The Migration of Common Green Darners

Common green darner by Ken Slade (TexasEagle on Flickr)

Common green darner by Ken Slade (TexasEagle on Flickr) — Creative Commons BY-NC licence

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.

green darner close-up by Linda Rae Duchaine

green darner close-up by Linda Rae Duchaine (Beadmobile on Flickr) — CC BY-NC-ND

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.

common green darners depositing eggs by Ken Slade

common green darners depositing eggs by Ken Slade (TexasEagle) — CC BY-NC

Through September 18, I observed an unusual number of common green darner dragonflies flying over First Field, and I suspected that on the 18th, 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.

green darner in flight by Jim McCulloch

green darner in flight by Jim McCulloch on Flickr — CC BY-NC

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.

mating pair of green darners by David Hofmann

mating pair of green darners by David Hofmann (davidhofmann08 on Flickr) — CC BY-NC-ND

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.

cover of A Dragon in the Sky by Laurence PringleProbably 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.

common green darner resting by Ken Slade

common green darner resting by Ken Slade (TexasEagle) — CC BY-NC

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.

Dragonflying

“Dragonflying is good for jaded birdwatchers. It presents new challenges,” Cynthia Berger told me as we watched darting dragonflies at Whipple Dam State Park one sunny day in late July. Berger is the author of Dragonflies, an excellent new book designed for beginning dragonfly-watchers.

These “glittering aerial acrobats,” Berger writes in her book, are similar to birds in several ways. Like birds, dragonflies are strong flyers and have distinctive and often colorful bodies. Many defend territory, guard mates, and are excellent aerial predators. The female and male of a species are frequently different in appearance, as different as male and female black-throated blue warblers, rose-breasted grosbeaks, or indigo buntings, and, like those birds, the males are the flashier. Several dragonfly species migrate south in the fall and north in the spring. They also can be watched through binoculars, although close-focusing binoculars, designed for both butterfly- and dragonfly-watching, are better than birdwatching binoculars as I discovered when I used Berger’s on our Whipple Dam outing. Finally, dragonfly-watchers, like butterfly-watchers and birdwatchers, can make original discoveries about species’ distribution and behavior.

Dragonfly-watching has really taken off in this century with the publication of two user-friendly, dragonfly field guides: Sidney Dunkle’s Dragonflies Through Binoculars (2000) and Beginner’s Guide to Dragonflies by Nikula, Sones, Donald and Lillian Stokes (2002).

Dunkle is an odonatist or odonatologist–someone who studies dragonflies–and earlier wrote two excellent guides to the dragonflies and damselflies of Florida. Back in 1978, Dunkle, along with Dennis R. Paulson and under the auspices of the Dragonfly Society of the Americas, dreamed up most of the common English names for all 425 North American dragonflies and damselflies. This made dragonfly-watching more accessible to nonscientists. Many names are site specific such as swamp darner, Ozark clubtail, and Pacific spiketail. Others refer to the dragonfly’s color–roseate skimmer, azure darner, coppery emerald, cinnamon shadowdragon. Then there is my own personal favorite, the species that sent me to the books, the comet darner.

Living on a dry mountaintop, as I do, I wondered why I often saw dragonflies coursing above the grasses of our First and Far fields. With the help of Dunkle’s guide, I identified common green darners, twelve-spotted skimmers, and common whitetails. All are large, showy species easy to see even through my birdwatching binoculars.

But back on August 19, 2003, I was dazzled by a new species. As I walked across First Field toward Big Tree Trail, dozens of huge, flashy dragonflies, which I later identified as comet darners (Anax longipes), zipped around me like miniature helicopters on speed.

“Heavenly Day,” lepidopterist Alexander B. Klots said when he first saw a male comet darner, “isn’t he a beautiful thing on the wing! With that emerald green of the thorax and blood red of the abdomen…” The male comet darner also has a greenish-yellow head and unusually long legs, hence the longipes species name that means “long legs.” Virginia Carpenter, author of Dragonflies and Damselflies of Cape Cod, published back in 1991, calls the comet darner “easily our most spectacular dragonfly…a fleet, powerful insect which flies with an easy, fluid grace…bird-like [in]size…and..rarely seen at rest.”

For more than three weeks, male comet darners hawked insects over First Field, and never once did one land while I was watching. Carpenter claims they are so swift that they are hard to catch. And they are fierce predators. She saw one with three dragonflies, half the size of its 3.2 inches, in its grasp. Dunkle wrote that the comet darner’s preferred habitat is borrow pits or semipermanent, usually shallow, grassy ponds without fish. We don’t have those close by, but apparently dragonflies do gather in swarms to feed on abundant prey miles from where you might expect to see them, which explains why I see so many above our fields. Members of a feeding swarm ignore each other and often consist only of males.

While the comet darner swarm only appeared here in 2003, the common green darners (Anax junius) are attracted to First Field every year in late summer. I sit on Alan’s Bench and watch them zip about for hours. Only slightly smaller than comet darners, common green darners have bright green thoraxes and azure blue abdomens adorned with a vertical dark brown stripe. The most widely distributed darner of its genus, it can be found in all 50 states, as well as in Canada, Mexico, the Caribbean, and even Asia. They also like to breed in temporary, fish-less ponds.

Some common green darners migrate; others are residents that remain in ponds as nymphs throughout the winter. Of the 300 dragonfly species in North American, only 16 species from two families–the darners and skimmers–migrate. Usually they fly in air currents high above humans’ sight. Many migrate singly, but sometimes huge swarms migrate. In 1992, more than a million common green darners were counted along Lake Michigan’s shore at Chicago, Berger says. Like birds they follow land forms and seem to know their way instinctively.

Often, hawk watchers report seeing dragonflies too. Late last August, Rudy Keller was counting hawks at Hawk Mountain Sanctuary, and he reported on the Pennsylvania butterfly and dragonfly listserv that “dragonflies were constantly moving by North Lookout or passing through binocular fields during my scans for raptors. Local redtails, passing kestrels, and even sharpies were stooping on, capturing and eating them as snacks on the wing all afternoon…” Keller was able to identify black saddlebags and twelve-spotted skimmers, both of which migrate. Merlins also prey on migrating dragonflies. Other bird species that relish dragonflies of all kinds are common nighthawks, swifts, swallows, flycatchers, and purple martins, all birds that catch their prey on the wing, using their mouths as dragonflies do when “hawking” food.

Dragonflies can fly for hours nonstop, averaging 25 to 35 miles an hour when they are migrating, and they travel thousands of miles. “Dragonflies,” Berger writes, “can take off backward, launch vertically like a helicopter, hover motionless for more than a minute, execute an unbanked turn, make a series of dazzling zigzag maneuvers, and stop on a dime.”

They also have eyes that wrap around their heads so they can see in almost every direction at the same time. Their color vision is better than ours–they can see four or five to our three primary colors, including ultraviolet light. Because of their superior eyesight, they only use their eyes to find prey. They eat flying insects including other dragonflies and need at least ten to fifteen percent of their body weight in food every day.

The insect order to which both dragonflies and damselflies belong–Odonata–means “toothed ones,” which refers to their sharply serrated lower jaw that they use to grab their prey. While dragonflies are big and stocky with wings that spread out flat when they are perched, damselflies are little and dainty and fold their wings over their backs when they are perched. The ebony jewelwings or black-winged damselflies (Calopteryx maculata) sit on vegetation overhanging our stream bed looking, as Virginia Carpenter observed, “like so many little black bows tied gaily to the tips of branches.”

During my time at Whipple Dam State Park with Cynthia Berger, we saw three species of damselflies–familiar bluets along the side of the lake, eastern forktails, which Berger says are monogamous, over the water, and ebony jewelwings at the edge of the outlet stream in the shaded forest.

“I always think they’re fun to see,” Berger commented as a couple chased back and forth. Both were males because of their black wings and striking, dark, metallic green bodies. Females have a white stigma (a blood-filled blister at the tip of each wing) on each brownish wing and dull, bronze-colored bodies.

Male ebony jewelwings defend territory for as long as a week from ten in the morning until four in the afternoon and mate with numerous females over that time period. After a fluttering courtship display, a male ebony jewelwing grabs a female at the top edge of her thorax with four claspers from the tip of his abdomen. Then they fly in tandem, the male leading and dragging the female along until they can form what is called a “copulation wheel.” She grasps the tip of his abdomen with her legs and folds her abdomen up under her thorax, pushing the tip of her abdomen against his genitalia. Dragonflies do this while flying; damselflies while on secluded perches.

To my delight, Berger and I spotted a pair mating on the leaves of a hickory sapling beside the stream. The “wheel,” though, looked more like a Valentine heart. We also saw another male open his wings and then clap them shut. Berger says that both sexes may do “wing-clapping” to signal their location, to cool their bodies on a warm day, or to improve the intake of oxygen.

We didn’t stay to watch the female lay her eggs on floating or submerged water plants while the male guarded her and tried to court other females at the same time. I was content to have seen the three damselfly species and three dragonfly species–twelve-spotted skimmers, common whitetails, and widow skimmers. But Berger was disappointed.

“Oh, it was so much better the last time I was here,” she told me.

Spoken like a true birdwatcher!

Cynthia Berger’s Dragonflies is published by Stackpole Books and not only covers the life history of dragonflies and damselflies, but tells readers how they can attract them to their backyards. The book is beautifully illustrated in color and includes paintings and species accounts of the most common dragonflies and damselflies. Its comprehensive Resource section should help anybody interested in becoming a dragonfly-watcher.