Wood Thrush, adult © Brian E. Small/VIREO
What do a Wood Thrush, a Monarch butterfly and a Common Green Darner dragonfly have in common? Each spring they all migrate great distances to the Northeast from warm southern climates to breed. We’ve known where thrushes winter in Central America. We’ve known where Monarchs winter in Mexico for nearly 40 years now. But no one knows where the darners spend their winters or how they repopulate the Northeast each spring. We cannot manage and conserve a migratory animal without knowledge of its full annual cycle.
I have joined a group of biologists that are hot on the trail of discovery. I have teamed up with the Migratory Dragonfly Partnership, a group of scientists from across the United States, Canada and Mexico, to better understand dragonfly migration. Pete Marra from the Smithsonian Conservation Biology Institute and I are leading a ground-breaking study using stable-hydrogen isotopes in the wings of dragonflies to trace spring migrants back to their natal origins, unlocking the geographic scale and connectivity of these populations.
Dragonfly migrations have been observed on every continent except Antarctica, with some species performing spectacular long- distance mass movements. The Wandering Glider dragonfly is the global insect long-distance champion; making flights across the Indian Ocean that are twice the distance of Monarch butterfly migrations. In North America, dragonfly migrations are seen annually in late summer and early fall, when thousands to millions of insects move from Canada down to Mexico and Florida and the West Indies, passing along both coasts of the United States and through the Midwest. North America may have as many as eighteen migratory dragonfly species, some engage in annual seasonal migrations, and others are more sporadic migrants.
Monarch, male © Rick Cech
Spring movements northward by dragonflies are rarely seen, presumably because it occurs over a wider front, over a longer time period, and with fewer individuals than in the fall. We know it happens because dragonflies appear early in spring in places where nymphs have not yet emerged.
The best-known migrant dragonfly in North America is the Common Green Darner. This species appears in early spring at northern latitudes, often seen flying before any local dragonflies have emerged. These are migrants from the south, returning from perhaps Florida, the Caribbean, or Mexico. These individuals breed soon after they arrive in spring, and their nymphs develop quickly in wetlands warmed by the summer sun. Many adults emerge in August, and instead of maturing and breeding at the same site, they begin a southward movement that may take over a month. Their destination is at present unknown but presumably the same areas thought to produce spring migrants. Migrating individuals may breed at their final destination or along the way.
Although migration is common, it is not obligatory for Common Green Darner. Populations in more northern areas are known to contain both resident and migratory individuals. These phenotypes overlap in space, but exhibit strikingly different annual phenologies appear to limit temporal overlap in breeding. Migrants arrive at breeding ponds in March – April and larvae develop into adults in 4-5 months. Residents begin their breeding cycle roughly one month later in June – July and larvae overwinter in ponds, finally emerging as adults in May-June in the following year.
There is some evidence air temperature plays a role in the maintenance of this phenotypic variation. Final-instar larvae of migratory phenotypes reared in the laboratory required a minimum water temperature of 8.7oC to develop into adults. In Ontario, resident phenotypes required 20% more accumulated degree-days than migrants to complete development. These thresholds suggest the relative size of migratory populations could vary with latitudinal gradients and temperature.
Green Darner © E. R. Degginger, Color-Pic, Inc.
Despite the fact that it spans several countries and has been documented since at least the 1880s, North American dragonfly migration is a poorly understood phenomenon. Knowledge about migratory cues, flight pathways, population connectivity and the southern limits of overwintering grounds is still seriously lacking. This knowledge gap prevents development of international management plans and coordinated conservation actions to sustain and protect the migration. None of the dragonfly species known to be migrants in North America is currently endangered, but identifying the habitats on which migrating dragonflies rely for their transcontinental flights may help us better protect these important systems. The threats to wetland habitats, including the effects of global climate disruption, could alter environmental cues for migration, affect larval development and adult emergence times, disrupt migratory corridors, or render overwintering habitat unsuitable.
The overarching goal of this study is to understand the geographic scale and connectivity of dragonfly migration. Remarkably, we can do this by examining the chemistry locked in the dragonfly’s wings.
Stable-hydrogen isotopes are ideal for inferring natal origins dragonflies because they reflect the latitude at which body tissues were grown and because they are chemically inert once bound. Vaporizing a tiny piece of a wing in a mass spectrometer gives us the figures we need to determine the latitude of the pond where they grew up. With the help of volunteers in the field and museum collections from the past, we are sampling Common Green Darners from Mexico to Texas, over to Florida and up the eastern half of North America into Canada in a quest to better understand what might be one of North Americas most amazing animal migrations.