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  BBC 10 May 06
 Tiny tags trace dragonfly paths
National Geographic 10 May 06
Dragonflies Migrate Like Birds, Study Says
James Owen for National Geographic News

Dragonflies are long-distance fliers that travel similarly to migrating birds, a new study shows.

Researchers who followed radio-tagged dragonflies by plane say the insects' journey shows "astounding similarities" to that of migratory songbirds.

Dragonflies that migrate appear to build up fat reserves, wait for favorable winds, take rest breaks, and reorient themselves when they lose their way, according to the study.

The insects also followed the same flyways as migrating birds, with one dragonfly traveling 100 miles (160 kilometers) in a day.

Martin Wikelski, a professor of ecology and evolutionary biology at Princeton University in New Jersey, led the study. Wikelski's team investigated the migration of green darner dragonflies as part of research funded by the National Geographic Society Committee for Research and Exploration.

Green darners are one of nine long-distance migratory dragonflies in North America. They fly each summer to the northern U.S. and Canada, and their offspring return south in the fall. While green darners have been recorded as far south as the Gulf of Mexico, Wikelski says, it isn't known where they ultimately end up. "We don't know if they go all the way to Florida or continue to Venezuela," he said. "We are unbelievably ignorant about [their] migratory phenomenon."

Mini Transmitters

The team's research, published today in the journal Biology Letters, focused on green darners as they migrated south from Cape May Point in New Jersey.

The dragonflies were fitted with tiny radio tags that were glued to the insects' undersides. 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.

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.

BBC 10 May 06
 Tiny tags trace dragonfly paths
The epic journeys taken by dragonflies searching for warmer climates have been revealed by scientists in the US.

The team, led by researchers from Princeton University, found that the insects are capable of flying up to 85 miles (137 km) in a day. Writing in the journal Biology Letters, the group describes how it tracked the movements by attaching tiny radio transmitters to the insects.

A scientific posse followed the signals from a receiving aeroplane. Other researchers monitored the insects' progress from the ground. Tricks of the trade The dragonflies' route took them along the east coast of America towards the warmer south.

The data revealed that the dragonflies' migration patterns are strikingly similar to those of songbirds, suggesting there is a strong evolutionary link to their behaviours.

"Insects have been around far longer than birds, therefore we suspect that they have been migrating far longer than birds," said Professor David Wilcove of Princeton University and one of the authors of the paper.

"It is just possible what we are seeing here are the basic primitive rules of migration and that birds converged on the tricks of the trade," he told Science In Action on the BBC World Service.

Billions of common green darner dragonflies ( Anax junius ) migrate every year but until now hardly anything was known about their routes or strategy.

The new research shows there is method behind the insects' flight behaviour. For example, the dragonflies tended to only move after two nights of cooler temperatures, indicating a cold front was approaching with favourable north-westerly winds.

They also tended to change direction when they met a large body of water, ensuring they never became stranded out over the sea. Two insects were recorded flying out over the ocean before reversing and skirting down the coastline.

The detailed flight path information was made possible by tiny radio transmitters developed by the team.

Final destination

Each transmitter weighed about a third of a gram and had enough battery life to track an individual for 10 days; but tagging such small creatures is far from easy.

"The challenge is first catching the dragonfly," said Professor Wilcove. Once caught, each transmitter was attached with a couple of drops of superglue and some eye-lash adhesive.

However, the final destination of the green darners is still not known because of the power restraints of the transmitter set-up; and some individuals simply flew out of range of the scientists.

In the future, the team hopes to refine its techniques and make the tracking even more hi-tech. "The dream scenario would be to get a satellite to pick up the signals from these transmitters," said Professor Wilcove. "If you had a satellite like that you could [follow the migration of] all kinds of birds, dragonflies, and locusts; and, I think, it would shed tremendous light on the movements of these organisms."
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