New study shows changes in fall migration patterns of birds using 50 years of data

The research which was published on The Auk: Ornithological Advances was based on Black-throated Blue Warblers

New research showed that the spring migration of birds has been happening earlier in recent decades. The research which is from The Auk: Ornithological Advances based on Black-throated Blue Warblers, which is a common songbird and they migrate from Canada and also from the eastern US to Central America and again they go back every year. The study was conducted using fifty years of bird-banding data that added another piece to the puzzle, revealing that little-studied fall migration patterns also got shifted over time.

Loyola Marymount University's Kristen Covino and her colleagues made use of the data available at the USGS Bird Banding Laboratory on the migrating Black-throated Blue Warblers ranging between 1965 and 2015. All over the United States, scientists who are working with this program safely capture migrating birds, collect data on them, and fit them with metal leg bands with unique codes which allow them to get identified if they're captured again.

Covino and her colleagues analyzed almost 150,000 individual records

Blue Warblers
Black-throated Blue Warblers have shifted the timing of their spring and fall migrations over the past fifty years Kyle Horton

Analyzing almost 150,000 individual records, Covino and her colleagues found that the timing of the birds' spring migration has advanced over the last fifty years, with early migrants passing through banding sites approximately one day earlier each decade. Crucially, their data also covered fall migration, which has been less well-studied, and found that while the timing of the peak of fall migration hasn't changed, fall migration takes longer today than it did fifty years ago.

The North American Bird Banding Program is one of the most expansive historical datasets on migratory birds, including records for over 38 million songbirds banded since 1960. "My coauthor Sara Morris and I were already working together on another paper on Blackpoll Warblers using data we'd requested from banding stations across North America.

We wanted to take a similar large-scale approach for this study, but we wanted to demonstrate that we could do this approach with data that is completely available from the Bird Banding Lab," says Covino. "We selected Black-throated Blue Warblers because it's relatively straightforward to determine their age and sex, which means that the data this species generates are both accurate and powerful."

Findings can't be explicitly linked to climate change

Although the researchers emphasize that their findings can't be explicitly linked to climate change without incorporating climate or environmental data, they believe similar methods could be useful for tracking the effects of climate change on birds.

"The protraction of fall migration means that the season is getting longer overall, but it could also mean that the breeding season may be shifting, ending earlier for some individuals but later for others. To determine what this means in the context of breeding season shifts in timing, additional studies that incorporate both arrivals on the breeding grounds and, importantly, a departure from them are needed," says Covino.

"More studies of these patterns of fall migration timing and, even more so, both spring and fall migration timing across years are needed to gain the complete picture of how species are changing migration timing."

(With agency inputs)