It was a dark and windy night, but millions of moths migrating over Britain could still tell which way they were going.
Radar showed that silver Y moths heading south for winter selected winds sweeping them in the right general direction, says Jason Chapman of Rothamsted Research in Harpenden, England. Moths even seemed to adjust their flight direction to compensate for somewhat off-course winds, he and his colleagues report in the April 8 Current Biology.
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“This is the first good evidence for some kind of compass in nocturnal migrating insects in the wild,” Chapman says.
A Rothamsted engineer custom built radar systems to study insects flying between 150 and 1,200 meters overhead. The radar systems don’t scan around like conventional ship radar but instead look straight up while moving slightly. The arrangement reveals extra details, explains Ian P. Woiwod, also of Rothamsted. This radar detects the rough size and height of an insect winging overhead plus the orientation of its long body axis.
Two of Rothamsted’s radar systems run continuously, and from 2000 to 2003 they recorded 42 bursts of Autographa gamma, or silver Y, migration. The researchers didn’t identify the species just from radar but used other methods, such as traps attached to balloons in the region.
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In fall the moths fly south-southwest toward the Mediterranean. The year 2003 was a boom year for silver Ys, and some 200 million migrated across England. Chapman says entomologists aren’t sure exactly where the moths go for the winter but their ultimate refuge probably lies somewhere in North Africa. When that region dries up months later, the silver Ys fly north again.
After analyzing moths whisking over the radar, Chapman, Woiwod, and their colleagues contend that the silver Ys can manage sophisticated orientation. On fall nights, the winds at migration-height can blow in any direction, but the moths flew only when winds blew roughly in the right direction.
On a given night, moths flew at the height with the fastest winds, according to detailed weather service extrapolations. And if the wind blew more than 20 degrees away from south-southwest, the moths oriented their bodies in a direction that compensated, the researchers report.
The moths can’t see the sun, and they probably aren’t finding direction by the moon since it was still below the horizon during some of the migration flights, says Chapman. He doesn’t expect moth vision to resolve stars well enough for navigation, so he speculates that they have a magnetic sense. Nocturnally migrating birds do, but “there’s not any really good evidence for free-flying nocturnal insects,” he says.
Robert Dudley of the University of California, Berkeley remains skeptical. He says he would like to see the researchers measure wind speeds on-site instead of relying on the weather service models.