The Migration of Common Green Darners

After a hot, humid day in early September, a large swarm of common green darner dragonflies hunted for food above the barn bank. Our son, Dave, had alerted me to the phenomenon, and we stood watching as the dragonflies darted about. Dave tried to catch one in my insect net, but every time he zigged, the dragonfly zagged. We also attempted to count them but again they were too fast for us to tally.

We stood mesmerized by their aerial display—flying sideways, backwards, and forwards as they hunted. Sometimes they hovered, like helicopters. Then, they would beat all four wings together and accelerate as fast as 30 miles an hour, according to researchers. They can also stop instantly by lowering their abdomens and wings.

Added to their flying expertise is their amazing eyesight. Those bulging eyes that cover much of their heads have more than 28,000 facets so they can see above, behind, and around themselves. No wonder Dave couldn’t catch one.

Because of the heat, I was almost certain that we were watching a feeding swarm. On such days, common green darners might be pausing in their migration and waiting for a cold front from the north before pushing south. Or, on the other hand, they may have been permanent residents.

Common green darners, like some of our birds, have two populations. The permanent residents mate in the summer and lay their eggs in leaves underwater. Their nymphs hatch in about three weeks and spend their winters in ponds and other slow-moving water. By June they have hatched into dragonflies and are coursing over water in search of food.

The migrators mate in spring as soon as they return, and brand new dragonflies are ready to migrate by August. But those migratory populations alternate generations between breeding in the north and breeding in the south. Often, they migrate as individuals, but some years they migrate in huge swarms, especially along the Atlantic seaboard.

Like birds, they follow mountain ridges and the shores of the Great Lakes and ocean. Last fall—2011—was a big year for the migration of common green darners. The Internet was filled with reports of swarms in West Virginia and Ohio backyards and especially in New Jersey. One report, from the panhandle of West Virginia, occurred just a couple days after our swarm, and I wondered if it was the same one we saw. The video they made of the swarm looked exactly like the one Dave had made of ours, flashes of dragonflies moving too fast to see.

Through September 18, I observed an unusual number of common green darner dragonflies flying over First Field, and I suspected that on the 18^th, which was cool and clear, I was watching migration. On that same day raptors were also on the move south, and I sat at the top of First Field on Alan’s Bench above a tableau of gold as 38 acres of goldenrod waved in the breeze.

Dozens of monarch butterflies and common green darners flew up the field, heading south. Then, a sharp-shinned hawk flew up First Field, and over the spruce grove, followed by a merlin. Merlins and American kestrels seem to time their migration to that of common green darners because despite their excellent eyesight, the dragonflies have blind spots to their rear and below so merlins and American kestrels, and farther south, Mississippi kites, swoop beneath them from the rear, catch and eat them.

It was professional hawk counter and bird bander Frank Nicoletti, working at the Hawk Ridge Nature Reserve in Duluth, Minnesota, who first noticed that common green darners migrating down the North Shore of Lake Superior were providing ample food for migrating American kestrels later in the day. In September 1995 Nicoletti counted 1,106 kestrels and 10,330 common green darners. And observers at our own Hawk Mountain also have great views of migrating common green darners as well as American kestrels and merlins in September.

But why do some of these large, blue and green dragonflies, popularly known as “devil’s darning needles,” “mule-killers,” “snake doctors,” and “mosquito-hawks,” migrate? After all, many common green darners do perfectly well living much longer lives in the north. Researchers are still trying to answer that question.

And common green darners are not the only dragonfly migrators. Of the 5200 dragonfly species worldwide, at least 25 to 50 are migratory. In North America, with more than 300 species of dragonflies, approximately nine species in two families—the darners and skimmers—migrate including the twelve-spotted skimmer, blue dasher, wandering glider, spot-winged glider, and black saddlebags. Sometimes a few of these species, predominately black saddlebags, join a common green darner migration or feeding swarm.

While many researchers are studying bird migration, the study of dragonfly migration is in its infancy. But Martin Wikelski, a professor of ecology and evolutionary biology at Princeton University as well as a licensed pilot, along with five colleagues, decided to study whether dragonfly migration is similar to bird migration.

