Birds aren't the only animals on the move right now. Some dragonflies are also migrating, and as they do you might see swarms of them flying overhead.

Not all dragonfly species migrate, but a few do. The ones you are seeing flying around in such large numbers right now most likely are common green darners. These are a common and familiar dragonfly species in Illinois and beyond. They are between 2 1/2 inches to 3 inches long with a green thorax, green eyes and a green face, according to the Illinois Department of Natural Resources. They also show flashes of bright blue on their long bodies, with five of the six segments on their abdomens blue in color, with the first segment being green. 

At this time of year, it's not uncommon to see large groupings or even swarms of the common green darners clumsily flying over prairies, roadways and other open expanses. The dragonflies aren't actually migrating when we see them aloft. Instead, they are feeding and mating during pitstops on their migration, WTTW reports.

Their migratory flight pattern is less noticeable to us because they are not as close together and they are all flying in the same general direction, according to WTTW. On their pitstops, they bunch together while feeding and mating, and this makes it easier for us to see them. When you see them near water, they are likely engaged in mating. Groupings of the dragonflies removed from water are more likely to be hunting for food.

Not all green darners migrate. A segment of the population remains here all year, overwintering as nymphs, but the remaining dragonflies will begin to migrate south in August or September each year. It is believed that the migratory population spends the winter in Florida, eastern Mexico and the West Indies, IDNR reports.

The common green darners that migrate south for the winter are not the same ones that will return here next spring. Three generations of the insects complete the full migratory journey. To start the cycle, first-generation common green darners emerge as adults from lakes and ponds in the southern United States, Mexico and the Caribbean, the Smithsonian Magazine reports. Those dragonflies will make the long journey north to their breeding grounds. Once they arrive, they will lay their eggs and die. 

The second generation of the insects will then hatch, reaching maturity between July and October. Once mature, they will begin their migration south, arriving to lay their eggs and die, according to the Smithsonian. Those eggs represent the third generation, and those dragonflies live their adult lives in the south then lay their eggs. It's these eggs that will hatch and make the northern migratory journey in the spring.

As with other migratory animals like monarch butterflies and many birds, scientists do not fully understand how the dragonflies know when to migrate and where to go, the Smithsonian reports. It is thought that temperature and changes in daylight influence the timing of the migration, but how they know where to go even though they have never made the journey before remains a mystery.