“Hundreds of huge dragonflies swarming the parking lot and grassy area. I just sat in the car and watched them fly! They were amazing! When Ann got there about 20 minutes later they were still going strong. We watched for at least half an hour and they finally started to dwindle in numbers. Just flying round and round, zigging and sagging, bobbing and weaving – and me without a camera! Ann tried but they were flying so fast and not one of them landed.”
Such is the preliminary report I recently received from Sally Engle, who observed this event on Wednesday evening, August 18, 2010 at the parking lot for Cedar Bog Nature Preserve near Urbana, Ohio. We shared several more e-mails and a phone call to discuss this phenomenon. Sally said there were clearly two different species and perhaps a third or fourth. While not a “dragonflier” (yet), Sally is a pretty keen observer of the natural world. We jointly concluded that her descriptions of the insects were (in order of numbers): Common Green Darner (Anax junius) and Wandering Glider (Pantala flavescens). Sally thought the third might be some very late Swamp Darners (Epiaeschna heros), with no clear idea on the possible fourth, but both of these identifications are much more tentative. Following is some more of her observations.
“It was clear to partly cloudy, very warm and relatively high humidity. I arrived at about 7:15 PM. They thinned out around 8:30. The masses were moving over the grassy area north of the building, the parking lot and north “pacman” area and the old field area north of the parking lot. They continuously circled the area, some flying low enough to buzz my head and the car and others staying high.”
“One thing that I noticed that I found interesting. There were some birds – and I wish I was better at species but they were small and had reddish orange breast coloring, too high to really see – that were circling the dragonflies. Several times a bird would approach as if to try snatching one up, but seemed to be intimidated by the sheer mass of dragonflies. It would veer off and continue to make a wide circle around the dragonflies. One did finally get brave and try and two dragonflies seemed to go after the bird – the bird flew off. Is that normal?”
“They seemed to just fade away, but did not fly off into the distance. I was wondering if they all decided to land somewhere in the tall plants north of the parking lot. I plan on going back out tonight – with camera – to see if by chance they are there again.”
“I admit, I was curious about the feeding and Ann and I beat the plants to stir up the insects when the masses seemed to be thinning. Suddenly the sky was filled with dragonflies again. We did that again when the swarm seemed to thin and once again it worked. I think that kind of proved the feeding theory. LOL Ann had her camera and tried to get pictures but they were just too fast! She got a couple but I haven’t heard yet if they came out well enough to identify. Oh for a really good camera!!! Or a net!!! I should have gone in the building and gotten ours to catch and release one! It ended up I had plenty of time but I didn’t know it then.”
Feeding Swarms and Migrating Dragonflies
The event Sally observed is a feeding swarm. These are characterized by “Just flying round and round, zigging and sagging, bobbing and weaving” which they do as they try to snatch up swarms of midges or other tiny flies or insects that they are feeding on, which are often too small for us to see. Mike May of Rutgers University (and head of the former Migration Taskforce) confirmed my belief that many such feeding swarms in late August and September are part of less frequently seen migration swarms. In migration, you see the hundreds, thousands, or even greater masses flying in a single direction – most frequently in front of a storm front. The direction need not necessarily be south; even though that is the general direction they are headed. Especially in evening, they may be heading for a roosting site for the night.
“Watch the Skies!”
Keep your eyes open, and if you see either feeding or migrating swarms, let me know asap. Record as much information as you can about species, weather, speed, direction and density of the flock. If possible, either collect sample individuals or get some photos. Mid August through September are the prime times for these mass assemblages.
We have had a handful of migratory swarms reported in Ohio that I am aware of during the last two decades. In 1993 I gathered seven reports of large feeding swarms plus three reports of directional, migratory swarms. Probably the most exciting of these migratory swarms came from Mike Greene in Summit County. On August 29th of that year Mike observed a swarm for twenty minutes, like “a flock flying in formation.” Mike and I pieced together data from his observation regarding the density of individuals, speed of flight as well as the width and height of the “flock” and from this made a conservative estimate of between 100,000 to 1,000,000 individual Common Green Darners. Wow!
Starting in 2005, a team of researchers from Princeton and Rutgers University started experimenting with gluing very small radio transmitters on Green Darners and several other large dragonfly species. The transmitters weighed only 0.3 grams, and the Green Darners 1.2 grams. The transmitters were 25% the weight of the dragonflies – but observations of feeding after they were released suggested they handled this weight well. The team put transmitters on 14 green darners and tracked them for ten days (estimated maximum battery life 12 days). The dragonflies followed an approximate pattern of resting/feeding for two days and migrating on the third day. Migratory flights averaged 58 km (35 miles), but one individual in one day flew 150 km (90 miles). All flights took place in daylight, and on two occasions dragonflies started to cross the Delaware Bay, flew out 5 km, then turned around and skirted the shore until they could see land across the bay.
In contrast, a study on some off shore islands near China observed migration in the Wandering Glider. Here an undescribed “searchlight trap” captured 42,161 Wandering Gliders in one night. These migrated at night instead of during the day, so different species may migrate with different patterns. Many songbirds migrate at night and rest and feed during the day. Since these dragonflies were island hopping, they tracked them migrating 150 to 400 km (90 to 240 miles) in a single flight. Of course, the genus Pantala is well known for it prodigious, floating, effortless flight.
Just last year a researcher documented migratory flights of mixed species (but 98% were Wandering Gliders, P. flavescens) that moved with the monsoon rains from southern India, to the Maldives Islands, to southern Africa – and likely back again, “a round trip of 14,000 to 18,000 km” (8,750 to 11,250 miles). Pantala breeds in rain pools and can mature from egg to adult in less than two months – so these were not necessarily the same individuals making the round trip. Some of the individual flights over ocean were 800 km (480 miles), to 2700 km (1,620 miles), to a maximum of 3,800 km (2,280 miles).
In North America, at least 12 or 14 species of dragonflies appear to be somewhat involved in migratory flights. Where they go, how they navigate and the overall biology of their epic journey remains mostly a mystery. Unfortunately, Mike May tells me the radio transmitter experiment has ended as their key member returned to his former home in Germany. Perhaps they can re-group and continue the work. Perhaps you will someday be able to record significant observations that might help us understand this phenomenon better. As the science fiction writers of the 1950s would say, “Keep your eyes on the skies.”