They glued miniature radio transmitters four-tenths of an inch long and .01 ounce in weight to the undersides of the thoraxes of 14 common green darners—seven males and seven females–and followed them during their autumn migration using receiver-equipped Cessna airplanes and ground teams. They took the dragonflies from five sites in New Jersey. All were newly-emerged adults still strengthening and developing when the urge to migrate started them on their way south.

The fourteen dragonflies took off within three minutes after the transmitters were attached. The researchers hypothesized that since the male common green darners often carry the females during copulation and also capture prey larger than themselves, they would have no trouble carrying a transmitter weighing the equivalent of a paper clip.

Although they followed them by receivers on the ground using tracking vehicles and handheld devices and by aerial surveys, it was easier to keep track of them from the air, Wikelski told reporters, “Because they move so much. From the ground it’s almost impossible to follow them.”

They found that most of the dragonflies traveled on average 38 miles a day, but one did fly 100 miles in a day. The common green darners followed the same flyways used by songbirds and raptors along the Atlantic seaboard, gathering around shores and mountain ridges. The researchers stopped surveying individual migration movements when the dragonflies moved out of their driving or flying range, approximately 84 miles from Princeton.

Like songbirds, common green darners did not migrate if surface winds were more than 15 miles an hour. Also, they flew on days when the previous night was colder than the night before that and they stayed put when the nights were warmer. Although they headed south from Princeton, when they reached the Delaware Bay at its mouth, just like songbirds do, they turned north again until they could find a narrower place to cross. They also did not seem to compensate for wind direction but flew with it as the migrating songbirds also do. Unlike songbirds, though, common green darners moving northward in the spring have little wing wear, suggesting that they are newly-emerged adults that have never flown the route before.

Probably the best book to read about common green darner migration is a lavishly illustrated children’s book by Laurence Pringle entitled A Dragon in the Sky. He says that migrating common green darners fly above the treetops and catch strong southeast breezes that enable them to glide much of the way like raptors do. Since they can eat on the wing, they can keep flying even while they catch insects and ballooning spiders. Each night they roost in trees. If it rains, they hunker down and wait for sunshine and a cold front.

Northwest winds often push them toward the Atlantic coast, hence the huge numbers seen in New Jersey. Eventually, most end up in Cape May, most notably on September 11, 1992 when an estimated 400,000 dragonflies, mostly common green darners, flew over Cape May and then turned northward again to avoid flying over Delaware Bay just as Wikelski, et. al. discovered in 2005. Instead, they flew along the eastern shore of the Delaware Bay and, in mid afternoon, turned and flew west over the bay where it was only four miles wide.

Farther along, when they reach the southern tip of the Delmarva Peninsula, Pringle writes, they follow the Chesapeake Bay Bridge Tunnel, thus avoiding the 17 miles of open water across the mouth of the bay.

Often they pause to feed in swarms over farm fields, marshes, and other open places with lots of midges, craneflies, gnats, and mosquitoes. They rest on cloudy or rainy days because they use sun compass navigation, flying according to the sun’s position in the sky with the assistance of an internal clock that makes up for the earth’s rotation and keeps them on track.

They rest by hanging under the leaves of trees or roosting close to the ground in fields or shrubs. During their migration they go from tenerals or immatures to full adults. The males’ purplish-brown abdomens gradually turn blue while the females have gray-green abdomens. Their reproductive organs are developed, and they are ready to mate in their southern retreats in Florida, Texas, or even as far south as Mexico, having taken as long as two months to get there.

Presumably, it is their offspring that head north in the spring to begin again the cycle of common green darner life, either mating and laying eggs immediately or becoming permanent residents by waiting until summer to mate.

But whatever their life style, the common green darner—Anax junius—which means “Lord of June,” is seen from April to October in Pennsylvania, flying over ponds, marshes, and pools as well as over open fields, meadows, and uplands far from the nearest still water, such as our First and Far fields.

Be sure to click on photos from Flickr to see larger, sharper versions.