From the Literature: Mass Migrations in Dragonflies

dragonfly swarm

Dragonfly swarming behavior. Photo copyright Steven Young and taken from

Wow!  I’m running way behind on getting a new post up.  I’ve been getting a ton of dragonfly swarm reports in the last week, so I’ve been scrambling around trying to keep up with that.  Couple that with some writer’s block and the start of my field season, and Ms. Dragonfly’s life has been a bit hectic!  I’ll try to get something up a bit more quickly next time…

I’m going to do a short series on dragonfly swarming behavior since this seems to be a topic that a lot of people are interested in this summer – and one that I’ve become rather obsessed with over the last few months.  (Seriously – ask my husband, my Ph.D. advisor, or my coworkers how many times I’ve brought up dragonfly swarms in conversation over the last few weeks!)  After combing through the literature, I’ve discovered some great information on dragonfly swarming and I think I have some good explanations for why we see these swarms in nature.  Today’s post will cover a paper that came out in 1998 about the mass migratory swarms observed in dragonflies.  For those of you out there who have seen hundreds of thousands or millions of dragonflies all flying together in a single direction, a sort of river of dragonflies, this paper describes and attempts to explain this behavior.  And what an amazing behavior it is!  I would dearly love to see one of these giant migratory swarms.  If any of you happen to get this behavior on video, I strongly encourage you to upload it to YouTube so that others and I can see it!  But let’s get to that paper.

Over the years, several dragonfly researchers have witnessed massive swarms of dragonflies flying together in what appeared to be migratory swarms.  Each time, they consisted of several hundreds of thousands or millions of dragonflies, enough to nearly blacken the sky and make themselves really obvious – if you happened to be looking up at the right time to see them!  In 1998, Robert Russell, Michael May, Kenneth Soltesz, and John  Fitzpatrick combined their eyewitness accounts of these migratory swarms and wrote one of the first papers describing this behavior in detail.  At the time of the paper’s release, very little was known about dragonfly migratory patterns.  25-50 species of all dragonflies were thought to be migratory and only 18 migratory species had been reported in the eastern United States.  In their paper, the authors set out to describe the massive swarm migrations they’d observed in dragonflies, but they also wanted to review the literature concerning migrations of dragonflies to see if any patterns emerged.  They were hoping to determine why these swarms formed and understand how they worked.

Anax junius adult

Green Darner (Anax junius) male – notice the large wings and huge eyes!

First the authors described three swarm migrations in the United States that three of the four authors had witnessed themselves: one swarm each in Chicago, Illinois; Cape May, New Jersey; and Crescent Beach, Florida.  In each place, the observers reported the same things:

  1. Nearly all of the dragonflies in each swarm were moving together in the same direction, though not all swarms moved the same direction.
  2. The swarms were largely made up of green darners (Anax junius – pictured to the left above), though several other species were present in the Cape May swarm to a much lesser extent (black saddlebags, twelve-spotted skimmers, swamp darners, Carolina saddlebags, wandering gliders, spot-winged gliders, and blue dashers).
  3. The swarms were very large, at least 200,000 dragonflies (Florida) and up to an estimated 1.2 million in the Chicago swarm!  (They compared the dragonfly swarms they observed to those observed in locusts and found that the number of swarming dragonflies was comparable to swarming locusts.  If you’ve heard about locust plagues, you have a good idea of what one of these swarms should look like!)
  4. The swarms tended to be very compact, that is they flew within a rather confined corridor rather than spreading far apart either horizontally or vertically.
  5. The swarms tended to follow very obvious landmarks such as shorelines, ridges, and coasts.
  6. All of the swarms took place soon after a cold front passed through the area, so they appeared to be distinctly weather-related.

The authors also reviewed the literature and collected field observations from other researchers to better determine how, where, and when these massive swarms were taking place.  They gathered 41 reports spanning the years 1881 through 1995 and found that these swarms generally shared the same characteristics as the swarms witnessed by the authors.  They further discovered that the swarms took place between July 30 and October 13, though the majority (28 swarms) occurred in September.  This was clearly a highly seasonal event.  All of these reports were also made in locations in the eastern United States, so it appeared that these swarms only occurred in half of the U.S.

