Swarming occurs when a large group of bees from a hive leave with a queen to form a new colony. This occurs when conditions in the hive cannot support the present number of bees. It can be seen as a form of reproduction. Before the swarm occurs a new queen is raised to replace the old queen. This is done with worker bees selecting one or more fertilized eggs to become a replacement queen. This new queen will stay with part of the colony and the old queen will depart with the remainder of the worker bees.
During a primary swarm the old queen leaves with approximately 50% of the workers and moves in a big mass to a temporary location. This can be thousands of bees. The sight of a large cluster of bees outside can be alarming. At this point swarming bees are not interested in stinging; they have gorged themselves prior to leaving which reduces their ability to sting. they are more concerned with finding a new home and are quite vulnerable. They have no home to defend and their queen is exposed. They will stay stationary in their cluster (for an hour up to a few days) while scout bees are sent out to find an appropriate location to settle. Once the scouts have found a new home the bees will travel and relocate there.
Secondary swarms can also occur which are smaller than the primary swarm. They are usually accompanied by one or more virgin queens.
What to do if you see a swarm
Do not swat at or disturb the bees or try to kill them. If the bees have set up a temporary location which is too close for comfort for people, please contact a beekeeper to come and safely remove the swarm from the area.