Some species of birds, fish, mammals, and insects can gather in large flocks consisting of millions to trillions of individuals.
Birds
In early 2020, ornithologist Noah Strycker from the University of Brook in the United States found himself walking among thousands of Macaroni Penguins on Elephant Island, a remote, icy land off the coast of Antarctica. He was there to conduct a survey of the penguin colony, which had not been properly surveyed since 1970.
Strycker’s latest survey, along with his colleagues, revealed that Macaroni Penguins actually form one of the largest bird flocks in the world, with around 2 million birds concentrated in certain locations such as the South Sandwich Islands. This discovery prompted Strycker to seek answers for an even more ambitious question: what is the largest group of animals ever recorded on Earth?
A flock of Macaroni Penguins on Elephant Island in Antarctica. Photo: Wolfgang Kaehler
Strycker embarked on his mission and found that the penguin flock at South Sandwich could easily be “surpassed” by another bird species known as the Red-billed Quelea, which lives on the grasslands of sub-Saharan Africa.
“They gather in very large flocks with millions, tens of millions, and can reach up to hundreds of millions,” Strycker stated. “I believe they are the most abundant bird species in the world.”
The Red-billed Quelea is so numerous that observers report it takes five hours for a flock to fly overhead. However, they are still not the largest bird flock.
“There are stories of Passenger Pigeons flying over observers for hours or even days. It’s insane!” Strycker added. A gathering in 1866 was recorded to stretch 1.6 kilometers wide and 482 kilometers long, estimated to contain around 3.5 billion Passenger Pigeons, based on the countable number per square mile and extrapolated by the size of the flock. Today, the Passenger Pigeon has been hunted to extinction.
Fish
Moving from the high skies to the depths of the ocean, there are records of one fish species—specifically, the Atlantic Herring—gathering in schools of over 4 billion individuals, far surpassing the previously most qualified candidate, the Passenger Pigeon.
Mammals
Mammal species do not reach the same large numbers as Atlantic Herring, but they are still significant to mention. These include Springbok and Blue Wildebeest in southern Africa. In the past, they have gathered in herds of over 1 million, parading across the grasslands for weeks.
But most impressive is the Mexican Free-tailed Bat (Tadarida brasiliensis). In Texas, there is a single cave that is home to over 20 million of these bats. When they all leave the cave to forage, they create a massive, undulating cloud in the sky.
A herd of Blue Wildebeest along with some zebras during migration inside the Masai Mara Reserve in Kenya. Photo: AfriPics
Locusts
The record for the largest flock in the animal kingdom belongs to locusts. In East Africa in early 2020, a cloud of desert locusts swept across the sky, spanning thousands of square kilometers.
“It looked like a black blanket covering the sky, so dense that it was very difficult to see through the clouds,” described researcher Emily Kimathi from the Insect Physiology and Ecology Center in Kenya.
This special event was the largest swarm observed in the Horn of Africa in 25 years. Experts estimate that they gathered at a density of about 50 million individuals per square kilometer, meaning that a single locust swarm could contain about 200 billion individuals. Desert locusts are known for their rapid reproductive capabilities, which can increase their numbers by 20 times within three months.
A man trying to chase away a swarm of desert locusts at a farm near Nanyuki town in Laikipia, Kenya. Photo: Reuters
200 billion is an astonishing number, but historical data suggests that locust swarms could grow much larger under ideal environmental conditions. In 1875, a meteorologist named Albert Child was astonished to see locusts flying across the sky in a massive swarm that covered much of the western United States. This species is the Rocky Mountain Locust, and Albert estimated they covered a sky area of 512,800 square kilometers.
This historical event is now referred to as “Albert’s Locust Swarm.” Based on the meteorologist’s estimates, their numbers could have reached up to 3.5 trillion individuals. This is considered the largest number of animals in a single flock ever recorded by humans.
Like the Passenger Pigeon, the Rocky Mountain Locust is now extinct, with the last living specimens observed in 1902 in southern Canada, but their historic flight remains a record that is very challenging to break.
