These Birds Aren’t Lost. They’re Adapting.

From what we can tell, the Steller’s sea eagle trekking across North America does not appear homesick.

The bird has strayed thousands of miles from its native range in East Asia over the last two years, roving from the Denali Highway in Alaska down to a potential sighting South Texas before moving eastward and back north to Canada and New England. Its cartoonish yellow beak and distinctive wing coloration recently attracted crowds of rapt birders to Maine before turning up on April Fools’ Day in Nova Scotia.

“We live in a world of very little surprise,” said Nick Lund, the outreach manager for Maine Audubon and creator of The Birdist blog. Catching a glimpse of a far-flung bird in one’s backyard, he said, “is like the purest form of joy.”

But the rogue Steller’s sea eagle isn’t just a lost bird: It is an avian vagrant, a term that describes birds that wing their way well beyond their species’s normal range of movement.

Humans have long marveled at such exotic stragglers—which experts also refer to as waifs, rarities, extralimitals, casuals and accidentals—and what they suggest about the biological importance of wandering. “The ‘accidentals’ are the exceptional individuals that go farthest away from the metropolis of the species; they do not belong to the ordinary mob,” Joseph Grinnell, a field biologist in California, noted in 1922. “They constitute sort of sensitive tentacles, by which the species keeps aware of the possibilities of aerial expansion.”

The peripatetic sea eagle wasn’t 2021’s only extralimital. Other fan favorites strayed from the parts of Central and South America where they are typically found: an Inca tern spotted in Hawaii; a small-billed elaenia captured in a net in Quebec; a heron-like limpkin recorded in Texas for the first time; and, a gray-breasted Martin observed in Prospect Park in Brooklyn.

“Each time there’s a vagrant it’s its own exciting story,” Mr. Lund said. “It has this treasure hunter’s charm.”

A new book, “Vagrancy in Birds,” extends this century-old notion — arguing that vagrancy does not always represent a tale of navigational avian misfortune, but can be one of the first visible signs of bird species adapting to human-driven alterations to Earth’s waters, lands and skies.

“We’re destroying and creating habitats,” said Alexander Lees, a co-author of the book and a senior biodiversity lecturer at Manchester Metropolitan University in England. “We’d expect wildlife to adapt to that.”

Migratory birds are equipped with tools to navigate what Heinrich Gätke, a German ornithologist, described in 1895 as “the whole vault of heaven.”

Some rely on potentially inherited “mental maps” refined during the first journeys of life.

Many employ an internal clock for measuring elapsed time. They may also have an internal compass for determining orientation that they calibrate using the sun, stars, patterns of polarized light and the Earth’s magnetic field.

Most will successfully migrate entire lifetimes without a hitch. But the avian navigation system is not error-proof.

For one, unusual weather patterns can rechart a bird’s regularly scheduled flight path. In 1927, for example, a cold spell combined with an “exceptionally high easterly wind” pushed a spray of Lapwings across the Atlantic Ocean to Newfoundland. More recently, extreme droughts have sent marsh-loving glossy ibises from the Mediterranean as far north as the British Isles.

Often, vagrants do not survive their detours. “They either die or disappear never to be seen again,” said Robert Davis, a wildlife biologist at Edith Cowan University in Australia. “It’s almost a novelty, a fleeting moment — then it’s gone.”

In June 2011, for example, a juvenile emperor penguin washed ashore on Peka Peka Beach in New Zealand, some 2,000 miles from its coastal Antarctic home. The marooned bird mistook wet sand for snow, and began eating it, landing him in rehabilitation at Wellington Zoo’s animal hospital (a live-streamed, monthslong stay involving a bed of crushed ice and a hand-fed salmon diet). The wayward penguin eventually returned to sea with a one-inch microchip tucked under the skin of his right thigh and a three-inch satellite transmitter glued to his lower back, both of which stopped sending signals after five days.

That’s a pretty typical ending for a vagrant — but that doesn’t mean there’s not more to the story.

“Vagrancy is not just something that just happens totally accidentally,” said José Ramírez-Garofalo, an ecology graduate student at Rutgers University. “Rather than thinking, ‘This bird shouldn’t be here,’” he said, “We should be thinking, ‘Why is it here?’”

Research has demonstrated that the long-term impact of a single avian vagrant can in fact, be ecologically profound.

