Barn swallows and dragonflies are two completely different species, yet they swarm for the same reason: food, specifically the small flying insects favored by both. Each must hunt and consume as many insects as they can during the warmer times of year. All the barn swallows and dragonflies in the area act on their species' biological impulses simultaneously, which quickly leads to swarms.
Yearly Cycle of Barn Swallows
Barn swallows are a common type of bird with a range covering all of the United States except the Gulf coast, and extending up to Canada. They are easily recognizable by their deeply forked tails. Barn swallows migrate to South America during winter, and then return once the weather is warm. During the summer, barn swallows race to complete a nest, and then breed and raise their young.
Barn Swallows Swarming
Barn swallows swarm in an effort to catch enough insects to feed themselves and their babies. Barn swallows are monogamous and share responsibility for the babies; while one mate is incubating the eggs or staying with newly hatched babies, the other must hunt enough insects to feed all of them. Sometimes the barn swallow must fly in circles adding up to 600 miles per day to catch enough insects, according to the Chesapeake Bay Journal article "600 Miles Just to Eat?"
Yearly Cycle of Dragonflies
Dragonflies are some of the most ancient flying creatures on the planet, according to the Bay Nature article "Why Do Dragonflies Swarm?" They are cold-blooded and need warm weather, 63 degrees F or above, to be active. Therefore, the warmer times of year are busy for the dragonfly with hunting, eating and breeding.
Dragonflies swarm to take advantage of the warmer months before having to migrate south to escape the cold. Warm, wet weather is ideal for dragonflies, and they will congregate in groups of thousands or millions around swampy areas or standing water to prey on mosquitoes and breed. Dragonflies do not bite and do not pose a threat to people or crops; they provide a valuable service in decreasing the mosquito population. Swarms are a short-lived phenomenon, according the Fox 6 News story "Dragonflies Swarming," the dragonflies will head south once the temperature drops.