When does fall migration begin? That depends on where you live. It may start in early September through late November, although birds in the Arctic begin migrating much earlier.
The beginning of November always makes me think of the fall migrations of birds. In the United States, this annual ritual typically begins around August 1 and lasts through November 30; although, some birds may migrate earlier or later, and migration can start as early as mid-June and last until early January.
But it’s November, in particular, that makes me notice the birds leaving. It could be because the falling leaves no longer block midair, the skies turn moodier and flapping wings can be seen en masse.
The time of year, though, isn’t the only thing that determines fall bird migrations. These departures are finely tuned to the birds’ complex and unpredictable environments. By studying the migrations of four, iconic species of cranes, researchers are now learning more about how animals use their surroundings to survive and thrive—and how global climate change will affect them.
By studying the migrations of cranes—such as that of the demoiselle crane, the smallest crane species—researchers are learning more about how animals use their surroundings to thrive and how global climate change will affect them.
Starlings, too, are showing us that migration isn’t just a simple issue of timing and follow-the-leader. Young, naive starlings look for wintering grounds independently of experienced birds. Although starlings are highly social, they don’t copy their migration routes from each other.
At the other end of the cycle from the fall migration is the spring migration. And one result of climate change is that spring is arriving earlier. However, some migratory birds are not keeping up with this development and arrive too late for the peak in food availability when it is time for breeding. By “encouraging” these birds to fly a little farther north, researchers have discovered that these birds can give their chicks a better start in life.
And here’s some more good news: some birds need no encouragement from us, at all. In an apparent reaction to pressures along their former migratory route, a population of Arctic geese has rapidly adjusted on its own, forming a new migration route and breeding location almost 622 miles from their original stomping grounds.
Although starlings are highly social birds, they don’t copy their migration routes from each other.
Cranes assess and then navigate their complex migrations
Published in the science journal Proceedings of the National Academy of Sciences in September 2024, a collaborative, global study of four species of cranes has shed light on the way that migrations are synchronized with complex and volatile environments. The research team, led by scientists from Germany’s Max Planck Institute of Animal Behavior and Connecticut’s Yale University, combined novel animal tracking technology; remote-sensed information about the environment; and a new, statistical framework to gain insight into four iconic species: black-necked cranes, common cranes, demoiselle cranes and white-naped cranes.
The researchers used tiny GPS tracking devices to follow the movements of 104 cranes in Africa, Asia and Europe. These devices included unique, solar-powered, GPS leg bands. The tracking data revealed the impressive migrations that cranes undertook. Some of the migratory routes exceeded 3,900 miles of travel round trip and required crossing barriers such as the Alps or the Himalayas, the deserts of the Arabian Peninsula or the Mediterranean Sea. In addition to the tracking study, the researchers also developed a statistical framework that revealed how the cranes’ movements relate to aspects of the environment, such as the presence of crops or water bodies nearby, and the temperature and vegetation cover on the land.
The scientists found that all four crane species experienced starkly different environmental conditions over a year, and that these periods coincided with important events in their lives. This was particularly pronounced when comparing resource availability and temperatures on summer-breeding and wintering grounds. For some, the migrations themselves entailed huge shifts in environmental conditions. For example, the demoiselle cranes migrated across the Tibetan Plateau and had to contend with massive fluctuations in temperatures while doing so.
Common cranes, also known as the Eurasian cranes, concentrate on agricultural areas during the late summer, a period that aligns with raising juveniles and preparing for fall migration. It’s exactly when we would expect them to want easy access to food.
The researchers suspect this has to do with different biological needs during these different times of the year. For example, common cranes clearly favored agricultural areas during the late summer, a period that aligns with raising juveniles and preparing for fall migration. It’s exactly when we would expect them to want easy access to food.
For other species, access to food may come at a cost. The black-necked cranes in the study had to decide between abundant resources or safe roosting habitats. Amazingly, say the scientists, the balance between these competing needs changed over the year depending on what the birds were doing. During migration, they opted for safer roosting conditions; whereas during breeding, they leaned towards abundant food.
While this type of shifting emphasis depending on what cranes need at any given time was expected, the scientists were blown away by how well the cranes used movement to resolve trade-offs among competing needs and to access certain environments during key periods of the year.
Black-necked cranes must decide between safe roosting habitats and abundant resources. During migration, they opt for more secure roosting conditions; whereas during breeding, they lean toward ample food.
Understanding how animals interact with their surroundings not only gives us a more nuanced view of how they survive in complex environments, it’s also crucial for developing management and policy actions to address the dual crises of biodiversity loss and climate change, say the authors.
