Author Archives: Bryan Pfeiffer

The Nuclear Option for Dragonflies

By Bryan Pfeiffer

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A Common Green Darner (Anax junius) / © Bryan Pfeiffer

On a crisp, sunny day in September, after what was probably a typical summer for a dragonfly (which involves flying around, killing things and having sex beside a pond), a Common Green Darner took off and began to migrate south. As it cruised past the summit of Vermonts Mt. Philo, with Lake Champlain below and the Adirondacks off in the distance, the dragonfly crossed paths with a Merlin.

The Merlin, a falcon that kills in flight, swerved, plucked the dragonfly from the sky with its talons and began to eat on the wing. As the falcon and its prey continued southbound, all that remained in their wake was a single dragonfly wing, falling like an autumn leaf toward fields at the base of Mt. Philo.

Eagles, hawks, falcons and Monarch butterflies aren’t the only migrants moving south past mountains this fall. Joining them are dragonflies. Although biologists know plenty about the fall raptor and Monarch migrations, we are only beginning to discover, with some creative chemistry, where these dragonflies go and how migration figures in their conservation.

Fly or Die

Most dragonfly species do not migrate. In fact, most are now dead, having already mated during the summer season, leaving behind eggs or larvae to survive the winter. The killing frost will finish off much of what’s still on the wing. But some survivors will leave.

Wandering Glider (Pantala flavescens) / © Bryan Pfeiffer

Wandering Glider (Pantala flavescens) / © Bryan Pfeiffer

Among the 460 or so dragonfly and damselfly species native to North America, at least five are classic migrants: Common Green Darner (Anax junius), Black Saddlebags (Tramea lacerata), Wandering Glider (Pantala flavescens), Spot-winged Glider (Pantala hymenaea) and Variegated Meadowhawk (Sympetrum corruptum). Each species is on the move this fall.

Dragonflies migrate for the same reasons other animals migrate: to avoid inhospitable conditions, in this case habitats that freeze or become too cold for the dragonflies themselves or their insect prey. Monarchs go to Mexico. Broad-winged Hawks leave for wintering grounds stretching from southern Mexico into South America. Dragonflies head south to who knows where.

Having studied birds for two centuries, biologists know well their breeding and wintering distributions , even to the point of learning the destination of a particular warbler or sparrow after it leaves us in the fall. Ornithologists catch lots of songbirds in nets and place around one leg a tiny silver bracelet embossed with a unique number – an avian social security number – and then release the birds to the winds. A small percentage of them, still sporting their bracelets, are later recaptured while in migration or on wintering grounds thousands of miles away. Better yet, we’re putting small electronic transmitters on large birds, such as Bar-tailed Godwits and American Woodcocks, and tracking their movements real-time with satellites.

On of my tagged Monarchs before heading off toward Mexico.

A tagged Monarchs before heading off toward Mexico.

We can even track the movement of a single butterfly. I myself have placed little stickers, each bearing a unique alpha-numerical code, on the hind wings of more than 1,000 Monarchs here in Vermont and elsewhere in North America, and then set each one free to fly toward Mexico, where many are later encountered by conservationists searching for the buttterflies once they arrive in Mexico. With each recovery, we learn more about Monarchs and how they migrate.

The “Heavy” Hydrogen

Dragonflies aren’t so obliging. For one thing, we’re clueless about where they go. Monarchs concentrate each winter in stands of Oyamel Fir in mountains west of Mexico City. So we know where to find them and how to protect them. Tagging or somehow marking a dragonfly would be like putting a message in a bottle and tossing it out to sea. Actually, I suspect we’d find the bottle before the dragonfly.

Yet it turns out that we need not tag or otherwise mark these migratory dragonflies because they themselves carry clues about where they have been. If the Merlin doesn’t get it first, we can catch any migrating dragonfly, analyze trace elements in its tissue and determine roughly how far it has flown.

Our marker is water, more to the point the two hydrogen atoms in water. Recall from high school chemistry that hydrogen nucleus normally contains a single proton and no neutron. But a tiny fraction of hydrogen atoms around the world carry one proton and one neutron. We call it “heavy hydrogen,” or deuterium. And unlike other such atomic variations among elements (which can be radioactive), deuterium is stable in the environment – a “stable isotope”– and stable in the wing of a dragonfly.

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Credit: Migratory Dragonfly Partnership

The amount of deuterium in water varies somewhat predictably in North America. You can map it. The ratio of deuterium to hydrogen in water falling as rain or snow changes on a gradient corresponding roughly with latitude. Water in Alberta, for example, carries a different deuterium-to-hydrogen ratio than water in Alabama.

Because dragonflies grow up as nymphs in water, they incorporate the local deuterium ratio into their tissue. It’s like a dialect or an accent that a dragonfly bears for life – whether as a nymph in water or a free-flying adult in migration. A Common Green Darner on the wing over Mt. Philo or Miami unwittingly carries a particular deuterium ratio, a birth certificate that tells us generally where it grew up. You are what you eat – or drink.

This science isn’t perfect. We can’t pinpoint a dragonfly’s natal waters in the way we know where a banded bird hatched or a tagged Monarch emerged. But stable isotopes are helping us track the range of migrating dragonflies. It’s “better living through chemistry.” After all, we can’t really know a bird or butterfly or a dragonfly – and what it might need in the way of conservation – until we know all the places it lives or wanders.

By the way, you need not be a chemist to help track dragonfly migration. We’re counting dragonflies in the same way we count migrating raptors during hawkwatches each fall. Learn how to do it and report what you find with help from the Migratory Dragonfly Partnership.

And while we’re out there counting, if a Merlin happens to catch a dragonfly first, we can still make a difference … by catching one of those dragonfly wings floating toward Earth.

Bryan Pfeiffer is a writer and field naturalist who specializes in birds and insects. He teaches writing in the University of Vermont’s Field Naturalist and Ecological Planning Programs.

Hobblebush on Fire

Viburnum alnifolium

 

By Bryan Pfeiffer

An entire season of fall foliage flares from a single plant. Find your fireworks on Hobblebush (Viburnum alnifolium). This understory gem may be the perfect shrub. It adds food and habitat diversity – for nesting birds and other wildlife – beneath forest canopy. And its blooms play a crafty game of deception each spring. Continue reading

The Forecast Calls for Snowberries

snowberry3Now awaiting a frolic through your senses is one of nature’s most delightful candies, a reward so discreet that you probably pass it by during walks on life’s long, green path. When you are next high on some mountain trail, in dense coniferous woods, or near a spruce bog, find an elegant vine with tiny, waxy leaves. Drop to your knees because here is your low-hanging fruit: a sweet wintergreen explosion known as Creeping Snowberry.

No wild food is more enchanting. Creeping Snowberry (Gaultheria hispidula), a northern and boreal member of the heath family (Ericaceae), dispenses its little white gifts in August and September. Although I’m reluctant to mention them in the same sentence, Creeping Snowberry fruits resemble Tic Tac® candies. But beyond their size and shape, there is no comparison. Not even close. Willy Wonka couldn’t have designed a more intoxicating experience. Continue reading

Icy Romance

During this arctic grip on Burlington, when almost anything outside seems to groan or crunch or crack, when the cold itself seems evil, a drama begins each morning in frigid waters off Perkin’s Pier. In Lake Champlain, Common Goldeneyes are getting hot.

These perky ducks bob and dive, lunge and flutter, cavort and compete. Nearly four months before our woods will glow with a rainbow of migrating songbirds, Common Goldeneyes are already courting – proof that icy water doesn’t necessarily put a chill on carnal desire. Continue reading