Swarming is the reproduction of a honey bee colony. It is a normal and natural phenomenon, and rarely poses a danger to people or animals.
There is a variety of reasons why Honeybees will Swarm. The most common in managed colonies is poor hive management which includes crowding, disease and a lack of hive maintenance. In feral colonies if the colony is not diseased the most common reason for swarming is lack of room or crowding within whatever the bees are located in such as a hollow tree or wall of a building. Genetics also plays a very important role in swarming as many colonies have a very high tendency to swarm which guarantees that the colonies genetics will live on.
When honey bees have good weather and plenty of food, their populations can increase dramatically. When a colony becomes crowded inside their current home (a bee hive, hollow tree, or other cavity) they will begin to raise a new queen bee. When this new queen is almost mature, the old queen will leave the hive, followed by about one half or two-thirds of the worker bees and some of the drones. This queen bee will land nearby, on a tree, shrub, fence post or even a building. The worker bees cluster around the queen to protect her and keep her warm. However, a number of scout bees are sent out in all directions to search for another suitable home for their colony to move into. When a satisfactory site has been chosen, the entire swarm of bees will take to the air and move off to their new home. Once inside, the workers will begin to secrete beeswax, and build new combs for the queen to lay eggs and for the storage of honey.
Meanwhile, back in the old hive, a newly emerged queen bee will take a series of mating flights, then begin to head her inherited colony. She will never leave the hive again, unless she too leads a swarm to a new home. Soon after the new queen begins laying eggs, the population of bees in the original colony will begin to grow.
They are focused on finding a new nest, not on attacking. Because they do not have any brood or honey to protect, a new swarm is usually very gentle in temperament and they rarely sting. Unless the swarm poses a specific threat or inconvenience to people, they can be left alone and admired from a distance.
A swarm may stay around for a few days, depending on how quickly the scout bees find a suitable new home. This could happen very quickly, even within a day.
If, however, you come across a bee swarm that really is too inconvenient to tolerate, then firstly: