Bumble bees rarely swarm, mainly because they live in small numbers, however, it is a very common sight to see honey bees swarming. Swarming is essential to increase the number of honey bee colonies, especially as many natural colonies die out each year. (In IE you can hear the swarm buzz).
Bees swarm in late spring in search of a new home. During the winter the queen has been laying her eggs and the colony prepares to rear their young. As the weather becomes warmer and the early flowers appear, the raising of young bees increases rapidly. By the end of spring the colony becomes overcrowded. When a hive becomes too large, the bees create new queens and swarm. A large number of the bees, including the old queen, leave the hive and the swarm then sets off in search of a new home and to establish a new colony.
The bees will rest during swarming, usually in a tree or bunched together, draping from a bush, the bees are at their mildest at this point of the swarm. They will all wait with the queen while the scouts go hunting for a suitable new home. Sometimes the swarm will make their new home out in the open, but typically they will find a more protected and suitable hive site.
In the old colony that the swarm leave behind, a new queen emerges and takes over the roll of the old vacated queen and she will experience the same swarming instincts the following year. And so the cycle of activity goes on.
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.
Swarms of bees sometimes frighten people, though they are usually not aggressive at this stage of their life cycle. This is principally due to the fact the swarming bees have no hive to defend and are more interested in finding a new nesting point for their queen. This does not mean that bee swarms will not attack if they perceive a threat. Most swarms will move on and find a suitable nesting location in a day or two.
A swarm that does not quickly find a home and more nectar stores will starve. This happens most often with early swarms that are cast on a warm day that is followed by cold or rainy weather in spring. The remnant colony after having cast one or more swarms is usually well provisioned with food, but the new queen can be lost or eaten by predators during her mating flight, or poor weather can prevent her mating flight. In this case the hive has no further young brood to raise additional queens, and it will not survive.