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What's the biggest group of animals ever recorded on Earth?

For these gregarious animals, ‘alone time’ is a fantasy.

In early 2020, ornithologist Noah Strycker found himself walking amongst several thousand chinstrap penguins on Elephant Island, a remote blip of snow-covered rock just off the Antarctic Peninsula. He was there to carry out a census of the island’s penguin colony, which hadn’t been properly surveyed since 1970. “I’ll never forget the sight, sound, and…smell,” joked Strycker, a graduate student at Stony Brook University in New York, as well as a professional bird watcher, and author.

The survey that he and his colleagues eventually produced revealed that chinstrap penguin numbers are in decline. But despite this, this species actually forms the biggest colony of penguins on Earth – gathering in the millions in some Antarctic locations. But counting these animals doesn’t daunt Strycker, who has actually developed something of a hobby for this task. 

It started a few years ago when he found himself pondering how many starlings were contained in the magical murmurations that these birds form, and which swell and undulate across the evening sky in many parts of the world. “They are quite beautiful. It almost looks like smoke,” Strycker told Live Science. “And it just gets you wondering, how many of them are there?” The answer, he discovered, was that there are roughly 1 million in the average murmuration, all soaring and swooping in unison. That discovery spurred Strycker on to answer an even more ambitious question: beyond birds, what’s the biggest group of animals ever recorded on Earth?

Related: What’s the first species humans drove to extinction?

Answering this question takes us to some very interesting places — back into the past, up into the sky, down into the ocean and sweeping across desert plains. It offers magnificent proof of the abundance of animal life on Earth, but it also points to humanity’s role in reducing — and, unexpectedly, increasing it too.

Thousands, millions, billions

When Strycker embarked upon his unusual quest, he shared his discoveries in his book called “The Thing with Feathers: The Surprising Lives of Birds and What They Reveal About Being Human” (Penguin Random House, 2014). As the title suggests, birds are high contenders for the title of most numerous group. At 1 million per flock, starling numbers are jaw-droppingly high – but they’re easily outnumbered by chinstrap penguins, which can reach 2 million on the South Sandwich Islands off Antarctica. 

But those charismatic penguins fall far behind the red-billed quelea: this small species that can gather in single flocks of several million over savannah and grassland areas in sub-Saharan Africa — so huge that they seem to roar as they pass overhead. “I think they’re considered now to be the most abundant species of bird in the world. And they do make very large flocks in the millions — tens of millions, maybe hundreds of millions,” Strycker said. Their explosive success as a species may be helped by agriculture’s spread: these birds consume grass seeds, but they’ll also settle for fields of cultivated grain. As such, they’re loathed by embattled farmers who lose huge shares of barley, buckwheat and sorghum harvests to these birds every year.

Quelea are so numerous that observers say it can take five hours for a flock to pass overhead. But here is where this species yields to an even more populous bird that once was abundant across American skies: the passenger pigeon. “There are stories of people standing there and watching a single flock of passenger pigeons fly over them for hours or days at a time, which is crazy!” Strycker said. One gathering in 1866 was recorded as 1 mile (1.6 kilometers) wide and 300 miles (482 km) long, and was estimated to contain about 3.5 billion birds, based on the number of pigeons per square mile and extrapolated across the size of the flock. Of course, that was before hunting drove this successful species to extinction

So surely with that grand tally, this pigeon of yore takes the prize for most populous creature on Earth? Not so fast: there are quite a few other contenders to consider still. 

Related: Why are there so many pigeons?

Shifting our gaze down from the skies, and into the ocean’s depths, there are records of fish species — specifically Atlantic herring — gathering in schools that exceed 4 billion, Strycker explains in his book — the passenger pigeon’s closest contender for the reigning title so far. Other species don’t come close to the numbers tallied up so far — but they’re still so impressive to behold that they deserve a mention. These include migratory mammals like springbok and wildebeest in southern Africa that have, in the past, gathered in herds exceeding 1 million, forming vast processionals that march across the sun-beaten savanna for weeks. These are further outstripped by their winged mammalian cousins: in Texas, there’s a single cave that’s home to more than 20 million Mexican free-tailed bats, whose closely-packed bodies transform the cave’s interior into a rippling, writhing mass.

Yet there’s one animal whose enormous gatherings leave all these other contenders behind in a trail of dust. (Or rather, a trail of decimated vegetation and ravaged crops.)

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