Dragonfly Swarms: Static Feeding Swarms

At long last, I am finally getting to the information about static swarms in dragonflies!  Apparently posting once a week is a bit ambitious for my current schedule and even my weekend became one giant black hole of work.  I’ve had no time to blog!  Better cram this post in quick while I have a few free minutes…  :)

Before I jump into what’s known to science about static dragonfly swarms, take a moment to ponder the amazing swarm depicted in this image I found online:

Isn’t that glorious?!  I can’t be sure that this is a real photo, but based on what others have reported and what I’ve observed myself, this is certainly not outside the realm of possibilities!  The people in the photo aren’t enjoying the experience, even though the dragonflies in the swarm were completely harmless.  I wish I were in their place!  I would run out and stand in the middle of the swarm, soaking up the sound of the dragonfly wings fluttering against one another and the sight of several thousands of my favorite animals flying in a single location.  This is what my heaven looks like!  (And yes, you read that right: I did just say that my heaven involved massive numbers of flying insects.  What can I say?  I’m an entomologist!)

Now for the science!  If you’ve read any of my swarming series, you know that dragonflies migrate, sometimes in mass migratory swarms, and that migrating dragonflies have patterns of behavior similar to migrating birds.  I have also speculated, based on my own observations and those of people who reported swarms to me early this summer, that the non-migratory swarms (what I call static swarms) are feeding swarms.  Today, I’m going to go over what’s known about these swarms.  Unfortunately, most of this information is either buried in the scientific literature where it’s largely inaccessible to the public or in a $100 dragonfly book that, while truly brilliant, is also largely inaccessible.  And let’s face it.  Apart from college libraries, only odonatologists (scientists who study dragonflies) and other entomologists are going to shell out that kind of cash for a dragonfly book.  For those of you who don’t LOVE DRAGONFLIES SO MUCH that you’re willing to run out and fork over 100 bucks for a book (i.e. you’re normal), I’ll summarize what is known about static swarms here!

First of all, the static swarms are almost always feeding swarms!  I’ll go over the reasons why you might see these feeding swarms in a particular area in a moment, but first a few interesting facts.  First, this behavior appears to be exclusively anisopteran.  This means that the behavior is observed in dragonflies only, not their close relatives the damselflies.  This is probably because dragonflies exhibit vastly superior flight compared to damselflies. However, among dragonflies, many species of both perchers and fliers will take part in the swarms and fly for extended periods of time.  Both males and females swarm, though males are more commonly observed.  Swarms can be made up of several different species, and can even include other organisms, such as the vertebrate birds and bats.  If birds or bats are present, they will usually be found just above the dragonflies, feeding on the same insects the dragonflies are eating rather than on the dragonflies themselves.  And for those of you who are shocked at the number of dragonflies in the swarms you’ve see, you likely only saw the tip of the iceberg!  In one of the only studies looking at the number of individuals swarming within a population, only 16% of the dragonflies in the area participated in the swarm.   Just think: if you see 1000 dragonflies in a static swarm, that means that there were likely 6250 total dragonflies nearby!

You’ll most often see dragonfly swarms near dusk or dawn, and it’s thought that the dragonflies can see flying prey (most often mosquitoes or non-biting midges, but also termites, ants, and honey bees)  better when the sun is close to the horizon.  During these times, you may notices that dragonflies appear very suddenly, fly in circular patterns over a very specific area for some time, and then disappear as quickly as they arrived.  This is because the dragonflies are attracted to large groups of prey organisms.  Once the prey numbers drop or they become less active (e.g. as it get darker), the dragonflies move on.  If the prey return the next day, the dragonflies likely will too.  In some particularly productive areas where prey are consistently available, you might even see a swarm form nearly every day for months.

There are several reasons why dragonflies might congregate in one area versus a similar area nearby, but in nearly every case there is an abundance of prey species present in the area containing the swarm.  Dragonfly swarms will form where other insects are swarming.  Most people have seen a big swarm of gnats at one point.  Those swarms are like a dragonfly buffet!  The dragonflies will swoop in and out of the fly swarm, picking off flies and eating them on the wing before going back for more.  This is likely what happened in August when the dragonflies descended on Milwaukee.  Massive numbers of mosquitoes in the area drew dragonflies into the city and large swarms formed where the mosquitoes were congregating.  Dragonfly swarms often form when there is a seasonal emergence of ants or termites as well.  This was the case in the swarm I witnessed.  The ants and termites were emerging out of the grass and the dragonflies were catching and eating them as they emerged.

Dragonflies might also be attracted to objects that attract other insects.  If prey insects are consistently attracted to a particular object, dragonflies can learn to associate that object with a good meal.  In one study, dragonflies were found over a set of traps intended to attract other insects, feasting on the insects flying in toward the trap.  The prey insects eventually stopped coming to the traps, but the dragonflies returned for several more days.  In this case, the dragonflies were attracted to the traps and not the insects themselves because they’d learned to associated the traps with an abundance of prey.

Dragonflies might also be observed swarming in areas where a weather front has just passed through.  Insects often get trapped in fronts and are pushed along with the winds for some time before being deposited somewhere else.  When they finally free themselves from the front, they might find the dragonflies ready!  Dragonflies take advantage of these windfalls of prey by forming swarms and eating the insects as they arrive.  This sort of feeding also happens during migratory flights.  The same fronts that deposit large numbers of prey insects in an area help the dragonflies fly long distances, so prey is readily available when the dragonflies stop to rest and feed.

In wooded areas, many insects will congregate in sunny, open patches.  Lots of dragonfly swarms form in small sunny patches to take advantage of the other insects that are attracted to the spaces.  The swarms of prey insects will move as the sun changes position, os the dragonflies will move too.

In high winds, insects will congregate in lee areas (areas protected from the wind).  Lees promote dragonfly swarming behavior because of the high abundance of insects found in these areas.

Finally, and I think most remarkably, some dragonflies will swarm in areas where insects are being stirred up due to some sort of disturbance.  Mowing your lawn?  It disturbs the small insects living in your grass and cause them to fly around more than they usually would.  The prey draws the dragonflies in and swarms form.  Similarly, some dragonflies have learned to follow large, slow moving objects (these could be people, bicycles, cows, cars, etc) because they disturb prey insects as they move and encourage the prey to fly – often into the eager grip of a hungry dragonfly.

So all of this boils down to one simple concept: any time you have an abundance of dragonflies in an area as well as a significant prey population, you are likely to see dragonfly swarms.  The behavior is thus fairly common in many different species of dragonflies.  That said, the chances of a single person seeing more than one or two swarms in their lifetime in a single area can be quite low.  The conditions have to be just right for swarms to occur, perfect for both a large number of prey insects AND a large number of dragonflies to exist in the same area at the same time.

Next time (and I’ll get the post up much more quickly this time), I’m going to discuss some of the references available for identifying dragonflies, both in print and online.  In the meantime, keep sending me swarm reports!  I am beyond thrilled with the participation in my swarm project, so keep ’em coming!

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Have you seen a dragonfly swarm?

I am tracking swarms so I can learn more about this interesting behavior.  If you see one, I’d love to hear from you!  Please visit my Report a Dragonfly Swarm page to fill out the official report form.  It only takes a few minutes!

Thanks!

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Want more information?

Visit my dragonfly swarm information page for my entire collection of posts about dragonfly swarms!

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