Migration Isn't Just For The Birds

 

Wandering Glider dragonflies do more than wander...

Most people who are interested in nature know that each spring many species of birds migrate north, and each fall they migrate south.  But birds aren’t the only animals that participate in this amazing natural phenomenon, as even some insects (other than the well-known Monarch butterfly) participate in long-distance migrations as well.  While insects are the most species-rich and abundant group of macroscopic organisms on the planet, understanding many aspects of their annual cycles and behavior drastically trails behind the ability to identify and classify them. 

Migration is defined as the cyclical travel of an animal as it returns to its place of departure.  It can be accomplished in one or multiple generations.  Some animals don’t migrate at all, and others move only short distances south or north.  While the guiding factors for navigation are still being discovered, migration is typically triggered by local climate, food availability, seasonality, or reproduction.

Adult Common Green Darner

Interestingly, about nine dragonfly species are known to migrate, including the Common Green Darner (Anax junius), Wandering Glider (Pantala flavescens), as well as some species in the Skimmer family such as meadowhawks and saddlebags. These dragonflies are known to travel up to 1500 miles over the full cycle of their migration, taking three generations.

Adult Wandering Glider

The secrets of dragonfly migration were discovered and confirmed by biologists measuring the chemical makeup of their wings.  In their immature or nymph stage, dragonflies grow their wings while still developing in local water.  Because the stable hydrogen isotope concentration varies in concentration by latitude, biologists can estimate the geographic region where adult dragonflies developed as nymphs.  

The female Common Green Darner (bottom) is held by
the male (top) after mating, when ovipositing in water.

From February to August the first generation emerges in the south and migrates north.  From June to October the second generation emerges in the north and migrates south.  Both of these generations have the ability to undergo rapid development from egg to adult.  But from November to March the third generation emerges in the south and is non-migratory, focusing mainly on producing another first generation for the cycle to start again.  It is this generation that employs a different strategy, developing slowly in late fall, entering diapause or actively induced dormancy over the winter, and emerging in early spring.

Dragonfly migration was not well understood until very recently.  Research published in 2018 detailed the journeys of Common Green Darners by analyzing 852 wing samples from eight different countries and utilizing 21 years of citizen scientist observations to link each specimen to its place of origin and establish its migration history. However, it is important to note that the timing of migration and the development of nymphs are both highly temperature dependent, so continued climate change could lead to fundamental changes in the migration cycles for dragonflies and other migrating insect species. So as you can see, migration isn’t just for the birds!

An Overwintering Texan

Rufous Hummingbird (male), Selasphorus rufus.

Late August into September typically marks migration season for hummingbirds, when most individuals move from their northern breeding grounds to their southern wintering grounds. Several factors affect this seasonal movement including amount of daylight, the angle of the sun relative to the bird’s location, availability (or lack of) food resources, and local weather patterns. Mature birds often start their migration earlier than juveniles, and males typically migrate a few days before females.  But the longest migration of any hummingbird species belongs to the Rufous Hummingbird (Selasphorus rufus), a species that can typically travel from as far away as Alaska to spend the winter in Mexico. 

A fairly small hummingbird with a nearly straight, slender bill, fairly short wings that don’t reach the end of the tail when the bird is perched, and a tail that tapers to a point when folded, the Rufous is like no other hummingbird in terms of color or behavior.  Males are bright orange on the back and belly with a vividly iridescent copper-red throat, while females are green above with orange-washed flanks and often a spot of orange in the throat.  They are the feistiest hummingbird with a gift for fast, darting flight and exceptional maneuverability, tirelessly chasing away other hummingbirds wherever they feed.  Males court females with elaborate flight displays, including J-shaped dives and nearly horizontal figure 8s.

Rufous Hummingbird (female)

In recent years, the Rufous has become the most common overwintering hummingbird in the southeastern United States, particularly along the Gulf Coast.  For the last several years we have kept a small hummingbird feeder on our back porch filled throughout the fall and winter, and have been regularly rewarded with an overwintering Rufous.  This species seems particularly able to handle the colder temperatures, perhaps because they go into ‘topor’ overnight, a reduced physiological state where their body temperature and metabolic rate are reduced.

While it has been proven that this species has an excellent memory for location, which may explain why they find our feeder year after year, it remains a mystery to scientists as to why these birds don’t complete their traditional fall migration to the Pacific coast of Mexico.  While providing a nectar feeder does not delay a hummingbird’s migration, scientists are investigating the theory that established shifts in climate and flower-blooming times are affecting their typical patterns.  Not only do these shifts appear to affect where these birds overwinter, but they also affect the timing of the clockwise circuit they make each year as they move northward up the Pacific coast in late winter and early spring, and travel southward along the chain of the Rocky Mountains in late summer. There is still much to learn about these migration patterns, and why these hummingbirds show up in places we don’t expect them to stay in winter.    

Regardless of reason, we feel fortunate to have our yard brightened during the colder months with this colorful visitor.  Why not keep a hummingbird feeder filled in your yard this season, and you just might find you have an overwintering Texan, too!