In keeping with both its unofficial slogan — Keep Austin Weird — and its official one — Live Music Capital of the World — Austin will celebrate the 7th Annual Bat Fest on Saturday with live music and a celebratory viewing of the city’s 1.5 million resident Mexican free-tailed bats as they head out for their evening meal of moths, mosquitos and millions of other unwanted pests.

Austin has been the seasonal home to the world’s largest urban bat colony since sometime in the 1980s, when the city reinforced a downtown bridge with beams that, unintentionally, offered an ideal habitat for bats. Locals were initially unnerved when hundreds of thousands of bats arrived each spring to nest, but worries subsided once wildlife groups pointed out that bats are a great natural pesticide.

Since then, tourists and locals have been gathering downtown each summer evening at sunset to watch a sky-darkening curtain of bats emerge from the bridge.

Last year there was so much rain in Austin that the bats often didn’t emerge until after dark, so few visitors were able to see the bat show. But this year there’s a drought. “That means there are fewer crops in the fields and fewer pests, so the bats have to forage longer and emerge earlier,” said James Eggers, director of education for Austin-based Bat Conservation International.

On an average summer night, between 2,000 and 3,000 bat-watchers gather in downtown Austin to watch the bats emerge. During Saturday’s free Bat Fest, however, organizers expect a crowd of up to 40,000 bat fans.

Will all that music and partying bother the bats? “Well, despite the thousands of vehicles that cross the bridge each day, this colony has chosen this bridge,” said Eggers. “They’re apparently happy in that home. Even with the music and all those people, we don’t see that bats emerge any differently.”