Growing Research Finds Evidence of Long Flights
Bird migration is well documented and a familiar concept to most individuals, but birds are not the only creatures that depart from cool climates in search of warmer ones in the fall, and the reverse in the spring. Monarch Butterflies are also well-known long-distance migrants, but so are many other insects, including other species of butterflies, moths, locusts and dragonflies.
Exactly what is known about the phenomenon of dragonfly migration, and how was it discovered? Migration by dragonflies has been recorded sporadically for several centuries. European records of this phenomenon date back to 1494. Recently, there has been a renewed interest in this topic and in 1998 an extensive review of observations and literature about massive swarm migrations of dragonflies in eastern North America concluded the following: Most swarms are sighted between late July and mid-October, with a peak in September. Most of the large flights occur along topographic features such as lakeshores and coastlines. Massive swarm migrations are correlated with northerly winds following the passage of cold fronts.
The Common Green Darner (Anax junius) is the predominant species in the majority of these flights. A new collaborative, the Migratory Dragonfly Partnership, has greatly increased what we know about dragonfly migration by attaching miniature radio transmitters to the thoraxes of migrating Common Green Darners and following them for up to 12 days. Information has also been gleaned from the study of isotopes.
There are approximately 326 species of dragonflies in North America. Of these, only about 16 to 18 are regular migrants, with some making annual seasonal flights while others are more sporadic. It is not always easy to identify migrant dragonflies as opposed to those that aren’t migrating, but dragonflies flying together in large swarms that move in a common direction for sustained periods are considered migrants. In addition to the Common Green Darner, well-documented, annual, long-distance migratory species of dragonflies in North America include the Wandering Glider (Pantala flavescens), Spot-winged glider (Pantala hymenaea), Black Saddlebags (Tramea lacerata) and Variegated Meadowhawk (Sympetrum corruptum), and all but the last are found regularly in New England.
Radio telemetry reveals that the Common Green Darner, weighing about one gram, travels more than 400 miles over a two-month migration. There are interesting similarities between bird and dragonfly migration behavior. Like migrating birds, dragonflies stop over at feeding spots occasionally to refuel along the way.
In fact, during migration Common Green Darners often spend as much or more time feeding as they do making long flights. On average they migrate in a southward direction every three days, covering roughly 30-40 miles in 5-7 days. Common Green Darners migrate exclusively during the day, regardless of wind direction, but only after two nights of successively lower temperatures. Like many migrating songbirds and hawks, dragonflies appear to avoid flights over extensive open water, even if it means going miles out of their way.
Dragonflies begin their adult lives in the fall with very little fat, undeveloped ovaries and functional but incompletely-developed flight muscles. They quickly increase muscle mass and fat stores — more so than local breeding dragonflies that don’t migrate — and some species periodically lay eggs in ponds along their migratory route during their southward flight. The extent of the southward migratory flight of dragonflies has not been determined, but it is known that they reach Florida and even Mexico in substantial numbers.
Large swarms of dragonflies are a more common sight in the fall than in the spring. Various indicators lead entomologists to believe that there is a spring migration, though it may be more protracted than the fall migration. Adult dragonflies in various locations in New England have a history of appearing several weeks before the first sign that resident adults are emerging, indicating that the early arrivals are migrants. Mature dragonflies have also been observed mating and laying eggs in northern areas in early spring in locations where they could not have yet emerged. Substantial numbers of Common Green Darners have appeared with warm air masses in early spring in the Northeast, remaining at a given site for only a few days, indicating migratory behavior. Unlike birds, Common Green Darners observed migrating northward in the spring do not have much wing wear, which strongly suggests that they are not the same individuals involved in the autumn migration.
Even with this all of these observations, research and documentation, much remains to be learned about dragonfly migration, including their migratory cues, flight pathways and the southern limits of overwintering grounds. Citizen scientists can contribute to the growing bank of knowledge about this phenomenon by contacting the Migratory Dragonfly Partnership.
Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England,” “Milkweed Visitors,” and “Ferdinand Fox’s First Summer.” Her natural history blog is at www.maturallycuriouswithmaryholland.wordpress.com