The honey bee cluster moves up in winter and down in summer. The downward movement is easiest to see in a feral colony built into a tree or a building. The combs start at the top and are added in layers, one beneath the other, as the colony expands. When bees start at the top, there is no place to go but down.
Unlike feral colonies, those in a vertical hive, such as a Langstroth hive or Warré hive, sometimes have the option to move up. If they have that option in winter, up they go. The reason is warmth. Because warm air rises, the area just above the winter bee cluster is the warmest place in the entire hive aside from the cluster itself.
In fact, it is so comfortable, it’s the first place the bees head when they are looking for food in winter. Even if food is close by — below or to the sides of the cluster — the bees will go to the food that is the warmest.
Reversing brood boxes in spring is a common routine that is not necessary and can actually be detrimental to the colony. If the ascending colony straddles two boxes, as it often does, reversing brood boxes separates the colony into two parts. When split in two, there may not be enough adult bees to keep the brood warm.
Only the Winter Bee Cluster is Kept Warm
It helps to remember that a honey bee colony makes no attempt to keep the hive interior warm in the way we heat our homes. The bees’ only concern is keeping the brood warm. When the outside temperature drops to 64°F, the bees begin to form a loose cluster with the brood at the center. At 57° F, the cluster tightens into a compact sphere that surrounds and protects the brood. As long as brood is present, the core of the cluster is kept between 92-95°F, but without brood, the bees conserve energy by keeping the core at a cool 68 degrees.
Again, think of a feral colony hanging from a tree branch. It would be pointless to try to heat the whole outdoors, so they concentrate their efforts on the cluster itself. If you monitor temperatures in different parts of a hive, you will find it warmest just above the cluster, a bit cooler right beside the cluster, and coldest beneath it. The areas furthest from the winter cluster are often just a few degrees warmer than the outside.
Upper Entrances and Cleansing Flights
The cold temperature beneath the cluster is one reason beekeepers often give their bees an upper entrance in winter. On those days that are balmy enough to take cleansing flight, the bees can stay warm until they exit the hive. From the top entrance, they can quickly take off, circle around, and come back. When they return, the bees meet warm air as soon as they reach the entrance, so the time spent in cold air is very short.
If they have only a lower entrance, they must travel down through the cold hive, fly, then once again travel up through the cold hive. Because of the length of time in the cold, those bees are much less likely to survive.
Because warm air escapes through the top entrance, it may seem counterintuitive to provide one. But the warm shortcut to the outdoors can make a big difference in colony health because bees with easy access to the outdoors are much less likely to get dysentery. Like everything else in beekeeping, you must consider the trade-offs. If you have occasional warmish days in winter, an upper entrance is a great addition to a hive.
If you live in a colder area, or if your hives are highly shaded, you can keep the upper entrance closed until you begin getting some warmer days in spring. But don’t switch back and forth. Bees need time to adjust to a new entrance, and they can become disoriented when the location changes — a bad thing on a cold day.
Expansion and Contraction
The winter cluster itself expands and contracts with the temperature. The bees bunch closer together as the temperature drops, and they space themselves further apart as the temperature rises. Like a balloon, the sphere shrinks and expands with changing conditions. Bees spaced further apart allow more air to flow through the cluster, which improves ventilation and lowers the temperature.
If you have a vertical hive, it is important to keep a supply of food right above the cluster in the coldest months. Once spring temperatures begin to rise, the cluster will expand and the outer bees may run into honey stored beside the cluster. Also, as it gets warmer inside the hive, the retriever bees — those that fetch honey and bring it back to the brood nest and the queen — are more likely to leave the cluster and scout the inside of the hive for food.
Keeping Them Fed
To keep food directly above the cluster of bees, you can rearrange the frames of honey or add more. You can put a candy board directly above the upper brood box, or you can simply add sugar patties or a bag of sugar with some slits cut into it. It is best to avoid cooking the sugar, as heating increases the amount of hydroxymethylfurfural, a material that is poisonous to bees.
If you have a horizontal hive, like a top-bar hive, it is best to move the colony to one end of the hive before winter sets in. That way, all the honey can be put on one side of the colony. As winter passes, the cluster will move toward the honey and eat their way to the other end of the hive. But if you begin winter with the cluster in the center of the honey stores, the cluster has to go one way or the other. Once it reaches the end of the hive, it will be unable to reverse direction and travel to the other end to get the rest of the honey. Many colonies have starved with food only inches away.
If you want to know where your colony is, simply pull out your varroa tray to see where all the debris is landing. The pattern can tell you both the size and location of the cluster. A thermal imaging camera works well, too. Remember, the winter cluster is not static but easily changes in response to winter conditions.
What have you noticed about your winter bee clusters? Did they move up, down, or side to side? Did you provide an upper entrance? How did it work for your bees?