Songbird study reveals changes in fall migration
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Brooks Hays
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New research suggests black-throated blue warblers, a passerine species that travels back and forth between the Caribbean and the forests of the Eastern United States, have shifted the timing of both their spring and fall migrations during the last fifty years. Photo by Kyle Horton

 (UPI) -- As a number of previous studies have revealed, the timing of spring bird migrations in North America has shifted in response to climate change over the last several decades. Now, new research suggests fall migrations are being affected, too.

The new analysis, published this week in the journal The Auk: Ornithological Advances, showed the fall migration of black-throated blue warblers is getting longer.

"This means that some birds are leaving earlier while others are leaving later," lead researcher Kristen Covino, an avian biologist at Loyola Marymount University, told UPI in an email. "So while the peak of fall migration is staying the same, the period during which these birds are migrating is getting longer."

For the study, Covino and her colleagues utilized data from the North American Bird Banding Program. The program, administered jointly United States Geological Survey and the Canadian Wildlife Service, boasts one of the most expansive historical data sets on migratory birds.

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Scientists decided to study movement patterns of the black-throated blue warbler because the long-distance traveler may be representative of other species. The wealth of data on the species was also a motivating factor. Covino and her research partners were able to analyze more than 150,000 records of individual black-throated blue warblers.

"Also for this species, it is relatively straightforward to determine age and sex of individuals during both spring and fall, so we were confident in the reliability of those pieces of information since the data were initially collected by so many different field researchers," Covino said.

Scientists confirmed -- as previous studies have shown -- that birds have been flying north to their breeding grounds earlier and earlier each spring, an average of a day earlier each decade.

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Researchers suspect that the changes in the bird's spring migratory patterns could be responsible for the lengthening of the species' fall migration period.

"One possibility is that the early departures are young from the first brood -- chicks from a nest -- of the earlier arriving adults and the later departures are a product of some adults attempting, and some succeeding, in a second brood or reproductive attempt in the same season," Covino said. "This species does not always attempt two nests in the same season so it is possible that the breeding season is getting longer and allowing for an increase in this behavior."

Because spring migration patterns are a significant driver of reproductive success, scientists have focused exclusively on the links between climate change and spring migration shifts. Covino hopes their latest research while inspired future studies to consider changes in both fall and spring migrations together.

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"We stress that investigations into migratory phenology should include both migratory seasons in order to holistically encompass all the changes occurring within a species," she said. "Our follow-up study is to expand this approach to include several additional species."

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