Six-legged Sky Jewels

By Zoey Greenberg 

What if we told you there was a creature that, as a newborn, prowled ponds and streams using part of its face as a projectile sword to stab and eat everything in sight? What if you knew that the same newborn grows into an adult that hunts the skies with multi-directional compound eyes, eats hundreds of mosquitoes per day, and can migrate up to 11,000 miles. Would you believe us? Dragonflies do all of this, and more. At Hawk Mountain, we count migrating dragonflies as they fly south, adding knowledge about when, where, how, and why they migrate. Slowly but surely, answers are surfacing.  

Ask any novice birder and they will tell you that hawk watching is a test of patience and eyeball mechanics. Throw dragonflies into the mix, and you’ve got yourself an eyeball bootcamp. Dragonflies are famous for their speed and agility, and with 316 species in North America, identifying and counting is quite the test. 

Green Darners are a widespread species, being the state insect of Washington as well Eastern North America’s most noticeable dragonfly migrant. Their Latin name, Anax junius, means “Lord of June, “ and aptly reflects their noble appearance. These blue, green, and purple predators undertake impressive journeys that would seem impossible for an insect with a lifespan of only one year, most of which is spent underwater. Researchers theorize that it takes multiple generations of darners to complete one full migration. Adults likely deposit eggs during migration, which then hatch into nymphs, take wing, and subsequently fulfill an additional leg of the journey. Recently, animal tracking devices have become small enough that researchers can deploy them on dragonflies. This will yield new information on where these insects are traveling, and when they stop. For the time being, however, we rely on visual observations. 

Dragonflies are often seen traveling individually, although on several historic days large swarms have been seen moving south. These congregations are likely a result of climatic and geographical funneling effects that similarly direct the when, where, and how of bird migration. Hawk Mountain Sanctuary for example, is located on a ridge top because this serves as a “leading line” for migratory birds by providing free rides on updrafts and thermals that orient them towards their destination to the south. On long, energetically taxing journeys, such energy saving opportunities are hot commodities and evidence shows that dragonflies may be using the same tricks of the trade. 
Places like Cape May, New Jersey and Chicago, Illinois also serve as leading lines for migrants, based on their proximity to large bodies of water that naturally, migrants hope to avoid. On September 11, 1992 Cape May experienced a major cold front followed by northwest winds. One hour later, a dragonfly swarm pushed through town. This surge in dragonfly numbers was also accompanied by an unusual increase in the number of migrating Sharp-shinned Hawks and American Kestrels, two raptor species that utilize optimal winds to make headway during migration. Experts agree that the correlations between dragonfly and bird movements are not coincidental. 

Robert W. Russell and his coauthors were some of the first researchers to investigate the ecological incentives for dragonfly migration in 1998. They reviewed reports of swarms and unveiled some interesting findings in the process: first, movements occurred during the same time as bird migration, between late July and mid-October; second, most of the swarms were observed moving south in coastal locations and lakeshores which serve as leading lines; third, there was a link between the passage of large-scale cold fronts, northwest winds, and the movement of dragonfly swarms. Green Darners were the primary species in each of these events.