A climate-sensitive bird hints at global warming's lasting impact

The black-throated blue warbler's migration patterns are changing, and ornithologists can't help seeing the signs of climate change.
A black-throated Blue Warbler
A black-throated blue warbler.Kyle Horton

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By Denise Chow

It’s hard to be a bird these days.

Take the black-throated blue warbler. The migration patterns of this small songbird have been slowly but steadily changing over the past 50 years, according to a study published Thursday in the journal The Auk: Ornithological Advances.

The timing of the bird’s flight patterns has been advancing, with its spring migration occurring around one day earlier per decade. While the warbler isn’t facing the same risks as some of its other feathered friends, it’s the kind of small but unignorable change that ornithologists are becoming all too familiar with.

“There’s a reason the saying ‘canary in a coal mine’ is out there,” said Andrew Farnsworth, a research associate at the Cornell Lab of Ornithology, who was not involved with the new study. “Birds are very susceptible to changes, and they are really good indicators of what’s happening around them in their physical environment.”

This has made birds a particularly important part of understanding the growing impact of climate change. The report on warblers does not directly link their migration changes to global warming, nor does it incorporate temperature and climate data. But within the field of ornithology, it is becoming difficult not to see echoes of climate change across the spectrum of research.

Kristen Covino, an assistant professor of biology at Loyola Marymount University in Los Angeles, and lead author of the warbler study, said the findings have been received within the ornithology community as another example of the effects of global warming.

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“That’s the general consensus,” Covino said. “In some species, the change that is occurring in the timing of migration is coinciding with changes in environmental conditions.”

Many other studies have drawn a much more direct connection between birds and climate change. In a separate study published in December, scientists found that birds have been getting smaller as the planet warms. And earlier that year, researchers found a dramatic decline in the bird population in the United States and Canada since 1970 and included climate change among the causes of that staggering decline. Separately, the National Audubon Society found that two-thirds of North American birds face a risk of extinction due to global warming.

In some ways, though, ornithologists are in a unique position. They can tap a deep well of historical data thanks to amateur birdwatchers — a particularly passionate and dedicated group of citizen scientists — said Robert Rockwell, a research associate in the American Museum of Natural History’s ornithology department, who was not part of the warbler study.

“A number of studies get published based on birding observations,” Rockwell said of birdwatchers. “They’re a peculiar group because they become fanatics ... They become absolutely religious about keeping records and they’re quite willing to share those records with you.”

For the warbler study, Covino and her colleagues used data from the U.S. Geological Survey’s Bird Banding Laboratory. The archive, which extended from 1966 to 2015, consists of almost 150,000 individual records of warblers during their spring and fall migrations.

The researchers noticed some differences based on the bird’s sex and age — males tended to fly north earlier than females, for instance, and younger birds typically arrived at the breeding grounds later than more seasoned travelers — but the biggest change was in the overall timing of the migration.

“The earliest components of spring migration are getting earlier, in addition to overall spring migration getting earlier,” Covino said.

Jill Deppe, senior director of the National Audubon Society’s migratory bird initiative, who was not involved with the research, said the new study provides a valuable look at behavioral changes to a common songbird — changes that are likely having a similar impact on other bird species.

“It’s unique in that they looked at both fall and spring, but also across the whole geographic range,” Deppe said. “When you start looking at a pattern of consistent change over the entire migratory route of these birds, it lends additional credence to the results.”

While an average change of one day per decade may not seem like a huge difference, Farnsworth said it’s important to consider that change in its broader context.

“If you think about Christmas shifting forward a day every 10 years, then 50 years from now, Christmas would be on Dec. 20,” he said. “Then 50 years from then, it would be on Dec. 15, and further down the line, suddenly Christmas would be at Thanksgiving. If you start thinking about it like that, it’s not very much time from an evolutionary perspective.”

The study found fewer fluctuations in the birds’ fall migrations, though they did observe that the fall migration period is longer in recent years.

The findings demonstrate how interconnected birds are with their habitat, according to Deppe.

“The system is so complex that what’s impacting birds in one time of year impacts them at other times of the year, and what impacts them in one place also impacts them elsewhere,” she said.

Covino and her colleagues stress that their specific findings can’t be tied directly to global warming without examining these patterns against long-term climate trends. And she noted that changes observed in birds could be symptoms of major climate-related issues, which in turn influence the ability of birds to adapt and evolve,

“Are these changes in migration the same as changes in peak food availability?” she said. “Ultimately we want to know if changes in migration are keeping pace with environmental changes, because that’s ultimately going to be the driver of evolutionary changes.”