Swarming is nature’s way of increasing the number of honey bee colonies not the individual bee. Bees swarm when there is an abundance of food coming in, the hive is full of brood and honey bees. The result is two colonies, each with about half the original number of bees relieving the congestion within the original colony. The new colony is headed by the old queen who leaves with the swarm, but the old hive is not without a queen, she will hatch from her cell, mate and resuming responsibility as head of the old colony. If the remaining bees think that there is still congestion after the primary swarm has left, a second or third swarm may be issued, this time a virgin queen will go out with the bees. These swarms are known as after swarms and are smaller than the primary swarm. The swarming period is during the spring and early summer.
As the swarm leaves the hive, they fly in large seemingly unorganised circles until they settle onto one spot. Scout bees fly out to look for a new suitable nesting place. The settled swarm can move location until this happens, a few metres to 500 and might occur every half hour or several days, especially on hot days. They nest in peoples house, trees, rock crevasse or any other place that might make a good home.
Swarms are frightening to people but generally the bees are not aggressive as they have no home to protect. Beekeepers are alerted to swarms and ask to remove them and the bees generally accept their new home they are desperately looking for. Good beekeeping reduces the effect of unwanted swarming amongst the beehives as they are a nuisance and affect the original colonies honey gathering capacity for the season. An old English ditty says:
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.