A specialized whir dance performed by worker bees to initiate swarming for the purpose of colony division.

When the population of a colony becomes larger than what can be sustained by the hive and its surrounds, scout bees are sent out to search for a site at which to locate a new colony. Scouts communicate possible locations for new sites to each other and then do cross-inspections of each others' sites until they agree a site at which to locate the new colony.

Meanwhile the worker bees will be preparing to divide the colony by constructing queen cells into which a new queen can be born. Workers also engorge themselves on honey to build up a reserve for the move.

Then the Schwirrlauf (the German word for whir dance) begins: workers move in straight lines across the comb while every few seconds vibrating their partially spread wings. From time to time the dancing bees come into contact with one another at which time they make a high pitched buzzing sound.

The colony then divides, some staying behind with the new queen (who may still be in a larval state) in her queen cell, while others travel with the existing queen to the new site.

reference: http://gears.tucson.ars.ag.gov/beepop/wild.html