Some of my hives got a late start last season, and some of them I started late. Despite fall feeding, many were a little light going into winter. It's been cold lately, but temperatures are forecast to be above average in January and February. I wasn't sure how that would impact colony food reserves. For those with similar concerns, what follows is a summary of the research, as I understand it.
Honey bees evolved differently than most insects in the development of a social structure. This led to differences in their overwintering technique. Where most insects hibernate, honey bees remain active by clustering together to trap the heat produced by their metabolism. As outside temperature drops, the outer shell becomes more compact. At the less dense interior, a portion of bees vibrate their flight muscles to produce additional heat and compensate for any that escapes.
All of this clustering and heat production requires energy, which is sourced from stored honey. The metabolic burning of honey produces energy, some of it in the form of heat. As outside temperatures decreases, the bees' metabolism increases as they work harder to stay warm. Put another way, the bees' consumption of honey is like throwing another log on the fire. With adequate honey (fuel) available, a healthy cluster of 15,000 bees can maintain a core temperature of 95F indefinitely at more than 20F below zero. This is why you may have heard that bees don't freeze to death, they starve. And just as colder temperatures result in higher honey consumption, warmer temperatures result in less.
There is, of course, a limit to this simple formula. When the temperature is warm enough for bees to break cluster, energy demand begins to rise again. The pivotal temperature is around 50F. Generally speaking, the bees' metabolic rate increases the further their body temperature gets above or below this point. Ambient temperature inside a standard Langstroth hive is only a few degrees warmer than the outside air. Therefore, average daily temps of 40-45F should keep the colony loosely clustered, resulting in minimal demand for heat/energy/honey.
All other factors being equal, a mild winter should help colonies stretch their food reserves. This has implications in the decision to wrap or otherwise insulate hives, since reducing the effect of cold reduces the need for honey (below 50F). At the same time, warmer temps are likely to help stimulate early winter brood rearing, which then increases honey consumption (and facilitates mite reproduction). Every benefit comes at a cost. It's also worth noting that smaller colonies are less efficient, consuming more honey per bee than larger ones. Special consideration should be given to the insulation of small hives in an effort to maximize reserves.
In a broodless winter cluster, bees have been observed at very low metabolic rates caused by hypoxia. Minimal additional heat is produced in the absense of brood, and cluster temperature is allowed to drop considerably. The bees can become so compact that ventilation is serverely restricted, causing a shortage of oxygen and excess CO2. There is very little research available on this phenomenon, which can last for a few days before it must be terminated. If the thorax temperature of a honey bee falls below 40F, she becomes immobilized in a state of torpor known as chill coma. Unless warmed by an external source of heat, death comes in a few hours or a few days.