Flight burns less fuel than stopovers

The first measurements of energy use in migrating songbirds have confirmed a paradox predicted by some computer models of bird migration: Birds burn more energy during stopovers along the way than during their total flying time.

NIGHT FLYER. A thrush wired for radiotracking. Wikelski

Martin Wikelski of Princeton University and his colleagues monitored 38 Swainson’s and hermit thrushes during the nights of their spring migration through the northern United States. The researchers injected the radio-tagged birds with chemical-isotope tracers that enabled the scientists to measure the birds’ metabolism. The team members spent their nights driving a car, trying to keep up with a tagged bird. “We got stopped by a cop just about every night, not because we were speeding, but because they wanted to know what somebody was doing in a little town in Wisconsin at 4 a.m. with a giant antenna on the roof of a car,” says Wikelski.

A dozen birds took night flights covering up to 600 kilometers. The rest stayed put. The scientists determined that the birds that flew burned 71 kilojoules of energy on an average night’s flight of 4.6 hours. The birds that didn’t fly burned energy at 88 kJ per day.

Since the birds spent about 24 days and nights on stopovers during a typical 42-day journey from Panama to Canada, actual flying consumed only 29 percent of the total energy budget for the migration, Wikelski and his coworkers report in the June 12 Nature.

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Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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