Every year, hundreds of animals migrate across the globe. Some, like the Arctic tern (Sterna paradisaea), make spectacular journeys traversing the globe from north to south whilst others, like the merlin (Falco columbarius), make more modest journeys between Iceland and the UK. Migration is not just confined to birds, amphibians and mammals - some insect species are also known to migrate. Indeed, insects like the monarch butterfly(Danaus plexipus) make spectacular migrations across North America,leaving sites in the southern USA and Mexico in March and reaching as far north as Canada by May. In birds, mammals and amphibians a great deal of information is available not only about their migratory routes but also about the cues that trigger the migratory movements of individuals. By contrast,relatively little is known about the cues that trigger migration and the movements of individual insects during their migrations.
Like monarch butterflies, some dragonfly species are known to migrate north between March and June and south between August and October across the North American continent, travelling long distances. In preparation for these journeys, the dragonflies may accumulate fat stores just like song birds preparing to migrate. The rules that dragonflies use to guide their migration may also be similar to those of song birds. Martin Wikelski from Princeton University and his colleagues set out to determine what rules dragonflies use to guide their migration and whether they are similar to those used by song birds. During September and October 2005 they monitored the movements of 14 adult common green darner dragonflies (Anax junius) captured in New Jersey by attaching small radio transmitters to their ventral thorax. These radio transmitters were light enough (∼300 mg) that the green darners were still able to make their migratory flights. These flights were tracked both on the ground using conventional radio telemetry and in the air from Cessna aeroplanes.
All of the dragonflies made migratory flights during the period that they were being monitored. The migratory flights occurred during the day and tended to occur on days when the wind speed was low and there had been a drop in temperature during the previous night. Surprisingly, the direction of the wind did not affect whether the dragonflies migrated and, since the dragonflies'flight direction didn't differ much from the wind direction, this often resulted in zigzagging flight paths. The average direction of the migratory flights was almost due south, approximately 186° from north, taking the dragonflies to warmer climes. The dragonflies did not migrate continuously but instead had distinct stopover days. During their migratory flights some of the dragonflies encountered the ocean, causing them to change direction and avoid prolonged flight over open water.
These rules governing the migration of green darner dragonflies are remarkably similar to the rules that govern the migratory behaviour of song birds. Song birds are known to take stopover days, to migrate in response to decreasing nocturnal temperatures, to avoid flying in high winds and to avoid extended flights over open water. The similarities between the rules governing dragonfly and songbird migration suggest that the rules are reliable and that variables, such as night-time temperature, are good indicators of changing weather patterns, which could trigger departure.