Dragonfly Migration

Migrating monarchs are so lovely and welcome that people pay a lot of attention to them. But they're not the only insects that migrate. Millions of dragonflies also make a journey south each fall. Massive swarms commonly follow the passage of cold fronts.

Counting All Dragonflies: A Food Chain Connection
Frank Nicoletti is a professional hawk counter and bird-bander. His eyes have seen millions of raptors throughout the world, and his hands have held thousands. He spends every fall beside Lake Superior in Duluth, Minnesota, counting the hawks that fly past Hawk Ridge Nature Reserve.

During hawk migration, Frank noticed that both dragonflies and American Kestrels (our littlest falcon) migrate during the month of September along Lake Superior. He also noticed that kestrels seemed to migrate more on days when dragonflies were moving. But he couldn't find anything in the literature explaining this.


Green Darner	American Kestrel
Visit the Digital Dragonfly Museum	Listen! The kestrel's call sounds like "killy killy killy." Sound courtesy of Lang Elliott

It turns out that scientists knew very little about the relationship between dragonflies and hawks. So, in 1995, Frank started counting dragonfly numbers as well as hawk numbers. He used two mechanical clickers to keep count. For a whole month--over 8 hours a day--he recorded the total number he saw every hour. That September, he counted 1,106 kestrels and an amazing 10,330 Green Darners migrating past Hawk Ridge.

Frank wrote a scientific paper for "The Loon", an ornithologists' journal. He used his data to show that kestrel and dragonfly migrations are associated. Here's what he observed: During midday, when migration conditions are best for both, kestrels don't eat many dragonflies. (Probably because the kestrels are too busy flying!) But later in the afternoon, when the kestrels are flying low again, the majority of them are seen eating dragonflies. One of his observations may seem puzzling. How would you explain this?

You're the Scientist

Enlarge Graph for Printing
(Click Image)

Imagine that you're Frank Nicoletti. You've been asked to give a talk at a scientific meeting to explain your findings. Using the numbers from the graph on the left, describe the data you collected on Hawk Ridge in Fall, 1995.

What evidence could you give to show
that dragonflies and kestrels migrate together?

Frank is back counting hawks this fall--but he isn't counting dragonflies anymore. (Counting over ten thousand dragonflies in addition to a hundred thousand hawks is too hard on the eyes!) But he still pays attention to dragonflies. Last weekend, there were thousands flying along the Lake Superior shoreline. And sure enough, there were a lot of kestrels. The best days were September 5, 6, and 7, when Frank counted 104, 349, and 98 kestrels respectively.


September 5 104 kestrels	September 6 349 kestrels	September 7 98 kestrels
(Maps produced by Purdue University Weather Processor.)

Where Are the Dragonflies Going?
Two different populations of Green Darners live in Canada and the U.S. The RESIDENT population does not migrate. They breed in the north over the summer, and lay their eggs in northern water. The babies, or nymphs, spend the winter in that cold water beneath a thick layer of ice. In spring, they emerge from the water and spend the summer as adults.

The other population of Green Darners is MIGRATORY. They arrive from southern regions each spring to breed in the north. Their young emerge in late summer of that same year, and migrate south during August and September. Apparently the migratory population alternates generations between breeding in the north and breeding in the south, but both groups of this population get to spend the majority of the summer in the land of mosquitoes, and maximize the number of babies they produce.

Ornithologist's Answer to Journaling Question
What evidence could you give to show that dragonflies and kestrels migrate together? Frank Nicoletti's graph shows the numbers of kestrels and dragonflies every day in September. Every day with a big number of dragonflies has a big number of kestrels, and every day with a low number of dragonflies has a low number of kestrels. It's easy to see that the graph's peaks and valleys for both species happen on the same days. Scientists say these numbers are correlated.