The swarm envelops David Bree as he approaches the beach at Presqu’ile Provincial Park. Hundreds of dragonflies are whizzing around him, snap-turning in every direction as though attached to invisible yo-yo strings, then suddenly changing course, again and again.
He points his camera above and clicks, freezing the action. In the image: a seemingly chaotic squadron of tiny flying figures silhouetted against a pale blue background; T-shaped insects with thick round heads and bodies, rigidly perpendicular wings and long, thin tails, tilted at all angles, facing every way.
Bree had seen this spectacle before. As chief park naturalist at Presqu’ile for 16 years before retiring in 2021, he has come to view this annual gathering of common green darners, one of North America’s largest and most ubiquitous dragonfly species, as a highlight of the late-summer season. “If you walk through the fields and along the shoreline anywhere in Prince Edward County in the first week of September, you can often scare up hundreds of green darners,” he says.
At other times, getting a close encounter with even one of these colourful insects — named for their iridescent green face and thorax — requires more effort. In spring and summer, adult green darners fly solo, hunting small insects on the wing. But come August and September, they move south as one. The swarms at Presqu’ile and elsewhere in the province are participating in a great annual migration, much like that of the storied monarch butterfly. In winter, common green darners have been spotted in the southern United States, Mexico and Central America. However, while the monarch’s multi-generational migratory cycle has been heavily studied, inspiring an international effort to conserve the habitat these butterflies depend on, details of the green darner’s transcontinental journey are just starting to be understood.
“Monarch has gotten a lot of attention, and rightly so,” says Bryan Pfeiffer, a field naturalist and adjunct instructor at the University of Vermont who writes Chasing Nature on Substack. “But when I tell folks, even somewhat accomplished naturalists, that some dragonflies migrate, they’re like, ‘Wow, really?’”
In recent years, Pfeiffer notes, a growing number of birders have taken up dragonfly watching. The green darners’ group staging is a scene akin to that of the kettling of hawks and other raptors for fall migration. (In fact, the species are often seen together, with some raptors eating the dragonflies.) But there are big differences. “We take migration for granted when we think about birds,” says Pfeiffer, “but that this little insect that weighs not much more than a drop of water can move great distances is pretty cool.”
Scientists believe there is much to gain by filling in the missing puzzle pieces of the green darner’s annual cycle. “Dragonflies are key players in multiple ecosystems over their lives, both as consumers and as prey,” says Ryan Norris, associate professor in the department of integrative biology at the University of Guelph. “For us to understand our impact on different ecosystems, it’s important to understand the life histories of organisms that move through them. When are they there? How long are they there? Where else are they going? If we don’t have that information, it’s hard to make good conservation decisions.”
Marvelous migration: Common green darners travel from Ontario to as far as Central America in their fall migration
Green darners may have the word “common” in their name, but their annual migration makes them exceptional. Only about a dozen of the 135 dragonfly species native to Ontario migrate. Similarly, of the more than 330 species of dragonflies found in North America, just 16 are considered true migrants. Given that insects in the order Odonata (which includes dragonflies and damselflies) and their immediate ancestors among the first organisms to achieve flight roughly 300 million years ago, it is not surprising, says Pfeiffer, that some have evolved to fly great distances. But why only a few do is unclear.
One factor that has held back research on dragonfly migration is the insect’s size. Green darners are generally too small to be fitted with the radio and satellite transmitters commonly used to track birds on their migrations. However, in a study published in 2019, two Guelph students (supported by Norris) managed to put radio transmitters on 38 green darners captured on the Saugeen (Bruce) Peninsula. They discovered that one travelled 122 kilometres in a single day; another individual reached a (wind-assisted) top speed of 77 kilometres per hour. While the study was too limited in scope to support broad conclusions, it confirmed that green darners are capable of long-distance flight.
To date, most of what is known about dragonfly migration has come from two types of molecular study: DNA analysis, which reveals how much populations in different locations interact; and stable-hydrogen isotope analysis, which detects a geographic signature in animal tissue pinpointing the latitude where individuals were born, which enables researchers to calculate how far they travelled before capture.
A few years ago, isotope analysis yielded the biggest breakthrough thus far in understanding the common green darner’s annual migratory life cycle: that it involves three generations of dragonflies. The research was done by a team of U.S. scientists using wing tissue from more than 850 common green darners, including museum samples as much as 140 years old. They learned that the generation that emerges here migrates to the south, breeds and dies; their offspring hatch and emerge in the south and remain there; then a third generation is born and comes north in the spring, where they breed and die, and the cycle repeats. The distances they travel are impressive: upwards of 680 kilometres going south and at least 650 kilometres coming north.
Not only did the study confirm that the green darners seen leaving Ontario in the fall will never return, but the researchers also paired their isotope findings with community science observations to show that green darners’ northward migration tracked the seasonal advance of the 9 C mean daily temperature threshold. This is why common green darners sometimes show up in the north before all ponds and wetlands are free of ice. Those early fliers are migrants from the south, explains Colin Jones, provincial invertebrate zoologist at the Ontario Ministry of Natural Resources’ Natural Heritage Information Centre, whereas the first local nymphs that emerge as adults usually do not appear until June.
Tracking travel: Researchers seek to better understand migration behaviour
Isotope analysis has limitations, however. It can identify the approximate latitude where dragonflies emerge but not specific locations. It also says nothing about their actual journey.
“There’re still many unanswered questions,” says Smera Sukumar, Ontario Nature’s conservation science and stewardship director. “We need further research on how they’re navigating and to understand what threats they face in different parts of their life cycle.” That information, she notes, could support an international effort to protect wetland habitats, which play an important role not only in the dragonflies’ larval stage but also in their adult stage and migration. (Like all dragonflies, common green darner nymphs hatch from eggs laid in ponds, lakes, slow streams and rivers, later emerging as winged adults.)
The potential impact of climate change on green darner migration also merits study. As one of the first insects to fly in the spring, green darners are an important food source for early-migrating birds such as purple martins. “There have been years when a big weather event, like a snowstorm, killed all the common green darners that came up early. As a result, the baby purple martins starved,” says Ami Thompson, assistant professor of biology at North Carolina Wesleyan University. “With more data, we hope to understand if climate change is going to cause a mismatch in the abundance of common green darners as a food source for these birds.”
To that end, Thompson is in the final preparatory stages of a new common green darner tracking study in which she plans to use radio tracking tags much lighter than those used previously. Her aim is to partner with other researchers to tag large numbers of green darners as they migrate across the continent. “We don’t know much about how they move individually. That’s what these little trackers will tell us,” she says. “It’s like this is a book and this is just the first chapter. I want to read it and find out what’s going on.”
The article was originally published in the Spring 2025 issue of Ontario Nature’s ON Nature magazine. Top image courtesy David Bree; second image courtesy ON Nature magazine; final image courtesy Ryan Norris.