For those of you who have a pond (if I may paraphrase Vita Sackville-West), here’s an opportunity to make a personal contribution to science. The Xerces Society, a nonprofit group that supports the conservation of insects and other invertebrates, has announced a Dragonfly Pond Watch to help document the annual movements of two migratory dragonfly species, the common green darner and the black saddlebags. Both are common and widespread insects, found throughout California, and are easily recognizable even without binoculars. The darner has a particularly felicitous Latin name: Anax junius, the Lord of June. Volunteers are needed to observe a pond or other wetland on a regular basis and note the migrants’ arrival and departure dates (see www.xerces.org/dragonfly-migration/pondwatch for details.)
The phenomenon of dragonfly migration is unfamiliar to most people, even those who keep an eye on changes in the natural world. It’s similar to the migration of monarch butterflies in that no single individual makes a complete round trip. As Tim Manolis describes it in Dragonflies and Damselflies of California: “Observations suggest that, in late winter and early spring, [migratory] species begin to emerge in large numbers in Mexico and the southern border states (including the warmer areas of California) and move north into the northern United States and southern Canada. They breed in summer and then die. A late summer and fall emergence resulting from this breeding activity typically produces large numbers of offspring that migrate back south to breed in fall and early winter, and their offspring in turn emerge in spring to repeat the cycle.”
North America has 300-odd dragonfly species, only nine of which migrate. In our area, that would include the variegated meadowhawk, the wandering glider, and the spot-winged glider, in addition to the Pond Watch focal species. (The common names of North American dragonflies were standardized around 2000 by the Dragonfly Society of the Americas, with generally euphonious results.) Worldwide, migrants comprise 25 to 50 of the 5200 known species and have been observed on every continent except Antarctica. Flying swarms can be densely packed, with individuals only 3.1 meters apart. Their numbers can be jaw-dropping: 400,000 were tallied in a single day at Cape May, New Jersey. Author Scott Weidensaul on the fall passage through the Mexican state of Veracruz: “Each day, tremendous clouds of green darner dragonflies, probably numbering in the millions, would stream by us while we were counting hawks, the dry rattle of their wings sounding like sleet on dead leaves.”
Green darners have touched down on oil platforms in the Gulf of Mexico. Globe skimmers and a few other species fly all the way across the Indian Ocean from India to Tanzania and Mozambique, by way of isolated island chains like the Maldives and the Seychelles. A large aggregation of dragonflies, possibly globe skimmers, was spotted 900 miles off the coast of Australia.
In some ways, these mass movements parallel the seasonal migrations of birds. Groups follow ridges, cliffs, sea- and lakeshores, and other topographic leading lines. In North America, at least, their numbers peak after the passage of cold fronts. W.H. Hudson, the Anglo-Argentine naturalist, described flights preceding pamperos, seasonal winds of the pampas: “They make their appearance from five to fifteen minutes before the wind strikes, and when they are in great numbers the air to height of ten or twelve feet…is all at once seen to be full of them, rushing past with extraordinary velocity in a northeasterly direction…” Like birds, dragonflies fatten up before beginning their journeys. It’s not clear whether they feed en route.
A few years ago, Martin Wikelski of the Max Planck Institute for Ornithology and colleagues published their study of migratory behavior in the common green darner. They attached 300-milligram radio transmitters to the thoraxes of 14 darners captured in New Jersey, using eyelash adhesive and superglue. Once relaunched, the dragonflies were tracked from the ground and the air (with a Cessna 152 or 172.) The scientists found that their subjects traveled more often on calm days than windy days, and sat out days of high wind. Flying days always followed a night that was cooler than previous days. Like songbirds and some hawks, the darners were reluctant to cross large bodies of open water; some zigzagged to avoid Delaware Bay.
I don’t know if anyone has attempted a similar project on the West Coast, but there are anecdotal records of mass directional movements through Northern California. Keep watching the pond!