There is a crackling in the air, barely perceptible at first. With every step into the dense coniferous forest it becomes more audible: it swells to a rustling of millions of small wings. And then, suddenly: right in the middle! "It's overwhelming, a three-dimensional experience: They fly all around you. As high as you can see in the sky. Down to the ground. And they cover all the trees around you," says butterfly researcher Karen Oberhauser.