Starlings inherit their migration behaviors
The question of how migratory birds locate their migration routes has intrigued humankind for centuries. Biologist Albert Perdeck from the Netherlands aimed to find answers when he displaced thousands of migrating starlings by plane from the Netherlands to Switzerland and Spain in the 1950s and 1960s. Eleven thousand starlings were caught in trapping stations when they were passing through the Netherlands during their autumn migration. The adult and juvenile birds were ringed and then brought to Switzerland by plane, where they were released. Of these, 354 birds were recovered, either in the same year or in later years. The remarkable result was that the juveniles retained their original migration direction and thus ended up wintering in France and Spain rather than in England, where these birds were headed when caught in the Netherlands. The adult starlings, however, changed direction and migrated towards England after being released in Switzerland. This experiment has become a classic study on the migratory orientation of birds.
Now, 70 years later, a team of researchers at the Netherlands Institute of Ecology and the Swiss Ornithological Institute have confirmed Perdeck’s findings and were able to solve a long-lasting, scientific debate using his historical dataset.
Young, naive starlings look for their wintering grounds independently of those of their own species who are more experienced.
First, the scientists attached lightweight, metal rings with unique codes to the legs of some Spanish and Swiss starlings. Ring recoveries indicated that relocated young and adult starlings used different strategies to reach their winter destinations in the British Isles and France. Adult starlings were aware of this move and adjusted their migratory orientation to reach their normal wintering areas. Young starlings continued in a southwesterly direction—the direction they would have chosen when departing from the Netherlands—and reached “wrong” destinations in southern France and Spain.
Over the years, experts in the field of avian migration have been divided about the interpretation of Perdeck’s results. Starlings are highly social animals and, according to some scientists, the relocated young starlings could have joined a flock of local birds. The relocated starlings could then have copied the migratory behavior of their new friends, demonstrating that the migratory route is largely learned instead of inherited—a major difference.
By reanalyzing this historical dataset, the team showed that the migratory orientation of the relocated starlings differed from the local starlings. Thus, the birds are no social migrants or copycats, conclude the researchers, who published their findings in the scientific journal Biology Letters in July 2024. The alternative social explanation of Perdeck’s results has thus been debunked. Starlings travel independently and decisions about where to go are not overruled by the migratory behavior of others. Recently, another study showed that starlings migrate at night. This is in line with the 70-year-old findings, because how would you follow another in pitch darkness?
Even though starlings are numerous and widespread birds that have adjusted well to human-dominated landscapes, their inherited, migratory behavior is likely to be less flexible. That could be a problem in a time of rapid climate change.
Why does it matter if a behavior is inherited or learned? It’s important, especially in times of quick shifts in the global climate and land use. Inherited behaviors are less flexible to rapid changes. Although starlings are numerous and widespread birds that have adjusted to human-dominated landscapes, their migratory behavior is likely to be less adaptable.
Pied flycatchers—with help—learn to adjust their migrations to climate change
Around the world, global warming is causing problems for birds. Warmer springs mean that caterpillars hatch, grow and pupate earlier compared with just a few decades ago. This has consequences for birds that cannot eat caterpillars that have entered the pupal stage. Therefore, when the food supply runs out at an ever earlier time in the spring, more and more chicks starve during the breeding season.
This is a big problem for migratory birds that spend their summers in Europe and winters in Africa, as they can’t know how early spring arrives on the other continent. Could the problem be solved if the migratory birds simply flew farther north until they found a place with suitable, well-developed caterpillars?
The pied flycatcher is slightly smaller than a house sparrow. The male is mostly black on the upperparts and white underneath, with a bold, white patch on its folded wing. Females are browner. In Europe, the birds are summer visitors, spending the winters on the tropical coast of West Africa.
To test this in practice, Swedish researchers recently decided to help some pied flycatchers along their way. They caught some of the birds that had arrived prior to breeding in the Netherlands. The birds were then driven about 373 miles during the night to Vombs Fure, an area of pine forests outside Lund in Skane, Sweden, where they were released. The peak of caterpillar availability in Skane is about two weeks later than in the Netherlands.
Publishing their results in the journal Nature Ecology and Evolution in September 2023, the researchers showed that the birds that were given a lift from the Netherlands to Skane synchronized very well with the food peak. As they started to breed about 10 days earlier, they had a dramatically better breeding success than the pied flycatchers that remained in the Netherlands. In addition, it was shown that the chicks of the pied flycatchers that had received migration assistance did not stop in the Netherlands when they returned after their first spring migration. Instead, they continued to the area of pine forests outside Lund where they were born. Furthermore, they arrived earlier than the Swedish pied flycatchers and thereby had more well-fed chicks at Vombs Fure the year after the researchers gave the pied flycatchers a helping hand to find Skane.
The number of small birds, particularly migratory birds, has decreased drastically throughout Europe. By flying a little farther north, these birds, at least in principle, would be in tune with the food resources, bringing hope that robust populations of pied flycatchers and other small birds like them can be maintained, even though springs are arriving ever earlier, conclude the researchers.
With springs arriving ever earlier, pied flycatchers could better synchronize with food resources by flying a little farther north, bringing hope that robust populations of these small passerines can be maintained.
Arctic terns may have climate-change-resilient migrations
Other birds, however, are navigating climate change just fine by themselves.
