It has long been known that some species of dragonflies are migratory, but our knowledge of these insects was shrouded in mystery. Little was known about how far they migrated, what was their annual cycle, and what were the environmental cues driving this behaviour. A new study conducted by scientists at the Smithsonian Conservation Biology Institute and the Vermont Center of Ecostudies reveals detailed information about the life history and movements of the common green darner, a dragonfly found from Nova Scotia to Mexico. The scientific team used museum specimens and live-caught dragonflies to analyze the forms of hydrogen in the insects’ wings. This analysis was combined with 21 years of field observations by citizen scientists to map the life history and migration of the common green darner. I’ll explain more about citizen scientists shortly, however, let’s look at the intriguing results of this study.
The water in a dragonfly’s wing comes from the pond where they were born. Since a certain kind of hydrogen decreases in the environment from south to north, the wing analysis indicates the region of the individual dragonfly’s birth. The citizen scientists supplied observations on the date of dragonfly emergence from ponds along with habitat information. This collaboration provided the key to unlocking the mystery of dragonfly migration.
The study found that a first generation of common green darners emerge from ponds in Mexico and the southern United States from February to May. These dragonflies migrate north reaching as far as Nova Scotia and New Brunswick in the early summer. Here they lay their eggs and eventually die. A second generation emerges later in the summer and after a period of time migrates south to lay their eggs. A third, non-migratory, generation emerges in November and spends the entire winter in the south. Their offspring emerge in the spring and repeat the journey that their great-grandparents started the year before.
Other interesting findings showed that within a local population of common green darners, some are migratory, and some are not, and migrations are triggered by a very specific temperature, 9.17ºC in the spring and 9.54ºC in the autumn. Thus, these dragonflies have a strategy that reduces the risk of reproductive failure from drought by spreading the risk over a very large area. And with continuing climate change, more dragonflies may be coming our way, arriving earlier in the spring and staying longer in the fall.
So, returning to the question of citizen scientists, what exactly is citizen science? The Cornell Lab of Ornithology, that sponsors many citizen science projects, defines it as “projects in which volunteers partner with scientists to answer real-world questions.” A well-known example of citizen science is the Christmas Bird Count in which hundreds of counts by volunteers throughout North America within a specific period each year goes into a database that tracks long-term trends in bird populations. Indeed, citizen science has become very popular. Here in the tri-county area there are many opportunities to participate in citizen science: Project Feeder Watch, the Canadian Lakes Loon Survey, iNaturalist, hemlock wooly adelgid monitoring, the Great Backyard Bird Count, Project NestWatch, and the Christmas Bird Count to name just a few!
One of the most respected and trusted sources of citizen science information is eBird. Birders enter their bird observations online into this international database. The database in turns offers birders copies of their life list, the opportunity to share their lists online with others, and alerts which notify them when rare birds are nearby. The submitted observations are reviewed by a host of volunteer reviewers, aided by computer programs, to identify possible errors. A recent social science paper in the journal Society and Natural Resources cited the eBird review process as an example of the use of best practices in ensuring trust in the reliability of citizen science. Thus, eBird has now become a part of the data used in numerous scientific studies that concern the distribution, abundance, and movements of birds.