It’s about the time of year for the dragonflies to start moving south! I’ve already gotten several reports of big migratory swarms headed south from several locations across the eastern and midwestern U.S. and I expect many more – the season has just begun!
A few posts back, I discussed a paper that described mass migratory swarms in dragonflies. While the authors presented several unanswered questions and got the ball rolling on understanding how and why these swarms form, there has still been surprisingly little done in this field. As I suggested in my last post, this might have to do with the ephemeral nature of these swarms. The vast majority of swarm observations are “in the right place at the right time” sorts of observations and it’s extremely difficult to predict exactly when and where a swarm will form and/or travel. Depending on their location, any given dragonfly researcher might only see one or two mass migratory storms in his or her whole life! This is clearly a very difficult topic to study, and most accounts of swarms have been buried in the scientific literature. That means that there is very little information about dragonfly swarms freely available to the public. I think this is a sad state of affairs, thus today I’m covering another scientific paper on dragonfly migrations. This one is really fun!
(Okay, okay – I think it’s fun, but I’m also a huge bug geek… You can form your own opinion!)
In 2006, one group of researchers decided to answer one of the big unknowns: where do these migrating dragonflies go? The group, headed by Martin Wikelski of the Department of Ecology and Evolutionary Biology at Princeton, had noticed that though many insect species had been documented migrating, the ultimate destination and migration strategies of many of those species remained unknown. So, they decided to track swarming dragonflies. How did they do this, one might ask? With radio transmitters of course! Check out the photo to the left. That’s a green darner (Anax junius), the most common swarming dragonfly, with its radio transmitter attached. The researchers captured 14 darners in New Jersey between September and October, glued the tiny bug-sized transmitters onto their thoraxes, and released them. Then they tracked the dragonflies with radio receivers either by car or by Cessna plane for up to the 10 days of the transmitter’s life. In essence, this qualifies as one of the most awesome research projects ever! (Pardon me while I drool thinking about how amazing it would have been to track dragonflies from a plane in this study…)
Using this design, the researchers determined how far a dragonfly flew on any given day, how long it rested between flights, and the exact path it took during its migration. They then put all of their data together to determine how similar dragonfly migrations are to bird migrations and what rules dragonflies follow when making migratory decisions.
So what, then, did they learn? First, the dragonflies all migrated within within 4 days of receiving their radio transmitter, so they were still inclined to migrate even with the transmitter in place. They also learned that the dragonflies tended to move approximately once every three days. This means that the dragonflies flew one day, rested for two days, then flew again. Long stopovers were apparently necessary during the migrations.
What about the distance and direction traveled? The team found that there were three types of daily movements. Some dragonflies flew a short distance (1-4 km) and in all sorts of different directions. Others flew 8-12 km in a single direction. Still others flew 25-150 km (that’s just over 93 miles) in one day! Clearly these dragonflies were capable of flying long distances under certain conditions, though the average daily flight distance when all flights were combined was only 58 km (36 miles). As for the direction, some dragonflies flew west and some flew east at times, but the bulk of the movement was southwest.
Weather seemed to be important for determining when the dragonflies flew and when they did not. They were much more likely to fly in mild winds than in stronger winds, and no dragonflies flew if the wind speed was greater than 25 km/h (that’s just over 15 mph). They also tended to fly more on days when the wind was blowing from the north than on days when the wind blew in other directions. The dragonflies apparently depended on the wind to help them travel because the direction of the dragonflies and the direction of the wind on days where they flew were nearly identical. Curiously, there was no association of temperature and propensity to migrate on flight days: there was no difference in the daily high temperatures of flying days versus non-flying days. However, all migratory flights took place after a night with a temperature cooler than the previous night.
These data suggest that dragonflies have a set of simple rules they follow when deciding whether to migrate or not. The dragonflies move with the winds (but not in very strong winds) in response to cool night and take a few days off between flying days, presumably to hunt and/or rest. This in and of itself is pretty interesting, but it’s also interesting to place this information in the larger context of flying animal migrations. Nearly everyone is familiar with the annual migrations of birds and know that birds “fly south for the winter.” The data the dragonfly team collected revealed that the migratory patterns of dragonflies are remarkably similar to those of birds. Songbirds use the same sorts of weather cues to prompt their migrations, follow coastlines and other prominent landscape features in the direction of the wind, and make frequent stopovers, just like the dragonflies did. In essence, birds share the same set of rules governing migration that dragonflies exhibit. It is likely that other migratory flying animals follow the same rules.
The team finished their discussion of the dragonfly behavior by using their data to calculate the maximum migration distance these dragonflies might be expected to fly. Assuming a modest flight speed and a two month migration season, an individual dragonfly could be expected to fly 700 km, or 435 miles! This is a long way for what is ultimately a small animal to fly. Unfortunately, due to the limitations of the transmitters used (i.e. the battery life of 10 days), the team was never able to figure out exactly where the dragonflies ended up. If the dragonflies are traveling 435 miles, I’ve calculated that dragonflies starting off in New Jersey most likely end up in West Virginia or Virginia. This is much further north than previously suspected, which leads to at least two possible explanations for sightings of mass migratory swarms reported further south. 1) The dragonflies might fly faster than estimated, which would allow them to travel further during the 2 month migration season. Or 2) the dragonflies observed in locations such as Florida and further south might be starting off from a more southern location to begin with. Yet two more questions to be answered about this behavior! It may be possible to answer these questions using the techniques the dragonfly team developed. I suspect radio transmitters will play a significant role in answering some of the many outstanding questions about migratory behaviors in dragonflies.
Next time I’m going to post images and descriptions of the most common migratory dragonfly species so that people observing dragonfly swarms can determine which species they’re seeing. In the meantime, I hope all your easterners enjoy watching the dragonflies that are on the move in your part of the country! Based on the dragonfly activity in the north this year, it could be downright spectacular.
Wikelski M, Moskowitz D, Adelman JS, Cochran J, Wilcove DS, & May ML (2006). Simple rules guide dragonfly migration. Biology letters, 2 (3), 325-9 PMID: 17148394
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