Arctic terns—which fly on the longest migrations of any animal on Earth—live in near-perpetual daylight, breeding in the North of our planet and flying to Antarctica for the summer, covering enough distance in their lifetimes to travel to the moon three times.
In a recent study, published in the journal Global Change Biology in July 2023 and led by scientists from the United Kingdom’s University of Exeter and the Met Office, the researchers examined the likely impacts of climate change on Arctic terns outside of the breeding season, investigating changes to Antarctic sea ice, prevailing winds and primary productivity (which affects food availability) at key sites visited by Arctic terns.
A slender, small, gray-and-white bird with angular wings, the Arctic tern is well known for its long, yearly migration. It travels from Arctic breeding grounds to Antarctica, covering about 25,000 miles.
Arctic terns rely on sea ice for rest and foraging, prevailing winds during flight and productive oceans for food. Using multiple Earth system models and observations of ongoing climate change to project changes by 2100, the researchers examined the impacts of two emissions scenarios: “middle-of-the-road” and “fossil-fueled development.”
The fossil-fueled development scenario led to a projected decline of primary productivity in the North Atlantic—a key feeding ground for millions of seabirds and other marine animals. However, minimal changes to primary productivity were projected at three other key sites for Arctic terns: the Benguela Upwelling, the Southern Ocean and the Subantarctic Indian Ocean.
Meanwhile, the impact of Antarctic sea-ice decline on terns is uncertain, and the projections suggested small changes to prevailing winds would have “minimal impacts” on tern migration—except in the Southern Ocean, where strengthening winds may force the birds to shift flight routes.
Good news: the overall effects of climate change on migrating Arctic terns should be minor. They are likely to be resilient due to living their lives over such vast areas.
While poorer foraging in the North Atlantic seems likely to pose a threat for Arctic terns in the future, the study’s findings indicate that the overall effects of climate change for these migrating birds should be minor. They are likely to be resilient due to living their lives over such vast areas.
However, this is only part of a bigger picture, warn the scientists. Multiple small effects may still harm long-lived (up to 30 years) Arctic terns—and other species may be unable to escape local and regional changes. Meeting carbon emissions targets is vital, they say, to slow these projected, end-of-century climatic changes and minimize extinction risk for all species.
Pink-footed geese find their own, new migratory route
It appears that another group of migratory birds—a population of Arctic geese—have decided to confront climate change head-on and with their own two wings.
With buff-pink-and-gray plumage, stubby bills and bubblegum-colored legs, pink-footed geese are cute birds that live extreme lifestyles. In spring, they inhabit the remote cliffs and windswept tundras of Greenland, Iceland and Svalbard, where they breed. On migration and in winter, they form flocks of thousands in crop fields and grasslands of the United Kingdom and western Europe. They are rare visitors to the eastern United States and Canada, where they tend to fall in with groups of Canada geese.
A study, reported in the journal Current Biology in March 2023, shows that pink-footed geese in Norway’s Svalbard have quickly adjusted their former migratory route and relocated their breeding grounds almost 622 miles from the original location. What’s more, it appears the new route has caught on with other geese and even birds of other species via cultural transmission (social learning), and the new population already has grown to as many as 4,000 individuals.
Scientists had been studying Norway’s Svalbard population of pink-footed geese for more than 35 years. About 20 years ago, they started getting reports of geese turning up on migration in Finland and Sweden, which were confirmed as members of the Svalbard population. So, to learn more, the researchers went to Oulu, Finland, in the springs of 2018 and 2019 with a goose-catching team from Denmark. Their hope was to capture and outfit some pink-footed geese with GPS tags to find out where these geese were going, and they got an unexpected answer. Half of the marked individuals in Oulu migrated northeast to Novaya Zemlya in north Russia, which has experienced warming temperatures. The tagging information also demonstrated that females were breeding there. This site is more than 600 miles east of the Svalbard breeding grounds. While the new population is not demographically nor genetically isolated yet, it already qualifies as a separate population.
Such a rapid evolution—over the course of 10 to 15 years—of a migratory route and new breeding grounds by a bird species that is regarded as being very traditional in its behavior and site use was extremely fascinating to witness, stated the researchers. At a time when climate change and other human activities threaten many species, especially Arctic ones, social learning can be a behavior that can avoid some negative impacts—in the short term, at least. It could even have positive implications for hoofed ungulates, whales and wolves.
As the planet warms, animals that breed in the Arctic are at particular risk. But some are fighting the odds. In an apparent reaction to pressures along their former migratory route, a population of pink-footed geese has rapidly adjusted, forming a new migration route.
Birds design their own migratory flights
For literally millions of years, migrating birds have brought a flood of color and sound to awakening lands in spring and a river of songs and wings to sleepy terrains in the fall. In fact, in the vast mosaic of nature’s wonders, few spectacles rival the epic odysseys of bird migrations.
But far from being one sort of journey, every kind of bird, it seems, puts its own mark on the seasonal flows.
Here’s to finding your true places and natural habitats,
Candy
