It’s mid-October. The Sugar Maples (Acer saccharum) are on fire this year, with deep oranges and ruby reds. You might think that most of our migrant birds have all gone by now, but that’s not true. Among others, I have been seeing flocks of Yellow-rumped Warblers (Setophaga coronata) feeding on the berries of Spicebush (Lindera benzoin). They are heading south to winter in the southeastern United States. Dark-eyed Juncos (Junco hyemalis) and White-throated Sparrows (Zonotrichia albicollis), which spent the spring and summer in northern New England and Canada, are now arriving to spend their winter here in Connecticut.
What you might not expect to see now are Common Green Darner dragonflies (Anax junius). They are passing through too. Most people know that Monarch butterflies (Danaus plexippus) migrate south to spend the winter in the Oyamel Fir forests (Abies religiosa) in Mexico. But some dragonflies migrate too. In fact, Green Darners are one of at least 16 migratory dragonfly species in North America.
The Common Green Darner is a large dragonfly that grows to be 2.7 to 3.3 inches (6.8 to 8.4 centimeters) long, with a wingspan of 3.5 to 4.5 inches (9 to 11.5 centimeters). The male has a pale green head and thorax and a blue abdomen. The abdomen of the female and immature darners is green, brownish, or reddish. Darners get their name from their long, thin abdomens, which resemble a darning needle. Green Darners are one of the most common dragonflies and are found throughout the eastern United States and on the West Coast.
Dragonflies are skilled aerial predators. With their four wings they can maneuver quickly in mid-flight to catch a variety of flying insects. The immature nymphs are aquatic and feed on other small insects, and even small fish and tadpoles.
Unlike flocks of birds or a kaleidoscope of butterflies (yes, that’s what a group of butterflies is called), Green Darners often go unnoticed. They are some of the most abundant dragonflies on the continent—yet few people notice this mass migration. They too can swarm in large groups, particularly along the New England coast.
In a study published in Biology Letters in 2018, researchers from the Vermont Center for Ecostudies, the University of Maryland, and the Smithsonian Conservation Biology Institute described how the Green Darner completes its migration. They learned that most individuals traveled an average of 373 miles (600 kilometers) and that some migrated more than 1,553 miles (2,500 kilometers)!
The Common Green Darner has a multi-generational life cycle. The first generation hatches in the southern United States from February to May, migrates north, lays eggs, and then dies. The second generation emerges that summer in the north, migrates south, lays eggs, and then it dies. The third generation emerges there around November. This generation doesn’t migrate but will live through the winter in the South, lay eggs, and die. The cycle then repeats.
Understanding this migration gives scientists more information about the life cycles and ecology of insects. This is particularly important because of their declining populations and their role as a valuable food in ecosystems.
The researchers were able to track Green Darner migrations with a bit a chemical sleuthing. For two years, they captured dragonflies throughout eastern North America with the help of other biologists and citizen scientists. They also got permission to take samples from specimens in museums from Canada to the Caribbean. In both groups they looked for a hydrogen isotope, sampling more than 800 dragonfly wings. Most hydrogen atoms have a single proton, but a small percentage have a proton and neutron. This isotope is called deuterium.
The amount of deuterium in rainwater varies in North America along a north-south gradient. Because dragonflies are born and grow as nymphs in water, they take in deuterium in their bodies. The isotope is incorporated into the chitin in their adult wings. Measuring this deuterium told the researchers where each dragonfly was born.
The next time you see Common Green Darners in the fall, wish them a safe journey.