Bees are cold-blooded, so cluster to stay alive. They generate heat by flexing their wing muscles and take turns being on the outside of the ball. The queen is in the middle of the cluster, protected at all costs by worker bees surrounding her. She will not leave this cluster for several months. If their temperature falls below 6C the bees will die.
During the course of the winter the bee cluster will slowly move around the hive and consume the honey stores. During our winter inspections one of the main things we look out for is that there are always enough honey stores near the cluster - if the bees become disconnected from their foodstore they will starve as the cluster is not able to move quickly enough to bridge a gap.