You might be wondering why I am writing about migration in December. Surely fall migration in 2022 has been over for weeks. That’s what I thought too, until Thanksgiving Day. when at least 3 large flocks of Canada geese flew south over my house. They were good sized flocks too, not just a few stragglers that had gotten their timing wrong. It got me thinking that I hadn’t read up on migration science in a while. Then VT Center For Ecostudies (VCE) newsletter arrived with a short article about migrating green darner dragonflies. With that, I knew I needed to dive in.
The act of migration – the seasonal movement of creatures from one region to another – has been happening for millenia. It takes place all around us and at so many different levels. I grew up watching the annual monarch migrations and flocks of waterfowl passing overhead on the prairies of Iowa. In the years since then, scientists have been chipping away at migration’s mysteries. They know a lot more than they did when I first learned about monarchs in the 60s, but they also know there is much left to investigate. Let’s take a quick look at three species that live in Vermont and migrate each year – the common loon, the green darner dragonfly, and Daphnia, a common freshwater zooplankton.
Common Loons. Photo: Susan Wetmore, EOL.org.
Loons, like other waterfowl, depend on open water for survival. A loon on the ice is a loon in trouble. As lakes and ponds begin to freeze in the fall, loons head out to spend the winter in locations where the water does not freeze. For many local loons that means heading east to the coast. Loons from other northern lakes may make a trip further south to Florida and Texas. Some large northern lakes, like Champlain don’t freeze entirely in winter. Loons can live there year round. The Adirondack Center for Loon Conservation has tracked loons from lakes in northern New York and found that they overwinter at locations on the East Coast from Massachusetts to Florida (see their map) .
How does a loon hatched in northern Vermont know where to go? Loons will travel in small groups or make the trip alone. That sounds so easy, but really it is very complex. Birds have an amazing internal map that brings them to the right place. Though no one knows for sure how this map works, scientists believe that the Earth’s magnetic field helps birds figure out where they are with respect to latitude. They also use the sun and the stars to help them along the way. Some birds use physical landmarks like mountain ranges and rivers to guide their travels. Many young birds know the way to their winter homes without ever having been there before. A bird knocked off course by weather (or moved by an investigating scientist) can find their way back home from places they have never been before. Simply amazing! Learn more at Cornell Ornithology Lab’s All About Birds webpage.
Green Darner Dragonflies
Mating Green Darner Dragonflies. Photo: Liam O’Brien, EOL.org.
Though not many people know it, some dragonflies are major long-distance migrators just like monarchs. Studies by the VCE estimate that they regularly travel 600 – 700 miles between their northern and southern homes. This requires around 55 days of flying unless the wind gives them a big boost. Because dragonfly lives are short compared to those of birds, a full annual migration cycle takes several generations to complete. The southern generation of dragonflies emerges in spring and makes the migration north to their summer home. They will lay their eggs there and then die. The northern eggs from the previous summer hatch in the spring and grow into the adults that will make the journey south in the fall. These southbound adults make the journey, lay their eggs, and also die. Their eggs hatch into dragonflies that will not migrate. That winter generation lives its entire life in the southern habitat and lays the eggs that will hatch into the generation that flies north.
Confused? It’s quite a cycle and wasn’t something easily teased apart. VCE and their partners used chemical isotopes collected from the wings of museum specimens to identify where individual dragonflies hatched. They linked these locations together using citizen scientists observations of migrating dragonflies. You can read more about it here. The big question though, is how do the individuals know where to go when they migrate? They have never been there before and, in the case of the overwintering southern generation, their parents never travelled at all. Simply awesome! I wonder if they return to the same ponds year after year or go where the winds blow them. Maybe some day we will find out.
A Daphnia under the microscope. Photo: Center for Biodiversity Genomics, EOL.org.
Migration by Daphnia takes place on a daily basis (diel migration). They swim by the thousands up from the bottom of lakes and ponds at night and then swim down again in the morning. Scientists think that one of the main reasons to do this is to find food which is more abundant in the upper waters. By feeding at night, the Daphnia make it more difficult for their predators to find them. No matter the reason, they complete this journey every day, in every water body around the world.
They are not the only species to make this daily trek. Other zooplankton, fish, and insect larvae make it as well. Some reverse the journey, going down by night and up in the morning. Diel migration occurs in the oceans too. Masses of migrating zooplankton in freshwater or krill in marine locations are large enough to be seen by fish finders and radar. Changes in light are thought to trigger the movement, but even plankton in the darkest arctic winters migrate, so that’s not the full story. Just like the other two species I mention here, there is still a lot to be learned about Daphnia migration. Read more about how light influences diel migration here .
The Hazards of Migration
As you can well imagine, traveling long distances can be a dangerous activity. Many loons, dragonflies, and Daphnia that head out on their long trek don’t make it. Weather hazards like storms and winds can be deadly. Long distance travelers need to stop along the way to rest and refuel. They need safe locations that also have abundant food for these stops. All along the way, migrating creatures need to keep a sharp eye out for predators. They may arrive, tired and hungry, at their wintering sites only to find that the food and shelter there is gone. Still, over the millenia, species have survived and thrived after making these tremendous journeys.
Blue flag on Berlin Pond, VT. Photo: Angela Shambaugh.
People have made migration more difficult and dangerous. Development everywhere has changed habitats and resting stops. We’ve filled in wetlands, developed shorelines, and cut down forests around the world. There are fewer places now where migrating birds and insects can find those safe food-rich rest stops. Their southern or northern home sites can change drastically in a year, leaving them with nowhere to go. Like the monarchs that overwinter in Mexico, some birds have only one or two places that they overwinter. As those locations shrink, the number of birds returning to the north also shrinks.
You can make a difference in the life of a long distance migrator, be it bird or insect. All of the things that make our yards and properties havens for local wildlife can make it a rest stop for migrators. Think three dimensionally – birds need ground cover, understory and overstory plants for food and shelter. Dragonflies need shoreline vegetation and emergent plants for cover and resting. One big problem for all migrating creatures is the presence of light at night. Too much light and their internal compasses are messed up, making it hard to find their way. Limit outdoor lighting and make sure it doesn’t shine up into the sky (or down into the water). Learn more at Audubon’s Lights Out webpage. Let’s make Vermont’s lakeshores hot spots for migrating creatures of all kinds.