to face such a fortunate respite from rising temperatures. But still, we’ll take good news where we can.

So, am I an optimist or a pessimist? A little of both, I suppose, like any realist. Which migratory species, I wonder, will break under the strain of a changing climate? And which ones will surprise us, like those Dutch flycatchers?

I take some comfort in the fact that we’ve consistently underestimated birds’ resiliency. The fact that so many migrants are finding ways to at least partially compensate for the mess we’ve made of the seasons only adds to the sense of awe and wonder I’ve always felt for them. But I’m wary of complacency, because they can stretch only so far. In the end, the responsibility for migratory birds’ success—and the blame, should they fail—lies entirely with us.

This story originally ran in the Spring 2022��issue.��To receive our print magazine, become a member by��.��