How Songbirds Size Up|Their Trips South

(CN) - According to a new study funded by the National Science Foundation, birds in the United States make "migration risk assessments" before flying south for the winter.

The study found that the birds use a combination of factors, including body fat, weather and date to determine the least risky time to fly south.

It focused on three species of songbirds - red-eyed vireos, Swainson's thrushes and wood thrushes - as they crossed the Gulf of Mexico from coastal Alabama to the Yucatan peninsula, a distance of about 600 miles.

Songbirds spend about 30 percent of their lives migrating, and a small songbird can travel more than 3,000 miles twice a year.

The researchers published their findings in the Proceedings of the National Academy of Sciences.

To track the birds, researchers used radio telemetry as they migrated across the gulf. They attached transmitters to the birds' backs, and the transmitters emitted radio signals picked up by radio towers lining the migration route.

The team found that birds generally used two criteria, weather conditions and their own body fat levels, to make migration risk assessments. If humidity was too high, for example, the birds would not depart, and birds that do not carry large amounts of body fat would often change their routes.

Jill Deppe, a biologist at Eastern Illinois University and principal investigator on the NSF study, said in a statement that many studies have looked at birds' departure and arrival behaviors, but "this is the first time we've been able to take a subset of birds and gather data on both."

Although there are still many unanswered questions about bird migration habits, Deppe said they are not unanswerable, "They've simply been challenging to study, given current technological limitations. But that's changing."

The researchers believe their findings can form a basis for building better conservation strategies for migratory birds, such as protecting high-quality habitats along migration routes, where birds can eat and gain the fat they need to survive migration.

Michelle Elekonich, a program director in the NSF's Directorate for Biological Sciences, said in the statement that survival of migrating songbirds "has been declining for some time."

"This work will help us understand migration better and may allow us to better protect migrants," she said.

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