A winter cluster can be thought of as a ball of bees dissected by sheets of honeycomb. Clusters begin to form when the outside air temperature falls to about 57°F (14°C). Bees in these clusters are in no way hibernating but are actively moving, eating, and performing hive duties.
The comb in the very center of the ball contains a small amount of brood and the queen bee. The brood and the queen are kept at the correct temperature by the surrounding workers. The workers have several ways of regulating temperature and air flow throughout the cluster.
The cluster is not of uniform density. The outermost layers of the cluster form the densest portion, whereas the bees in the core are able to move freely and carry out the regular chores of brood rearing and caring for the queen. Although brood rearing may be almost non-existent in early winter, as the day length increases, so does the amount of brood rearing.
When no brood is present, the core temperature is kept somewhat less than 85°F (29°C), but brood needs to be kept warmer—at a constant temperature of about 93°F (34° C).
To warm the cluster, the workers vibrate their wing muscles—an action which burns calories and gives off heat. The temperature in the brood rearing area is further regulated by the expansion or contraction of the cluster. If the “nursery” becomes too hot, the cluster expands which increases the air flow around each bee and cools the nursery. If the nursery becomes too cold, the cluster contracts which decreases the air flow.
Bees on the exterior surface of the cluster become so cold that they appear motionless and dead. However, in a way that is not completely understood, these outside bees get pushed towards the center of the cluster by warm bees who then exchange places with them.
Clustered bees need a constant supply of food and, as the winter progresses, the cluster will slowly move toward stored honey. If the cluster loses contact with the stored honey, the bees can quickly starve. Clusters of bees that are too small—that is, they don’t have enough bee bodies to maintain adequate nest temperatures—will soon die as well.