The Great Monarch Migration

In mere weeks, a humble insect will begin an incredible journey. The current generation of monarch caterpillars, Danaus plexippus, is preparing to transform and make their annual migration. The butterflies take to the wind as the end of summer draws close, bound for warmer weather in Mexico. Most of the monarchs born in our stretch of the Blue Ridge will depart by mid October, though we may continue to see populations from further north passing through until the first frost. Once in Mexico, hundreds of millions of individuals crowd flowers and hang from trees in enormous roosts- also called bivouacs- while they wait out the winter weather. 

Some of the butterflies in our collection. The three in the bottom left box are monarchs, Danaus plexippus. The middle butterfly is a male, evidenced by the pheromone-producing black spots on the vein of the hind wing called his “pouches.” The top and bottom are females, who tend to have thicker black lines along the hind wing veins and lack these pouches. The box above them holds three commonly confused, but still very cool, Viceroy butterflies- Limenitis archippus. They mimic the coloration of monarchs and are similarly distasteful to predators. Notice the continuous black arc on the hind wing that runs parallel to the wing border present only on viceroy wings.

A female monarch visiting milkweed in our rain garden, likely selecting a suitable plant to host one of her young

When temperatures start to rise and days grow longer in northern Mexico, the monarchs prepare to make their return flight by increasing nectar consumption and beginning to mate. Come March, the butterflies depart- following the spring warmth as it stretches north. On their return flight, the butterflies fly low and stop often to feed and lay eggs. A female monarch may carry more than 300 fertile eggs at a time, and will continue finding new mates as she journeys onward. Mother monarchs are very particular about where they will leave these eggs. While the adult butterflies forage nectar from a variety of flowers, the caterpillars exclusively feed on milkweed species. She carefully examines each milkweed plant she finds and, should it pass her inspection, lays a single egg on the underside of a leaf near the top of the plant. This ensures her caterpillars have plenty of food to grow quickly without competing with others or decimating the plant. 

Monarch caterpillars feed exclusively on milkweed- a plant that is toxic to many other animals. The monarch stores these toxins in its own body so predators learn not to eat them.

Many of you are aware of the monarch butterfly migration; and we all are familiar with the four stages of their complete metamorphosis from egg to caterpillar, then chrysalis and finally adult butterfly.  However, the way these two life cycles interact may come as a surprise. In our region, D. plexippus experiences 4 generations every year. We have two generations of summer monarchs composed of small, short-lived individuals who will only know this land as home. The successors of the second summer monarchs are significantly larger, stronger, and longer-lived (Up to 9 months as a butterfly!) It is this generation, the fall monarchs, that will make the entire flight to Mexico and return more than halfway back, dispersing their eggs the entire return flight. From these eggs emerge the spring generation, who will complete the migration past the US’s northern border, recolonizing their entire native range.

Poster created by Paul Mirocha for Monarch Watch. This stylized map shows the seasonal ranges of monarch migration over North America. Our fall monarchs fly to Mexico before winter, then fly up into the South East in the spring. The next generation continues moving north, recolonizing the northern half of the continent. Summer generations breed in the north until the rise of the next fall generation, who will make the entire migration south to Mexico and halfway back.

Despite being one of the most extensively studied butterfly species, there are still many mysteries entomologists are working to solve. A community science project called Monarch Watch has volunteer participants help generate data about monarch populations during the fall migration. Unfortunately, the winter roosts in Mexico have been rapidly shrinking in recent decades, sparking much of the research and conservation efforts going on now. Leading theories of causes for this decline include habitat loss and resource scarcity. Much of their historic breeding and feeding lands have been converted to farms, making the butterflies’ journey even more perilous.

Monarch Watch is a community science project that began in 1992. Participants catch and tag fall migrating monarchs with a sticker that has a unique code. Data is recorded including the tag code, date, location, and information about the individual, before it is released. In the winter, butterflies are recaptured in Mexico and searched for tags. The data generated is crucial for estimating monarch population change over time as well as predicting migration times, routes, and survival rates.

Helping monarch populations can be an easy feat for anyone with the lawn space to grow a few native plants. Research some local long-blooming wildflowers, especially those that provide plenty of nectar and bloom through fall. Asters, columbine, vervain, mountain mint, and many others are fairly low maintenance native wildflowers that are great nectar sources for pollinators, including monarchs. Milkweed prefers to have a fair amount of space and well drained soil that gets plenty of water. However, there are several varieties that prefer different conditions, so researching a variety that would do well in your area is a great way to support the monarch population.

