This species, as other darters, is known to migrate regularly. David and Elizabeth Lack reported their pass south in the Pyrenees together with migratory birds, butterflies and hoverflies in October 1950 and reviewed the information available on dragonfly migration until then. Today, although the morning was warm with sunny spells, a large front with thunderstorms passed through all the afternoon, so it could be that the dragonflies were moving away from the coming rain.
Common Darters are known to form occasional irruptive, massive migratory swarms, although the reason for these are not well known. A large irruptive swarm of this species was reported in Ireland in August and September 1947 by Cynthia Longfield from the British Museum. She compiled the letters of many observers amazed by the sudden, unprecedented invasion, including interviews with schoolchildren, which helped document the spatial extent of the invasion. As, she was visiting Ireland at the time, she identified the dragonflies and further documented the invasion with her observations. She concluded that this mass invasion event probably came from Spain and Portugal, as no invasion was reported in France or England at the time. She calculated that the dragonflies must have flown over the sea overnight. It is hard to imagine what being there at such massive influx event would have been:
'They were flying low in compact formation. . . . The flight was orderly, in a northern direction, as if they were intent on reaching some known destination.'
'They seemed more to be drifting with the breeze than flying, but were keeping to a straight course, without zigzagging or undulating, coming in from-the sea to the south and going north. What was so remarkable was the witnessing of the actual arrival and passing of the swarm.'
'They seemed to be coming in from the sea and flying about two feet above the ground, though some were higher, to about eight feet up. All were flying in the same direction, northwards. It seemed a never-ending stream, but we had to move on, and passed out of their track, so I could not tell the duration of the flight. They flew considerably faster than we were walking. The density was about one foot, or rather less, apart.'
For the next day, 3rd September, Mr. Allen writes: 'Tuesday's invasion was nothing to Wednesday. Wind south-west. Before noon they commenced and continued until evening. They not only came singly, drifting as on Tuesday, but in great, thick black masses, like dark clouds. They really frightened the people.'
Here he found 'the people looking out to sea, and I saw a sight which gave me a fright at first. I heard a hum like bees and yet not quite like bees. The next thing the whole sky filled with large flies that resembled aeroplanes, millions of them'Cynthia hypothesizes that the reason these dragonflies were emigrating was the drying up of the marshes where they regularly bred. That these dragonflies somehow were prevented from completing their reproductive cycle in the region of origin is illustrated by the fact that many of them paired on arrival, and some were even observed laying eggs on the sea. Although these spectacular irruptions appear to be extremely rare occurrences, they illustrate that the more regular, seasonal migratory behaviour of some dragonflies, can possibly be exacerbated by unusual weather conditions in their region of origin - in a similar way that occurs with Painted Lady irruption years.
More informationCynthia Longfield (1948). A vast immigration of dragonflies into the South coast of Co. Cork The Irish Naturalists' Journal, 9 (6).
David and Elizabeth Lack (1951). Migration of Insects and Birds Through a Pyrenean Pass. Journal of Animal Ecology, 20, 63-67 DOI: 10.2307/1644