A lone goose detached from a flock of its own feather might link up with another waterfowl species, year after year, eventually producing hybrid offspring. Vagrants can also establish entirely new populations where they successfully breed, migrate or overwinter. Even if they die alone, wayward birds can seed new life-forms in a landscape by transporting parasites, pathogens, plants and even live fish eggs and brine shrimp cysts on their feet or in their feathers or guts.

Avian vagrants can also wreak havoc on other, potentially vulnerable, native bird species.

When a vagrant peregrine falcon lingered on Saint Paul Island in the Indian Ocean in 1999, it killed at least 27 MacGillivray’s prions, an endangered species whose entire global population is limited to a few square miles. In 2006, vagrants of the same raptor species killed 4 percent of endangered Laysan ducks that had just been translocated to the Midway Atoll from nearby islands.

On rare occasions, humans intervene to prevent such invasions. In 2017, a Nicobar pigeon suspected of flying across the Indian Ocean from Indonesia was discovered in northwestern Australia. Officials from the country’s agriculture department took the pigeon into custody, quarantined it, then transferred it to Adelaide Zoo. It was never released into the wild again. In another incident, last year, a racing pigeon named Joe went missing from Oregon and months later showed up 8,000 miles away in Melbourne, where officials deemed him a biosecurity risk and euthanized him.

Other scenarios — like ship-hitching house crows and hummingbirds, or the barn owl Believed to have traveled from England to Afghanistan in the hold of a military aircraft — demonstrating humanity’s role in punting bird species far from their origins.

Records of avian vagrants have surged in recent years. Australia, for example, logged a total of 329 vagrant birds over a 60-year period, from 1940 to 2000, Dr. Davis noted in a 2018 paper. That figure almost doubled — to 617 cases — from 2000 to 2017.

Experts have also noticed an uptick for some species in the United States. “We’re seeing a lot more vagrant birds showing up,” said Shannon Curley, a biologist at the College of Staten Island who studies the effects of climate change on bird distribution.

Part of that increase is attributed to more eyes on the sky. “No. 1, there’s just more birders out there than there were before,” Mr. Lund said. And he added that today’s bird-watchers are better equipped than ever to identify unfamiliar birds and record their appearances with publicly available citizen science tools like eBird and iNaturalist.

But he and other experts think there’s a more complicated dynamic for the many species — not just birds — known to wander long distances.

“We think of ranges as stable in space and time. But ranges are incredibly dynamic and they can change,” Dr. Lees, of Manchester Metropolitan University, said.

Vagrancy, the scientists argue, might help species chart an escape route from human-driven climate change and widespread habitat destruction. Instead of staying put and facing potential extinction, a few solitary pioneers can scout new habitats as their former homes become unlivable.

The critically endangered Chinese crested tern, for example, was presumed to be extinct after last being spotted in 1937. Then, in 2000, and again a few years later, biologists rediscovered the species at sites in China and Taiwan where it hadn’t bred before. In 2016, scientists found two nesting Chinese crested tern pairs incubating eggs on an uninhabited island in South Korea. Its tiny surviving population — only about 50 birds — is still threatening by egg-poaching humans and nest-destroying typhoons. But as one conservation officer noted in 2017, the Korean nesting site “means the future of this species looks more promising now.”

With growing attention to climate change, scientists emphasized the difficulty of unpacking the role of vagrancy in a species’s adaptation. “You can’t predict when and where a vagrant will show up,” said Lucinda Zawadzki, a zoologist at the University of Oxford. “They are, by nature, rare.”

For her own research, for example, Dr. Zawadzki set up 19 mist nets on Bon Portage Island in Nova Scotia to catch and study as many vagrants as possible. She netted 29 in two years — an impressive yield for the subject at hand. But she conceded that it was a small sample size for a scientific study.

In the absence of a solid understanding of their pioneering journeys, humans have typically written off avian vagrants as disoriented or windblown.

“There’s this historical narrative around vagrants that they have to be lost. They have to be aberrant. There’s something wrong with them,” Dr. Zawadzki said.

But faced with climate change, she said, the opposite might prove true: The ability to explore — or, seen another way, the opportunity to “get lost” — becomes a huge advantage.

“They’re more likely to survive,” she said.

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