The trees all bear fruit.
Maybe that doesn't sound all that amazing. After all, trees bear fruit everywhere. However, this fruiting is the only one that causes the largest gathering of mammals on the entire planet.
They fill the skies like a great black cloud, and when they finally land to rest their sheer numbers cause thick tree limbs to break off and fall to the forest floor.
Perhaps the greatest thing about this event is the mystery of its origins. Where do the bats come from? Where do they go when the fruit are all gone?
No one knows, and honestly, this makes me happy. In a world that is so often stripped of magic and mystery, its good to know there are still some unanswered riddles out there.