What are those bees doing during winter?

I found this great description of bees in their winter cluster online written by Shelley Stuart, who writes about beekeeping in the North East region for Examiner.com. She has also written for Weill Medical Magazine, and has articles published in the Cornell Chronicle Online and eCALS e-publications, both at Cornell University.

In the Northeast winter, honey bees need to accomplish two things: survive the cold and replenish the colony's population in time to forage for spring pollen and nectar.  As with most honeybee activities, they accomplish this as a collective.

The initial signal for the colony to shut down for the winter comes from the pollen they collect.  "Brood production stops when no net gain of pollen is introduced into the colony," explains Bucknell bee expert Dr. Elizabeth Capaldi Evans.   Evans notes that in her hives, some colonies have stopped raising brood (bee larvae) earlier in the fall than they did five years ago, thus prolonging the amount of time honey bees must rely on their winter stores.  While local beekeepers eye genetically modified crops as the potential culprit, Evans points out that the quality of the pollen, quantity of the pollen or the health of the colony's queen could all contribute to this phenomena. 

The lack of brood production, combined with decreasing outdoor temperatures, eventually cause the honey bees to huddle up in the "winter cluster" within the hive.  The hive itself is formed by sheets of honeycomb hanging 3/8" apart.  The cluster contains the queen, few (if any) male drone bees, and worker bees.    Bees will fill as much empty space within the cluster as necessary.   Some will fill the empty space between the honeycomb sheets, while others will dive headfirst into empty honeycomb cells.

The bees ultimately form a football-sized mass in the center of the bee hive, and use their wing muscles to shiver.  The vibrating muscles release heat, which warms the cluster.  Most of the bees in the cluster congregate in an outer shell, which help insulate the inner mass.  The temperature on the outside edge of the cluster will hover at about 40 degrees F.  The inner layer of the cluster's shell will reach about 75 degrees F.  The less dense center of the cluster allows bees to move around, and allows the queen to lay the eggs that will become the spring foragers.  Once the queen starts laying eggs, temperatures in the center of the cluster approach 100 degrees F.

The colder it gets during the winter, the more compact this winter cluster will become.  But when the temperature in the hive rises sufficiently, the bees are able to move around.  The bees can get to the outer  edges of the hive  and use the honey stored there.  How they use it remains a mystery, according to Evans.  One likely scenario is that bees in the outer edge of the cluster's shell tank up on honey, then return to the cluster and pass the honey around in a sweet mouth-to-mouth sharing of resources.

From December through March, the colony will gradually move from the bottom of the hive to the top, as it consumes the honey stores.  If conditions were right, in the spring the colony will emerge strong, healthy and primed to start another season.