Since bees need to raise their thoracic temperature to 30 degrees Celsius before flight, and are for that reason endothermic, colder temperatures can impose a high energy demand on individuals. Young worker bees prefer a body temperature within the range of 35-37.5 degrees Celsius while older workers prefer 31.5-36.5 degrees Celsius (Cahill and Lustick, 1976). These temperatures can be difficult for an individual to maintain at colder ambient temperatures. To combat these problems, bees work together in groups to raise their body temperatures . 
 

Starting at 14 degrees Celsius, bees will begin to form clusters by grouping together with other bees. Cluster temperatures range between 20 and 35 degrees and are mostly around 30 degrees Celsius (Cahill and Lustick, 1976). 

When the ambient temperature decreases - the cluster will contract so that the bees are closer together. This creates a smaller surface area for the cluster and it will retain heat more easily, compensating for the decrease in ambient temperature. 

When the ambient temperature increases - the cluster will expand to create a larger group surface area so that heat can more readily escape. This cools the cluster down a little to compensate for the increase in ambient temperature (Cahill and Lustick, 1976).