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.

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.

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 — 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.)

In East Africa in 2020, a veil of insects swept across the sky, forming a mass of spiky legs and fluttering wings that spanned nearly 930 square miles (2,400 square km). “It was literally like a black blanket that went over the clouds. It was difficult to even see the clouds,” said Emily Kimathi, a researcher at the International Center of Insect Physiology and Ecology in Kenya. 

That swarm was composed of desert locusts, a species that turns up in huge numbers sporadically in East and North Africa, as well as parts of the Middle East and South Asia. That particular event was the largest swarm seen in the Horn of Africa in 25 years. Experts estimate that locusts swarm at a density of about 50 million per 0.3 square miles (1 square km), so that means the single 2020 throng would have contained roughly 200 billion locusts, said Kimathi, who studies the desert locust. “[The species] can increase up to 20 times its population in a span of three months.” 

What Kimathi is concerned about is how much more frequent — and larger — these swarms could become. The desert locust needs two things to thrive: heat, and moisture, which is crucial for the eggs to hatch from the desert sands. And fortuitously for locusts, climate change is increasing these conditions across their vast range. “These areas are getting more arid, and when they do receive the rainfall, it’s torrential rain,” Kimathi said. “These conditions are becoming more frequent. And so these areas are becoming more favorable for locusts to breed.”

So Desert Locusts win this gathering!