The world's largest mammal migration that few travellers ever see

Every year, millions of straw-coloured fruit bats descend on Zambia's little-known Kasanka National Park, creating one of Africa's most extraordinary wildlife spectacles.

There's a storm on the horizon. Lightning flickers, illuminating the Central Zambezian miombo woodlands with brief flashes of silver. The sun is setting, the air smells of damp earth and somewhere ahead of us, millions of bats are waking up.

We park our vehicle in front of the Musola Hide, Kasanka National Park's prime bat observation point. The 10m (33ft) climb up to the wooden platform feels precarious, the ladder swaying slightly beneath my feet. Across the canopy, the sky glows orange and purple behind the distant thunderclouds. The forest beneath us begins to tremble.

Legions of straw-coloured fruit bats (Eidolon helvum) hang in the trees, packed together so tightly some seem to cling to one another rather than to the actual branches, which sag under their collective weight. The air fills with chatter, whistles and shrieks. And then the bats begin to fly.

At first, it's only a few, then, one by one, others take flight. Soon, the sky is a vortex of movement. Bats stream out of the forest in every direction, swirling and tumbling like smoke in an updraft. From our perch above the canopy, we see bats pour endlessly from the forest against the glowing evening sky. The air vibrates with the beating of millions of wings.

This is the Kasanka Bat Migration, the largest mammal migration on Earth. Each year, drawn from across Central Africa by a seasonal explosion of fruit, an estimated eight to 10 million straw-coloured fruit bats (Eidolon helvum) gather right here in the park. That's eight times as many mammals on the move as the Serengeti Great Migration.

The nightly spectacle takes place from late October to December. By the end of January, they'll be gone. Somewhere between 500,000 and 700,000 people visit the Serengeti-Maasai Mara migrations annually, but only around 800 will witness the bats while they're here in Kasanka.

Often overshadowed by Africa's larger, more famous protected reserves, Kasanka is one of Zambia's smallest national parks at just around 390 sqkm, and is tucked away in the country's central province, near the border with the Democratic Republic of Congo. There are no vast open plains here, no huge prides of lions, but also no endless convoys of safari vehicles with their camera-clicking crowds. Instead, Kasanka is feels startingly quiet, home to wetlands and lagoons, papyrus swamps and forests, wonderful bird species and just a sprinkling of traditional safari wildlife. Most visitors who make the journey come for the bats.

Researchers have discovered the small mammals disperse across enormous distances. Some travel up to 96km (60 miles) in a single night, consuming their own body weight in fruit before returning at dawn. Former Kasanka chief ecologist Frank Willems says the bats burn extraordinary amounts of calories on their night flights. A straw-coloured fruit bat weighs about 250g and can consume roughly its own body weight in fruit each night. Willems calculates that eight to 10 million bats will put away around 230-250 tonnes of fruit in a single night.

The sheer scale of........

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