Swarming is initiated by a special dance, the Schwirrlauf, the German
word for whir dance. During the Schwirrlauf, workers move without
stopping in straight lines across the comb. Every couple seconds,
they vibrate their partially spread wings. These dancers also make
occasional five second contacts with other worker bees. During this
contact the bee makes a continuous piping sound.

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

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Worker piping in honey bee swarms and its role in preparing for liftoff
by Thomas D. Seeley and Jürgen Tautz

Abstract: Worker piping, previously reported only in hives, was
observed in swarms as they prepared to liftoff to fly to a new home.
Pipers are excited bees which scramble through the swarm cluster,
pausing every second or so to emit a pipe. Each pipe consists of a
sound pulse which lasts 0.820.43 s and rises in fundamental frequency
from 100-200 Hz to 200-250 Hz. Many, if not all, of the pipers are
nest-site scouts. The scouts pipe when it is time to stimulate the
non-scouts to warm themselves to a flight-ready temperature (35C) in
preparation for liftoff. The time-course of worker piping matches
that of swarm warming; both start at a low level, about an hour
before liftoff, and both build to a climax at liftoff. When we
excluded pipers from bees hanging in the cool, outermost layer of a
swarm cluster, we found that these bees did not warm up. The form of
worker piping that we have studied in swarms differs from the form of
worker piping that others have studied in hives. We call the two
forms "wings-together piping" (in swarms) and "wings-apart piping"
(in hives).

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Peter Borst <[log in to unmask]>