At our latitude of around 36° 50’ N, monarch tagging begins at the end of the month- August 29. Peak abundance usually occurs in late September, and the last of this year’s monarchs might be seen into early November. As we look forward to watching another generation of butterflies follow in the wingbeats of their great-grandparents, let these beautiful creatures remind you of the enormous cycles that make up nature.

Canadian migrant arrives early on the New River

With the arrival of August it seems that summer is almost over, most birds have finished breeding, and cooler weather must be coming soon.  But one of the surest signs of the approaching end of summer is the first arrival of migrating birds that breed in the far north.   While on a bike ride to Byellsby Dam along the New River Trail in SW VA on July 26, I was surprised to see a solitary sandpiper feeding along a mud flat.  It would appear that this bird has already bred in the far north and is now returning to its winter home in Central or South America.    

It is always surprising to me that the breeding of these long distance migrants is so quick, and the distances traveled so great.  Conditions in Canada must be very conducive to breeding to justify the dangers of migration to accomplish this rapid reproductive cycle.  Yet how often do we fail to express our wonder at such amazing feats of avian athleticism and the remarkable evolutionary pathways that led to such migratory behavior?

Dragonfly Migration at Rocky Knob

Watching dragonflies as they fly southward across the pasture at Rocky Knob is for me the favorite part of the fall migration watch. I enjoy the flights of hawks, and swifts and swallows, and butterflies too. Perhaps a close 2nd to the dragonflies are the hummingbirds.

Many years ago it didn't occur to me that dragonflies migrated. I had heard of them swarming in large numbers, but had never seen that behavior until September 13, 1992. On that day at the Rocky Knob pasture, I saw several hundred per minute, and all of them were heading south. There were dragonflies across the entire field, and as high up as I could see with binoculars. After a while I began to realize that these were not swarming the way I thought of a swarm. First, I didn't witness any of them veering to catch some small insect. And second, I wasn't being attacked by gnats. As a wild guess, I believe that the minimum number of dragonflies I saw was about 250,000. I might have seen many more than that. That September afternoon is still the day, by far, with the greatest tally of dragonflies that I have seen.

I decided that day to learn what some of the dragonflies were, but it took me several years to be able to identify several species on the wing.

This photo is of Common Green Darner, the most abundant and frequently seen migrant most days at Rocky Knob. The species has been recorded at many of the hawk migration watch sites, and has been noted at the hawk site at Veracruz, Mexico in the many hundreds of thousands. Photo 

© Bruce Grimes

These are the most common species seen at Rocky Knob (listed in order of abundance):

Common Green Darner (Anax junius)

Black Saddlebags (Tramea lacerata)

Wandering Glider (Pantala flavescens)

Twelve-spotted Skimmer (Libellula pulchella)

Spot-winged Glider (Pantala hymenaea)

Black Saddlebags, number two in number at Rocky Knob, and it is quite easy to i.d. on the wing.

Photo 

© Bruce Grimes

In general these five species are quite easy to identify on the wing, though spot-winged takes a little more observing to be sure of the species. Observers at Rocky Knob have had a few days with over 10 to 20 thousand dragonflies. Green Darners and Black Saddlebags are the species most likely to be seen in the thousands at Rocky Knob. Numbers in the hundreds have only happened a couple of times, locally, for Wandering Glider, and a couple of times for Twelve-spots.

Twelve-spotted Skimmer. I think this species is my favorite dragonfly to watch in flight. The patches of white in the four wings can sometimes give a sort of "strobe light" effect. It really is dazzling when I catch sight of one while I am scanning the area with my binoculars. Photo 

© Bruce Grimes

The migration for Spot-winged Gliders occurs earlier in the season than we usually cover at Rocky Knob. But if you go there in July you may see several Spot-wings heading south, and maybe not many other dragonflies. The others reach peak movement usually sometime in August to mid to late September.

Spot-winged Glider is an early migrant at Rocky Knob, often showing up in some numbers in July. Note the spots in the wing are small patches close to the body. This species of rain pool glider dragonfly has a darker brown body than does the Wandering Glider which appears much more yellow or sometimes a bit golden on the wing. Photo 

© Bruce Grimes

Spot-winged and Wandering Gliders are two species you might encounter in parking lots. You might even get to see them lay eggs on your car windshield, a behavior that seems much more than wacky. If you ponder it you might conclude that this egg-laying practice is some sort of exercise is futility, with no chance of the eggs hatching. So why do female Spot-winged and Wandering Gliders do this?

I think I will leave the answer for later, and would encourage readers to reply, with their guesses as to why the egg-laying on cars. Better yet, I wish to encourage you to share your sightings of these two species, and the behaviors you observe. You can just reply to this blog posting after you have seen either of these species.

