Adapting to Climate Change, Bird Species Send Out Explorers to “Scout New Habitats”

(p. B1) 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 (p. D4) 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.”

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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.”

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“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 threatened 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.”

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“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.