Warm-up pushes migrating birds to earlier ETAs

UNC-CHAPEL HILL (US) — Rising temperatures are causing birds in eastern North America to pick up the pace along their yearly migratory paths.

Pushing migration earlier in the year could negatively affect birds over the long term, says Allen Hurlbert, assistant professor of biology at the University of North Carolina at Chapel Hill. “Timing of bird migration is something critical for the overall health of bird species.”

Using migration information collected in eBird, a citizen science database containing 10 years’ worth of observations from amateur birdwatchers, researchers analyzed when 18 different species of birds arrived at various points across their migration journeys. Since 2002, eBird has collected more than 48 million bird observations from roughly 35,000 contributors.

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The study results are published in the journal PLoS ONE.

Birds “have to time it right so they can balance arriving on breeding grounds after there’s no longer a risk of severe winter conditions,” Hurlbert says. “If they get it wrong, they may die or may not produce as many young. A change in migration could begin to contribute to population decline, putting many species at risk for extinction.”

To minimize these threats, Hurlbert says he hopes the findings would be used to increase awareness around bird conservation. The outcomes could also help scientists identify which parts of the eastern United States will experience the greatest migration shifts, as well as which species face the largest dangers because they will be least likely to adapt successfully to climate change.

Although eBird only contains a decade of amateur-submitted data, versus several decades of data compiled by select bird observatories, the information it contains provides greater geographic coverage.

Hurlbert’s team focused specifically on bird species that occur over the entire breadth of the eastern U.S. By reviewing the recorded temperatures and the exact dates on which bird watchers first noticed certain species in their areas, the researchers determined how closely bird migration tracks year-to-year variation in temperature.

Earlier ETAs

On average, each species reached various stopping points 0.8 days earlier per degree Celsius of temperature increase. Some species’ schedules accelerated by as much as three to six days for each rising degree. To date, the Northeast has experienced more relative warming than the Southeast.

The speed at which a species migrates is the biggest influence on how strongly it responds to increasing temperatures, Hurlbert says. Slow migrators, such as the red-eyed vireo or the great crested flycatcher, were the most adaptable to changes. Additionally, the length of the migration path impacts how quickly birds move from one location to another.

“It makes sense that if you take your time to move north, you’re sort of checking out the surroundings around you,” he says. “If the conditions seem too cold, you can decide there’s no point in moving on that day. Species that tended to advance quickly, as well as those migrating from greater distances, such as Central or South America, were less able to adapt to temperature changes.”

However, being a slow traveler doesn’t free a species from all climate change-induced migration challenges. Because they stay in one spot longer, such birds have heavier habitat and food requirements, making them more dependent upon the resources that are available along their paths.

That reliance could become a greater problem if climate projections for the next 50 years to 75 years hold true, Hurlbert says. Climatologists predict the Northeast will continue to warm at a faster pace than the Southeast, potentially forcing slow migrators to move even slower and put greater strain on their migratory routes.

“There’s a lot of concern in the scientific community about climate change and how it will affect living things,” he said. “This is a really useful data set that can likely address these anxieties around birds.”

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