Half-Century of Data Reveal Fluctuations in Fall Migration of Birds

(CN) – In recent decades, researchers have found that spring migration of birds occurs earlier and earlier, and a new review of migration data shows that the less-studied fall migration period is also in flux.

Loyola Marymount University researcher Kristen Covino and colleagues examined 50 years of data to understand exactly how birds’ fall migration patterns have shifted over time.

Researchers found that birds that migrate between North America to Central America each year, including black-throated blue warblers, have shifted the timing of their spring and fall migrations over the past 50 years. (Courtesy of Kyle Horton)

Data for the study released Wednesday was collected through bird banding by U.S. Geological Survey Bird Banding Laboratory researchers between 1965 and 2015.

With bird-binding, researchers capture migrating birds and place coded metal bands on their legs to track their migration patterns over a certain period.

Migration data for the black-throated blue warbler, a common songbird, was analyzed closely by researchers. This was the only species studied, though researchers said they plan to study others in the future.

The warbler’s annual migration pattern takes it on a round-trip journey from Canada and the eastern U.S. to Central America.

Covino and colleagues had previously examined more than 150,000 individual migration records, finding that birds’ spring migration had sped up over 50 years.

Migrating birds were passing banding stations approximately one day earlier each decade, researchers found.

In the new study published in the journal The Auk: Ornithological Advances, researchers also examined crucial fall migration data, finding that while the start of migration season has not changed, fall migration extends longer than it did in the 1960s.

Researchers handle female (left) and male (right) black-throated blue warblers as part of a study to understand bird migration patterns over the past 50 years. (Courtesy of Sara Morris)

The new findings were based on an analysis of over 38 million records of songbird migrations collected since the 1960s by the North American Bird Banding Program.

Covino said in a statement that with the new study, she and study co-author Sara Morris sought to expand on their ongoing bird migration research.

“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,” Covino said. “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.”

Covino said more research is needed to determine the effect of climate change on bird migration patterns.

“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,” Covino said. “To determine what this means in the context of breeding season shifts in timing, additional studies that incorporate both arrival on the breeding grounds and, importantly, departure from them are needed.”

Specifically, Covino noted Covino climate change could throw off nature’s food cycle, thus negatively affecting birds’ breeding process.

“A changing environment can alter the phenology of seasonal vegetation regrowth which also affects food availability, including insect availability,” said Covino in an interview. “If seasonally reproducing organisms are not properly synced with their environment, the timing of major events, like provisioning of young, will not properly coincide with peak food availability.”

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