The world’s biggest bat colony gathers in Zambia every year.

Everybody who visits Kasanka National Park in Zambia during “bat season” agrees that the evening emergence of African straw-coloured fruit bats from their roost site is one of the wildlife wonders of the world. The bats (Eidolon helvum) arrive at Kasanka every year around October. The numbers swell rapidly until they peak in November. By January, they were gone again.

Once they recover from the shock of the breathtaking spectacle, everyone also converges on the same question: how many bats are there? So many fly out so fast, it seems impossible to count them. Past estimates based on visual counts have ranged from 1 million to 10 million, a sign of how difficult the task is.

To crack the problem, we clearly needed a new approach. Using an array of small video cameras, we filmed the bats leaving their roost and then developed artificial intelligence to count them. This offers an inexpensive, fast, and repeatable way to count large numbers of moving animals.

Our average estimate for the Kasanka colony for five days in November 2019 was 857,233 bats. This makes it one of the biggest bat colonies in the world and one of the most important in Africa.