Dragonflies Migrate Like Birds, Study Says

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Each weighing just 0.01 ounce (0.3 gram), the battery-powered transmitters are the smallest ever used to track the migration of a wild animal (related news: "Dragonfly Migration Tracked With Tiny Radio Tags").

The team tracked 14 individuals for up to 12 days with Wikelski in pursuit in a Cessna plane.

"On the ground [the radio tags] have only about 400 meters' [0.25 mile] range, and the insects are tricky to follow, because they fly really close to the trees and other vegetation," he said.

"From a plane we had signals for up to eight miles [12.8 kilometers]."

The dragonflies migrated only during daylight hours, advancing an average of 38 miles (58 kilometers) over six days.

The team says their migration behavior appeared to be very similar to that of birds.

The insects followed the same flyways used by songbirds and hawks down the Atlantic seaboard.

Dragonflies that flew off course and out to sea appeared to realize their mistake and managed to reorient themselves as birds do, the researchers add.

Wikelski says the insects may use the lay of the land as a navigation guide.

"They gather around shores and mountain ridges," he said.

And as with migrating songbirds, colder nights seemed to trigger the dragonflies' journey south, with the insects using northerly winds to help them on their way.

"It's a pretty simple trick," Wikelski said. "As it gets colder you know the wind usually comes from the north."

Again like birds, green darners appear to build up their fat reserves before setting off, Wikelski added.

Many birds use so-called stopover sites during long-distance migrations where they rest and feed. The green darners were found to break their journeys in similar fashion every three days.

The study team spotted the insects resting up in oak and juniper trees before feeding again the following morning.

"Often they stop over for a day or two, so they don't fly every day," Wikelski said. "That again is similar to the songbirds."

One-Way Ticket

Up to 50 of the world's 5,200 dragonfly species are thought to migrate, though why they do so remains a mystery, Wikelski says.

"Our new study methods may some day allow us to find out," he added.

Some scientists speculate that, like birds, migratory insects navigate using an internal magnetic compass, says Kelvin Conrad, a dragonfly researcher at Rothamsted Research in Harpenden, England.

"[Insect] researchers have all the same sorts of theories that people have for birds, such as sun navigation and using major landmarks like water and forest edges," he added.

Unlike birds, however, migration is a one-way ticket for dragonflies, Conrad says.

"It's the offspring of the generation that's gone south in the autumn that's migrating north again in spring," he said.

Wikelski and his team say tracking insects using tiny radio tags could help in understanding the movements of endangered species, or in monitoring pest insects such as locusts.

Signals from mini-radio transmitters like those used in this study "could also be picked up from space, and would thus make possible the global surveillance of small organisms if a satellite system were installed," the researchers write.

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