Here, at Ballot-Flurin, the swarming of the bees is quite a normal event which allows the natural growth of bee colonies.
In spring at at the beginning of summer, if the bees feel too cramped in their hive, they will stop feeding the queen, this is the sign that the bees are ready to swarm.
The former queen bee having become a lot thinner will fly away accompanied by a part of the worker bees, all gorged with honey to relocate and build new honeycombs.
When the bees leave the hive with their queen, they form an impressive noisy cloud to then gather in a large cluster on a branch or another promontory. The scouts perform the danse of the swarm while others search for a new roof to develop the colony.
Meanwhile, the rest of the colony stays in the hive. They raise a new queen bee, that will be able to lay and replace the bees that left in the swarm.
To put it in a nutshell, when you see a swarm, it is the bees and their queen looking for a new place to call home. Welcome them well because it is a chance to have been selected by the bees. The bees of a swarm don't sting because they don't have a territory to protect and they are gorged with honey. You can therefore quietly observe the bees and call a beekeeper to come and direct the bees into a hive.
Hereunder you can see Pascal (beekeeper) providing the bees that had elected a tree as refuge with a warm "new home" in a hive.