Based on all of the reports together, the authors made some speculations about the massive migratory swarming behavior observed in several species of dragonflies.  First, their data suggest that at least some of the dragonfly species are migratory and are using these swarms to travel from north to south during the fall.  Where exactly they’re going is much less certain, but there are some reports of dragonflies crossing the Gulf of Mexico and massive swarms suddenly appearing in locations in Mexico.  The final destination could be as far south as Central America or as far north as the Gulf coast.  Regardless of where they end up, the dragonflies appear to be flying south for the winter, much like birds and monarchs do.

Second, the authors suggest that this type of swarming behavior might be unique to the United States and the regions where the dragonflies overwinter.  There are similar swarms in Europe, but the timing and recurrence of the swarms are very different than the yearly migrations observed in the US.  This also led the authors to suggest that the wandering glider, the only dragonfly species naturally found on all continents, might not form these migratory swarms.  While they have been observed in American migratory swarms, they typically only make up a small percentage of the total individuals within the swarm and are very uncommon in swarms reported in other locations.

Third, the authors proposed that weather may be the most important factor regulating the mass migrations of dragonflies.  Indeed, in nearly all of the accounts they collected, a cold front had passed through the area just prior to the appearance of the swarm.  A combination of season (autumn), north-westerly winds, and cold fronts may be necessary to entice dragonflies to fly from the cooler northern regions to warmer southern climates for the winter.  The timing of this behavior also suggests that these dragonflies are relying on aerial plankton for food as they migrate.  Aerial plankton is abundant during the fall, allowing the dragonflies to feed along the way.

Fourth, landmarks appear to be very important in directing dragonfly migration paths.  Mass migrations are almost always reported along major waterway, coasts, or prominent land features.  These “leading lines” help dragonflies orient themselves and follow the correct path south.  However, they also seem to reorient themselves under certain conditions, especially when they have to cross significant stretches of water.  This will be the subject of an upcoming post, so I won’t go into more detail here.

And last, there is some evidence that the dragonflies that fly south for the winter are not the same individuals that return north.  Dragonfly adults typically have a life span far shorter than the length of time the dragonflies overwinter, so it is very likely that migratory dragonflies fly south, mate, produce young, and die.  The offspring then make the return trip north in the spring.  There is a lot of uncertainty about what happens in the spring, however.  The route the dragonflies take on the return trip remains mysterious.  The migration north also does not appear to involve huge migratory swarms and may be comparatively inconspicuous, making it difficult to determine when and where the northward bound dragonflies are flying.

The authors ended their paper with a long list of questions that remain unknown about dragonfly migration.  12 years later, we still don’t know most of the answers to these questions!  For example, why exactly are these dragonflies forming these massive swarms when they migrate?  If it were only to protect the dragonflies as they fly (safety in numbers), then why do so few dragonfly species exhibit this behavior?  It doesn’t appear to be solely related to timing of adult emergence from the water either as emergence is not synchronous in the green darners and they are the most commonly observed migratory species.  Still, this paper went a long way toward explaining mass migrations in dragonflies.  Due to the ephemeral nature of swarms – a million dragonflies might fly over an area in a matter of minutes – we’re lucky there have been any studies of this behavior at all!

Over the next few posts, I’ll go over another paper about dragonfly migrations (one using radio transmitters!) and talk about what I’ve discovered in the literature about what I’m calling static swarms.  With these three posts, I hope to summarize most of what’s known about dragonfly swarming behaviors so that the information is available outside of the scientific community in an accessible way.  I am also going to post photos of all of the known migratory dragonfly species in the US.  Many people reporting dragonfly swarms to me have expressed an interest in identifying the dragonflies in their swarm, so I want to make that possible.  And, as always, keep sending me dragonfly swarm reports!  I have a huge collection of them going this summer and I couldn’t be happier with the level of participation in my project.

Literature Cited:

Russell, R., May, M., Soltesz, K., & Fitzpatrick, J. (1998). Massive Swarm Migrations of Dragonflies (Odonata) in Eastern North America The American Midland Naturalist, 140 (2), 325-342 DOI: 10.1674/0003-0031(1998)140[0325:MSMODO]2.0.CO;2


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