Bee swarms occur in almost every beehive once a year, usually in the spring. In managed, commercial hives the beekeeper can manipulate and divide the hive to prevent swarming.
In every hive there is a time in late winter/early spring when the hive begins to produce new queens. Just before the new queens emerge, the old queen will take a contingent of her offspring and leave the hive. This is the bee swarm. The queen will fly for a time, surrounded by a whirling mass of bees, anywhere from 5000 to 30,000 worker bees and drones (males). She will then land, usually in a tree or bush, and the other bees will also land on and around her to form a bee cluster. The swarm will stay in this new location from one to several days while “scouts” go out seeking a new location, which will be a suitable new home (hive site), i.e., a hollow log, hollow tree, box, inside the wall of a house, the attic, etc. When the new “home” is found, the queen is guided to the spot, and she along with her swarm move in and “set up house.”
Swarming bees are not dangerous, unless they are severely provoked. Bees sting in defense of their young or their food supply. Swarming bees have neither.
If the bees have moved into your house, they should be dealt with by a qualified exterminator (look in your phone book). If there is a cluster of bees in your trees or bushes, you can leave them alone, and they will most likely move on in a few days.