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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19 thoughts on “From the Literature: Tracking Dragonfly Migrations”
WOW!! Who would have thought? ~karen
Yeah, wow! What’s next, transmitters on mites?
Maybe they should test it out on velvet mites first. Those things are HUGE! And super cute, I might add.
I remember seeing some huge migratory swarms in Cape May, NJ, over two days in September 2008. Unfortunately I didn’t note the exact date or species composition. I do remember Common Green Darners being the dominant species and that several species were involved. The dragonflies were massed in a large open field on the south side of a brackish marsh.
I think the flight coincided with a falcon migration because I remember a dozen or so kestrels and merlins feasting on the dragonfly swarm.
If I see any swarms this year, I’ll let you know.
I got on to “google” what I was seeing this evening. It is August 22, about 7:45 and I went outside to check something and saw hundreds of large dragonflies swarming around. They aren’t all going in one direction like they are passing by but just circling around I think. I’ve never seen so many in one place. I live in a neighborhood and there is no water here on my street where I’m seeing them. I hear the cicada’s, so loud, so came on to see if they were chasing them, hunting them, if they eat them? It is in the 80’s here in KS this evening and still light. I live in a suburb of Kansas City.
These are about the largest I’ve ever seen and I would think if I was going to see so many in one place they would have been over a pond or something. There isn’t a pond for about 1/2 mile. Anyway, no bug expert just curious about nature, always.
Oh, couple more things you asked for. It rained hard last night but had been really dry before that, and no rain today. I’m guessing they are 6 -7 inches long. I see them from about 5 feet off the ground and then way up, higher than the trees. They are around the trees but not diving in. I can see lots of them over the street and cul-de-sac parking area. My husband tried to get pictures but I don’t know if they turned out yet. When he gets back from the pool, I will ask if they were over there (several blocks away) also.
Hope that helps.
I’m going to have to ask my family just down the road from you in Paola if they saw any swarms today. Thanks for the report!
What fascinating research! I had no idea that there were radio transmitters small enough to allow a dragonfly (even a darner!) to be able to carry it and still act normally. The rule of thumb with transmitters on birds is that it weigh no more than 3% of the bird’s own weight in order to be “invisible” to the bird (both sensory and metabolically). Which would be pretty tiny for a darner.
Also interesting that the dragonflies behaved in a similar manner to migrating birds. For birds, they use their rest stops to build up fat reserves. Bird banders at migratory stopover sites who recapture birds multiple times over the course of their four- or five- (or longer) stay notice a steady increase in the amount of fat the bird is carrying; a bird can put on something like 10-20% of its initial body weight at arrival in fat over a few days. The fat is what fuels the next leg of its journey, and it burns through that entire reserve usually in a single night of flight. Would dragonflies operate similarly, staying put for two days to build up fat reserves that they use to fuel their flight on the third?
It’s too bad satellite transmitter technology is still very large that only the larger birds such as ducks and raptors are able to carry them. Even the little sparrows and warblers are too small to carry the smallest available. It would be neat to put a sat transmitter on a dragonfly and see where it goes and how it spends the winter. I know that there are widespread Monarch Butterfly marking schemes that allow the butterflies to be tracked and resighted along their migration routes but primarily in their overwintering site. I wonder if dragonflies could be marked and tracked as well? Perhaps using nail varnish like they use when marking honeybees for a hive. Of course, you’d have to mark quite a lot of dragonflies to ensure that at least one or two of them might be resighted, but that’s how bird banding started…
Fat is definitely playing a role in the dragonfly migrations. However, unlike birds, dragonfly fat reserve measurements suggest that they stop flying before the fat reserve is exhausted. The dragonflies feed heavily during stopovers, so they must be rebuilding their fat reserves before they fly again, but whether stopovers are made specifically to refuel or because the cue to keep moving is no longer present remains unknown.
The trouble with the transmitters that are small enough for use on dragonflies is the battery life is very short. With current technology, you get about a week and a half out of a transmitter before the battery dies and you can no longer receive signals. This makes tracking them for more than about 500 miles or so really difficult. Marking dragonflies would be great, but I think it would be extremely time intensive and would yield, at best, only a tiny amount of data. With the monarchs, they all end up in the same spot at the end, so it’s easy to mark them at the start and check for them again at the overwintering site. They’re easier to catch than dragonflies too, so they can be captured by hobbyists en route as well. We don’t know where the dragonflies end up. Could be Texas, could be southern Mexico, could be southern Central America somewhere. It’s also likely that the dragonflies aren’t all ending up in the same spot but spread themselves out across a broad area as they do during the summers in the US and Canada. I think that even if you marked several thousand darners (which is no small feat – they’re very hard to catch!) you’d be lucky to get maybe 3 or 4 back at the end – and that would only happen if you had a very large network of people ready to look for them in the areas where they’re most likely to end up. You almost surely wouldn’t get any data while they’re en route because few people try to catch dragonflies due to the difficulty involved. All in all, I think that the transmitters will end up being the most effective means of tracking the migrations, but until the battery life of the tiny transmitters is improved, we’re likely to remain in the dark about where exactly the dragonflies are going. It will happen eventually. The bird people are constantly improving transmitter technology, so it’s only a matter of time before they come up with a tiny transmitter with a longer battery life.
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I’m just back from driving our son to retrieve his truck from a large field of corn and soybeans and the air was full of dragonflies. I had never seen them in migration so came home to google for information, found this site, and thought you would like to know they are swarming this evening, 8 p.m., August 29 near Miller, KS. How interesting! I’ll go back tomorrow and see if they are still around.
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Great post and blog! I have seen a few swarms of dragonflies but not this year yet. It is so amazing to watch. I sat down in an apple orchard and watched as hundreds were zigzagging about and could hear them bumping into each others wings. These were pretty low to the ground whereas other swarms I have seen were higher in the sky over a south field. Tree swallows were having a feast that year. i will let you know if I see any this year or is it too late now? ;>)
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Left my son’s house in LaPlata, MO last night at 7:15 pm, 9-2-12. Saw what I thought might be hummingbirds or dragonflys whipping around in the town when I stopped for a stop sign (I always stop at them). When I stopped at the stop sign for the 4-lane highway, it almost looked like clouds of locusts in the distance. I went south on Route 63, and started seeing HUGE dragonflys headed north, northeast and northwest. Thought they might be swarming or coming to MO because of being driven up by the storms and rain from Tropical Storm Isaac on Friday and Saturday. Called my wife who had left an hour before me, and she had seen them too. They would come at my windshield and divert up at the last moment and cause me to blink. Only a couple of them glanced off of the windshield, and they avoided getting smashed even so. They seemed to be moving very fast. Saw pockets of them all the way to my destination in Macon, MO, 20 miles south of where I started from. I am 57 and have never seen anything like that from dragonflys before.
Sounds like you saw a large migratory swarm! It’s quite possible that you were seeing the result of Isaac as you suggested. The dragonflies in Texas were headed north several days before you saw your swarm, so maybe you got to see them as they traveled?