We’re into February now, and the queens will probably have been laying for about 6 weeks. The workers will be feeding the babies from the stores of honey and bee-bread and keeping them warm by huddling on them and generating metabolic heat (carbohydrates, under digestion, break down to carbon dioxide and water, releasing energy in the process). They aim to have the outside of the cluster at about 57 degrees Fahrenheit and the brood area at about 94F, which is not much colder than you at 98.4! Most honey will liquify at this temperature.
The bees give priority to warmth over food. This is fine providing there’s not too much demand for food and, anyway, there’s plenty more in reach, but when the cold spell goes on for too long then they will eat everything nearby and won’t go the few inches to get more as the babies will get chilled. Then they die, clustered on their dead brood. This is called isolation starvation and I lose more colonies to that than to anything else.
My sample of apiaries to too small to be statistically significant, but I wonder how much difference it makes if the sun shines on the hive at some time during the day. I know, having placed hands in a National cedar hive on a cold day and felt the difference in temperature between the sunny and the shaded side that it does make quite a difference. This may be enough for the bees on the outside of the cluster to pop off to the kitchen and make a round of sandwiches to pass around their sisters. Too many of my hives are, I feel, too shaded.
Geoff Nantes, who was making hives before I was born, had a workshop, and above the doorway had some skeps on a glass floor so he could look up and see the cluster of bees. He sprayed the winter cluster with food dye and observed how the bees on the outside gradually made their way into the warmth of the inner cluster but eventually emerged again at the top, behaving as if they were in convection currents in a fluid.