FEAR NOT— we’ve still got plenty of summer here in Vermont and points north. So in this edition of VCE’s monthly field guide to nature, we’ll celebrate a few audacious summer insects. But we’ll also alert you to animals on the move. Yeah, the “M-word.” So if you’re not quite ready for fall migration, well, sorry… too late.
Bird Migration In August?
Afraid so. Take Tennessee Warblers, for example. They blast through Vermont in May on their way to boreal forests to breed, and now they’re trickling back through on their southward journey to wintering grounds in northern South America. “Fall” migration can begin for some individuals as early as July. They are gone long before Staghorn Sumacs and Red Maples catch fire and draw busloads of human migrants to Vermont. That’s one of the dirty little secrets about fall migration—it actually begins in summer. To be sure, these Tennessee Warblers are an early “fall” migrant, but it is by no means unusual. If you wait until next month for shorebirds, for example, you will have missed a huge part of their migration. Many have been departing Arctic breeding grounds since mid-July. So, in time for the shorebird migration, VCE research associate Bryan Pfeiffer offers us his annual “Solving Shorebirds” lesson and his “Headless Shorebird Challenge.” And, be sure to add your bird observations to Vermont eBird, a project of the Vermont Atlas of Life, to help us track their populations.
The Buzz on Northern Dog-day Cicada
The dog days of summer are upon us and so is the buzzing sound of the Northern Dog-day Cicada. Males are calling to females with a high-pitched buzz lasting about 15 seconds. It starts softly, gets louder, then tapers off—the only species in this area that sounds like a buzz saw.
These lazy, hot, dog days of August are named for Sirius (the “dog star”) in the constellation Canis Major. Sirius appears in northern skies around the same time that this cicada emerges and starts singing.
Males produce the buzz by using a pair of ribbed membranes, called tymbals, located at the base of their abdomens. A muscle pulls all the tymbal ribs inward. The ribs make a short, sharp noise when they snap inward and outward. The cicada repeats this 300 to 400 times per second to create a the buzz, which can be heard up to a quarter-mile away, and hopefully entice a female. If she’s interested in the song, she snaps her wings. The males hear this and move closer and closer, hoping to eventually mate.
Female cicadas use their ovipositor to saw small gashes into tree twigs, where they lay eggs. After about six weeks the nymphs hatch, drop to the ground, burrow down to a tree root and suck its juices for about three years to maturity. Then one July or August day, Dog-day Cicadas crawl out of the ground and climb up tree trunks as mature nymphs, where they split open and emerge as adults with long wings and bulging eyes—only to live for a few weeks.
Hear cicadas or other insects singing? Make a recording with your smartphone and submit them to the Vermont Atlas of Life iNaturalist project. Its fun and easy!
Monarchs Begin to Make their move
The Monarch flaps in my net as I reach in and carefully pull it out. I peel an adhesive tag the size of my small fingernail from a sheet and gently stick it on the middle of the butterfly’s wing. This monarch will now be known as CAK 682, the code on its new Monarch Watch wing tag. You can get tags too and help track their flight.
Where is this Monarch headed? Perhaps to the mountains of central Mexico where they overwinter at just 13 known sites. (A 1984 study found that there may have been as many as 60 overwintering sites, but commercial logging has eliminated many.) A single site may contain millions of Monarchs per acre and cover up to eight acres of forest.
The butterflies arrive from the north between November and late December and generally hang out on fir trees, metabolizing fat reserves that they built up during migration. Remarkably, they actually gain weight on migration and arrive on wintering grounds with fat reserves for the winter, unlike songbirds, which require huge fat stores to burn during migration.
Butterflies in overwintering sites begin to disperse in March and early April, and they then migrate to the Gulf Coast of the southeastern U.S., where females arrive just as milkweed is sprouting. They lay eggs on the fresh plants and then most butterflies die. One or two generations of Monarch caterpillars feed on milkweed before it becomes too hot and dry for the milkweed to persist. The newly minted adult butterflies then continue the northward migration and arrive in New England in June (sometimes May), just as our milkweed flourishes. The females lay eggs here and die.
It takes about 30 days to grow from Monarch egg to caterpillar. Beginning in mid-August, after several generations of monarchs have been produced here, decreasing daylight triggers a physiologic change in Monarchs, causing this generation to migrate back to the wintering sites in Mexico and complete the annual cycle. Incredibly, the adults that leave here have never seen Mexico—yet somehow they are guided back to these small sites.
How do Monarchs navigate to Mexico where their great-great-great-great grandparents wintered? Recent research has shown that Monarchs, using a clock in their antenna to keep time and eyes that record the azimuth of the sun, use a time-compensated “sun compass” to maintain a southwesterly direction during flight.
The winter generation of butterflies lives up to eight months while the successive spring and summer generations are lucky to live a single month. It takes four or five generations of spring and summer Monarchs to produce the final “super generation,” which migrates to Mexico in the fall and then back to the southern United States in spring.
How do we know that Vermont Monarchs actually make it to Mexico and are not enjoying the winter in Florida or the Caribbean where there are some resident, non-migratory Monarchs? One way is by tagging them.
With millions of Monarchs, the odds of a recapture are poor; however, there have been eight tagged Monarchs from Vermont found in Mexico. The first lucky tag was applied in Essex Junction on September 9, 1999, and the monarch was found in El Rosario, Mexico on March 1st, 2,320 miles away!
We also know that Monarchs from the Northeast winter in Mexico by testing their chemistry. Hydrogen isotope analysis shows that New England monarchs winter in Mexico. Rainwater contains slightly different amounts of these isotopes across North America, and this unique chemical signature is transferred from rainwater to milkweed to caterpillars to adult Monarchs. By selecting sites scattered across eastern North America, researchers have created a map displaying the chemical levels for each region. One study found that on average, only about 15% of the Monarchs overwintering in Mexico originated in the Northeast, while 38% are from the Upper Midwest.
Autumn Monarch migration begins at the very northern end of their range in mid-August and will be kicking into migration gear by the end of the month here in Vermont.
I check to make sure the tag has adhered to the Monarch’s wing. I note the date, location, and tag code in my notebook. The monarch flaps out of my grip and climbs high into the clear sky and glides out of sight. Maybe CAK 682 will be a lucky tag. Buen viaje, monarca!
It’s time for Mission Monarch across North America! Have you added a Monarch survey yet? It is so easy to do and takes as little as 15 minutes of your time! Find a milkweed patch of any size anywhere, survey the plants for eggs and caterpillars, add your data to Mission Monarch! And when Monarch migration begins, add your Monarch butterfly sightings to Journey North and help track their travels to Mexico.