The reason, according to University of North Carolina at Chapel Hill researchers, is rising temperatures due to climate change.
Using migration information collected in eBird, a citizen science program database containing 10 years' worth of observations from amateur birdwatchers, assistant professor of biology Allen Hurlbert, Ph.D., and his team in the UNC College of Arts and Sciences 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.
The study results were published in the journal PLoS ONE on Wednesday (Feb. 22).
Pushing migration earlier in the year could negatively affect birds over the long term, Hurlbert said.
"Timing of bird migration is something critical for the overall health of bird species," he said. "They have to time it right so they can balance arriving on breeding grounds after there's no longer a risk of severe winter conditions. 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 said he hoped the findings would be used to increase awareness around bird conservation. The outcomes also could 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 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.
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.
According to the review, Hurlbert said, the speed at which a species migrates is the biggest influence on how strongly it responds to increasing temperatures. 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 affects 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 said. "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 does not 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 said. 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."
The study's co-author was Zhongfei Liang, a former undergraduate student who helped Hurlbert analyze the data. The paper's title is "Spatiotemporal Variation in Avian Migration Phenology: Citizen Science Reveals Effects of Climate Change."
Study link: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0031662
Media note: Hurlbert can be reached at (919) 843-9930 or firstname.lastname@example.org
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