Strength in Numbers: 5 Amazing Animal Swarms

While a single animal may not catch the eye, when thousands or millions of them come together, it is an awe-inspiring—and sometimes fear-inspiring—sight to see. 

Swarms of Asian giant hornets have been in the news recently, after killing more than 40 people in northwestern China. (Read “Swarm Theory” in National Geographic magazine.)

But not all swarms are deadly. Animals congregate for different reasons: to migrate, find food, or defend against predators. These swarms are some of nature’s most magnificent spectacles. Read on to discover five incredible animals that swarm together to survive. (Watch a video of stellar animal swarms.)

Red Crabs

Each year, Christmas Island, a small Australian territory that is home to 1,500 people, gets blanketed in 60 million red crabs making their way to the sea.

Photo of a Christmas Island Red Crab mass crossing closed a road during annual migration.
Christmas Island red crabs cross a closed road during their annual migration. Photograph by Ingo Arndt, Minden Pictures/Corbis

At the beginning of the wet season in November or December, the crabs leave their burrows on the rain forest floor and crawl up to six miles to reach the ocean. There, the crabs mate.

The males march back inland; the females remain at the coast until their eggs develop. Just before dawn at high tide, the females release their eggs into the sea—up to 100,000 eggs each. After about a month, the larvae develop into baby crabs and leave the water to follow their parents’ footsteps back across the island and to the forest. (Related: “Pictures: Crab Swarms Overtake Island—Mystery Solved.”)

The baby crabs, only five millimeters across, spend their first three years hiding on the forest floor, until they are old enough to make the trek to breed and spawn themselves.

Free-Tailed Bats

Each evening from March through October, the skies of Austin, Texas, darken with bats on the hunt for food.

The Congress Avenue Bridge in downtown Austin is home to the largest urban bat colony in North America. Between 750,000 and 1.5 million Mexican free-tailed bats make their home there. Their nightly emergence has become a tourist attraction.

The colony consists mostly of pregnant females who have migrated north from Mexico to give birth, usually around June and July. By mid-August, the pups also begin flying, doubling the number of bats emerging from the bridge.

Mexican free-tailed bats have been known to fly up to 100 miles (160 kilometers) in an evening foraging for insects. It’s estimated that a colony of this size can consume 30,000 pounds of insects in a single night.

Desert Locusts

Notorious as a biblical plague, these insects are still capable of wreaking devastation on crops.

Photo of a Senegalese boy watching locusts swarm in Mbour village.
A Senegalese boy watches locusts swarm in Mbour village, northern Senegal, in 2004. Photograph by Pierre Holtz, Reuters/Corbis

Desert locusts are usually shy and solitary insects that live in dry regions from North Africa to India. But when conditions force them into close proximity, they make a remarkable transformation.

In this so-called gregarious phase, locusts turn from tan and green to black and yellow. They become aggressive, seeking out other locusts and coming together in a ravenous swarm. Locust swarms can contain a billion or more individuals that consume their own body weight daily. Large swarms can stretch for dozens of miles and strip entire fields of vegetation in minutes.

One of the worst locust swarms in modern times took place in 2004, when the insects swept across all of northern Africa, from Mauritania to Egypt, and eventually reached as far east as Israel and as far north as Portugal.

Monarch Butterflies

Each fall, tens of millions of North American monarch butterflies journey up to 3,000 miles to escape the cold winter months.

Monarchs east of the Rocky Mountains fly to central Mexico, while those west of the Rockies make their way to the California coast. These orange-and-black beauties migrate farther than any other butterfly, and they are the only butterfly to make a regular north-south migration like a bird. 

At their winter hibernation sites, monarchs cluster together to stay warm. Tens of thousands of butterflies can cluster on a single tree. (See more swarm pictures in National Geographic magazine.)

As they return north in the spring, the monarchs mate, lay eggs, and die. Successive generations continue to journey north, taking three to four generations to reach the northern United States and Canada.

Starling Murmurations

Videos of flocks of starlings in flight have gone viral. They feature a stunning example of collective behavior known as a starling murmuration. These aerial spectacles can be seen at dusk, when starlings flock together and seem to move as one. A murmuration can be made up of a few hundred to tens of thousands of birds.

starling picture
A flock of starlings. Photograph by Wolstenholme Images, Reuters/Corbis


What makes them mesmerizing is the strong coherence and extremely synchronized movements of the starlings. Scientists have recently determined that starlings in murmurations coordinate their movements with their seven nearest neighbors.

When one bird changes speed or direction, all the other birds in the vicinity respond in kind, and they do so nearly simultaneously. In this way, information spreads across the flock rapidly.

These graceful ballets are often responses to an approaching predator like a hawk, but also seem to occur spontaneously. Perhaps the starlings find their behavior as beautiful as we do.

Follow Mary Bates on Twitter and Facebook.

Mary Bates is a freelance science writer living in Boston. She has a PhD in psychology from Brown University where she studied bat echolocation. You can visit her website at and follow her on Twitter at @mebwriter.

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