Left unchecked colonies will swarm in May or June. This is their natural way of increasing the number of colonies. The queen lays eggs to create new queens in the old colony and then leaves with a large number of mature bees. They will cluster somewhere while scouts look for a suitable new home.
If you see swarming bees please call a beekeeper a soon as you can - local swarm collectors can be found vis the British Beekeeping website - www.bbka.org.uk