Fall Migration: Not Just for the Birds!

By Colleen O’Hara

Fall is a great time to spot migrating birds in our area, thanks to Arlington’s location along the Atlantic Flyway—a major route for migratory birds. But birds aren’t the only things on the move. The monarch butterfly, common green darner dragonfly, American eel, and the Eastern red bat are among the other animals in our area that also have interesting migration stories to tell.

Monarch butterflies. In the fall, monarchs that live in the northeastern U.S. and Canada will head south to the mountains of central Mexico where they will live together in large colonies high up in the Oyamel trees throughout the winter. These long-distance travelers are known as the “migratory generation” because they make the long trek south, traveling 50 to 100 miles a day, amazingly, to a place they have never been before. They will eat and rest during their southern journey, which takes around two months, but will not reproduce until the spring. The monarchs travel during the day along several flyways and will merge into a single flyway in central Texas before reaching Mexico. (Monarchs living west of the Rocky Mountains spend their winters in southern California).

As spring approaches, these overwintering monarchs will mate, lay eggs, and die, giving birth to a new “first generation” that soon will head north through Texas and the southern states laying eggs along the way. Over the summer, each successive generation travels farther north than the one before. It typically takes 3 to 4 generations to reach the northern United States and Canada. The last generation of the year is the migratory generation that will make the fall migration to Mexico. Summer monarch generations only live from 2 to 5 weeks, while butterflies in the migratory generation can live up to 9 months. Resource: https://www.fs.usda.gov/wildflowers/pollinators/Monarch_Butterfly/migration/index.shtml.

Common Green Darner. Did you know dragonflies migrate too? One example is the common green darner. You’ll likely see this dragonfly flitting around streams, ponds, and wetlands in our area during the warmer months. Similar to the monarch, the migration of the green darner consists of an impressive multi-generational journey that takes it from the south, to the north, and back again.

The first generation of the green darner emerges in the southern U.S., Mexico and the Caribbean, and makes its way north from about February to May. As they travel, the insects will mate and lay eggs in ponds, lakes, and slow-moving streams where the eggs will develop into nymphs. The second generation of the green darner emerges as adults around July and August (although some nymphs will overwinter in the north for 1 to 3 years). These dragonflies born in the north will begin their southern migration in September, where the cycle continues: they will mate, lay eggs, and die. It is the third generation of dragonflies hatched during the winter in the south, that will begin the journey north and start the cycle again.

To ensure a more successful migration, the green darner is able to change the color of its abdomen to a darker hue on colder days, so that it can absorb more heat and warm up its flight muscles. On warmer days, its abdomen will turn brighter to reflect the sun’s rays. Resource:  https://www.sciencenews.org/article/green-darner-dragonflies-migrate-bit-monarch-butterflies.

 American eel. If you’re lucky enough, you may have spotted an American eel in the streams of Arlington, the Potomac River, or Chesapeake Bay. And yes, they also have an impressive migration story.

The eels are catadromous, which means they live in freshwater as adults and spawn in the ocean. Their lifecycle is pretty interesting. Spawning takes place in winter and early spring in the Sargasso Sea in the Atlantic Ocean. Eggs hatch into larvae and drift in the ocean currents for 9 to 12 months during which they transform into “glass eels” and make their way to the brackish estuaries of Virginia and other locations along the East Coast. They continue to grow and change color over the next year taking on a brown and yellow hue. Many eels migrate into freshwater rivers, streams, lakes, and ponds while some remain in estuaries.

American eels transform into “silver eels” when they reach sexual maturity, which takes anywhere from 2 to 18 years. At this point, they begin their fall migration downstream back to the Atlantic Ocean and Sargasso Sea to spawn and die. The American eel is considered an endangered species in part because of habitat loss and barriers such as dams which block migration. Resource: https://www.vims.edu/research/units/programs/eel_survey/life_history/.

Eastern red bat. Eastern red bats are found throughout Virginia and across the eastern U.S. In fact, you may have witnessed their evening maneuvering as they forage for insects buzzing around streetlamps just after sunset. These mammals are mainly solitary, curling up and roosting (very well-camouflaged) in trees during the day, but will often travel in groups during their fall migration.

Throughout the summer, the bats bulk up on insects using echolocation to locate their prey. (They have also been known to eat spotted lanternflies.) Around October most eastern red bats will begin their migration in search of warmer temperatures and food, but some will stay in the northern part of their range and hibernate through the cold winter months. For those staying put, they will find shelter in hollowed out trees and under leaf litter and logs. Eastern red bats will emerge from hibernation in March and migrate to their summer spots in April. Baby bats (called pups) are typically born in May and June. Mating takes place in August and September, but sperm is stored in the female through the winter until fertilization, and subsequently birth takes place in the spring. Resource: https://loudounwildlife.org/2007/04/bats-loudoun-night-flyers-part-1/.

Much of the success of these migratory animals depends on having access to appropriate habitat and food sources. What can we do? Plant some native flowers and trees, opt out of pesticides and fertilizers that contaminate waterways, leave leaf litter, snags, and fallen logs, and turn off any unnecessary outside lights at night that may confuse nighttime migratory animals. Our efforts can help ensure that these amazing animals will be around for generations to come.