"A swarm of bees in May is worth a load of hay;
A swarm of bees in June is worth a silver spoon;
A swarm of bees in July isn't worth a fly."
Swarming is the natural means of reproduction of honey bee colonies.
Two or more colonies are created in place of the original single colony.
Beekeepers try to manage "domesticated" bees using techniques that artificially effect a swarm. Regardless, it still happens however alert you are. Beekeepers try to control reproduction by making nucs.
Swarming occurs when queen bees leave the colony with a large group of worker bees. The first or prime swarm goes with the old queen. As soon as the swarm is established as a new colony, the bees raise a new queen, or sometimes a replacement virgin queen is already present in the swarm. Afterswarms or Casts are usually smaller and are accompanied by one or more virgin queens. Sometimes a beehive will swarm in succession until it is almost totally depleted of workers
Swarming is mainly a spring phenomenon, usually within a two or three week period normally in May and June but recent years have seen them happen in April and August.
What to do if you think that it is a Swarm?
Assuming that it is a honey bee swarm, then it is likely to be friendly to start with but be careful and contact the Swarm Collection Coordinator or an experienced Bee keeper and remember that thes people do may collect the swarm voluntarily, are not insured and maight reasonably ask for a small payment for their work.
A Beekeeper collecting a simple to access Swarm.