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Every autumn like clockwork, bats have filled the night skies outside caves and other cold-air traps as populations leave their summer habitat and prepare for months of quiet hibernation. But time has run out for many colonies, and others are unwittingly practicing this instinct-driven swarm perhaps for the last time. This is a spectacle that few humans have ever witnessed, and fewer still completely understand.

Late summer/early fall swarming behavior begins long after dark each night and lasts for several hours. In the swarms, bats swirl and dive, wheeling with chaotic purpose above and around underground overwintering sites. Scientists believe that the swarms contain mixed sex flocks that are coming together for the first time all year so males and females can mate, ensuring future generations of temperate, cave-dwelling bats. There may also be a behavioral component to this melee where females teach young of the year about the locations of area hibernacula, or so the “genetic memory” is preserved in the population to document alternate winter roosts.

According to observations in September and October 2009, fall swarms have disappeared across much of the northeastern U.S. In February 2006, a fungus new to science was discovered growing on the faces and wing membranes of hibernating cave bats. First documented near Albany NY, the fungus has spread to bat populations in nine states, north to Vermont and New Hampshire and as far south and west as Virginia and West Virginia. Dubbed "white-nose syndrome" for the prominent halo of fungal growth on bat muzzles, the fungus appears to interfere with normal hibernation physiology. Bats need to maintain extremely low temperatures and metabolisms in order to survive on just a few grams of fat all winter. Infected bats become irritated by the fungus and wake too frequently, and burn through their limited fat reserves long before spring when their insect food is once again available. Bats are starving to death on the roost, or perishing on the winter landscape in failed attempts to find food to survive. Population declines up to 100% have been recorded for many colonies. This fall, the airspace outside their hibernation sites will be eerily quiet and whole species may become extirpated from large swaths of North America as this destructive fungus continues to spread.

Like the cliff swallows of San Juan Capistrano and the monarch butterflies in the mountains of Michoacan, northeastern bat species are disappearing. Passenger pigeons became extinct at the turn of the last century. Could cave-dwelling bats wink out of existence during this century? And, because this is a nocturnal phenomenon, will anyone notice? And, because it involves bats, will anyone care?