Bees swarm from mid spring, through summer. Usually swarming takes places because the colony (hive) has grown too large, and some bees will divide off and leave to find another site to establish a new colony. There will be a queen and a good few hundred (at least) workers. It may be the old queen, if the hive has decided she is not producing brood as well as expected, or if the existing queen is still vigorous, the queen may be new.
Often the air will turn dark with swarming bees. They will seek temporary respite, often clustered in the branches of a back yard shrub. Although they are usually â€śintoxicatedâ€ť with nectar and not interested in attacking or stinging, it is best to leave them well alone, and call a beekeeper.
During the time they are clustered outside, scouts will be looking for a more permanent home. In the wild, this will often be in the hollow of a tall tree. However, with much clearing of native forests for housing and other development, it can sometimes be difficult to find suitable tree hollows, and a decision may be made to establish the colony inside cavity walls of houses.