Swarming
Swarming usually occurs in the spring of the year. And the cause for it is that the hive is crowded or the queen is not  laying her eggs to the satisfaction of the rest of the bees. In the first instance the bees will take several eggs and nurse them to queen cells once a new queen emerges in 16 days from the egg. The old queen will take half the hive to look for a new place to set up her hive. This could be in a building or a hollow of a tree. The second reason is called a supersedure swarm, at that time a small cluster of bees will leave with the queen.

                                ,