Wandering Gliders, by the way, have one of the widest natural distributions, world-wide, of any insect. They occur on Easter Island for instance. They also are documented as having the longest distance inter-generational migration. Think Monarch butterfly, and add a lot more miles, and make the flight over the Indian Ocean.  Photo 

© Bruce Grimes

Here is a link to a Ted Talk about Wandering Glider migration (note that the dragonfly is referred to as a globe skimmer, actually a most apt name:

http://blog.ted.com/2009/12/17/discovering_dra/

Several other species of dragonfly have also been observed at Rocky Knob. I believe that most of these were migrating, but am not completely certain that all of them were.

Here is a list of these species.

Shadow Darner(Aeshna umbrosa). Note: members of this genus can be difficult to distinguish in flight, so we could have had more than one species occur at the migration watch site.

Comet Darner(Anax longipes). This species isn't noted as a migrant but we have a couple of records--individuals flying through the pasture when there were fair numbers of green darners on the move. We have been wide-eyed in surprise when we see one.

Swamp Darner(Epiaeschna heros). This species is more of a Atlantic coastal region migrant. We have only a smidgeon of records from the pasture.

Emerald species. Not identified to species at Rocky Knob. Our best guess is Clamp-tipped Emerald (Somatochlora tenebrosa). This species is not noted as migratory either. Maybe the few we saw weren't migrants. But they did head south, up and over the trees at the south end of the pasture. At a couple of other locations Bruce Grimes and I have found this species, and been able to identify. At one mountain in Greene County, Tennessee, Clamp-tipped Emeralds were definitely "in the mix" of migrants along with Common Green Darners, Black Saddlebags, Twelve-spots, a few Wandering Gliders, and as I recall, a Swamp Darner or two.

Eastern Pondhawk(Erythemis simplicicolis) We have several sightings of this species flying low, often just above the grasses, all flying generally southward. A few were mature males with bright blue bodies, and some were immature males or females.

Slaty Skimmer(Libellula incesta). Just a few records. Migrants? They flew south.

Great Blue Skimmer(Libellula vibrans). A couple of sightings. The individuals we have identified on the wing, actually perched for a little while so we could figure out what they were, then they flew on southward. Not sure if this species is really all that migratory or not.

Common Whitetail(Plathemis lydia). Generally not listed as a migrant. We have seen a few that flew southward as if migrating. Of course we aren't sure, more observations needed at RK and at other locations. This species is very common throughout the region.

Variegated Meadowhawk(Sympetrum ambiguum). Three sightings. I am still digging through the data pile to find the exact dates. This is one of the most common migratory dragonflies in the western US, but it is more like a waif in our area (as far as we know). The individuals at Rocky Knob were either females or immature males.

Variegated Meadowhawk was a surprise find at Rocky Knob because this small dragonfly is mainly found in the western states. We have no photo from our watch site, so no proof....just a sight record, but hey we know what we saw!  This picture is of an adult male. Photo 

© Bruce Grimes.

Other meadowhawks, unidentified to species, all individuals with bright red thoraxes, have also been found at Rocky Knob on a few occasions. None recently, but our migration coverage has been scant the past two years.

Carolina Saddlebags(Tramea carolina). A small number are seen each migration season at the pasture. This species has been observed in significant numbers on the Atlantic coast, but is a quite uncommon migrant at Rocky Knob. Large numbers though have been seen at ponds in Franklin and Patrick Counties in lower elevation sites.

We have observed several dragonflies that were not likely any of these species, but they all have that category of "unidentified species".

The Xerces Society has begun an effort to monitor dragonfly migration on a national basis. Here is the webpage about the migration of dragonflies, protocols for monitoring, and more:

http://www.xerces.org/dragonfly-migration/

We plan to submit our Rocky Knob data on dragonflies to this site, and starting in July or August will begin the season anew, with reports sent to the Xerxes project. It's hard to believe that the next migration season is just a little more than a month away.

You are most welcome to join us at Rocky Knob. The more observers the better is my view. We tally many species of birds, the migrant hawks, the warblers, thrushes, finches, hummingbirds and much more. We tally the migratory butterflies, and that's another story in itself. And of course we keep records of the migratory dragonflies.

We often see good migratory flights of raptors, particularly kestrels, when we see good numbers of dragonflies on the move to the south. Our only sighting of Swallow-tailed Kite occurred on a day when we had a fine flight of Black Saddlebags. The kite likely fed on several dragonflies that day.

I encourage you to try watching for migratory dragonflies, and sharing what you find. You can post a note in reply to this blog posting, or submit your data online to the Xerces site, or both.

Happy migration watching to you.