Formation of the Winter Cluster on Frames – Biology, Mechanisms and Apiary Practice

The formation of the winter cluster is a key adaptive mechanism of the honey bee, enabling the colony to survive in unfavourable thermal conditions through collective thermoregulation. Unlike many other insects, bees do not fall into lethargy but remain active, maintaining the temperature necessary for survival inside the cluster, using stored honey reserves as fuel.

Biological and physical foundations of cluster formation

The process of cluster formation is closely correlated with ambient temperature. The first signs of bees clustering in hives are observed when the external temperature drops below 12–13 °C. With a further drop to +7 °C to +8 °C, the cluster becomes fully formed and compact.

The structure of the winter cluster resembles an ellipsoid or a flattened sphere, divided into layers by the combs. Two main zones can be distinguished within it:

1. Shell (mantle): The outer layer consists of densely packed, less active bees whose task is thermal insulation. The thickness of this layer ranges from 2 to 10 cm and depends on the external temperature – the colder it is, the thicker and denser the shell becomes. Bees in this zone insert their heads into the tubular cells of the comb, which further increases the density of the cluster.
2. Core (centre): Inside the cluster, bees are arranged more loosely, allowing them to move. This is where heat is generated through micro-vibrations of the thorax muscles.

The temperature inside the cluster is not constant and undergoes fluctuations. In the first half of wintering, when there is no brood, the temperature in the centre ranges from 14 °C to 25 °C. The critical point is around 14 °C – when the temperature falls to this threshold, the bees in the core intensify their metabolism, generating a so-called temperature spike to approximately 25 °C, after which a phase of slow cooling follows.

Location of the cluster on the frames

The natural location of cluster formation depends on several factors:

• Entrance and ventilation: Bees instinctively choose a position opposite the entrance, where they have access to fresh air rich in oxygen.
• Last brood: The cluster most frequently forms on those parts of the combs from which the last autumn brood emerged.
• Comb quality: Dark brown, „warm” combs, in which several generations of bees have already been raised, are preferred because they have better insulating properties than fresh foundation.
• Sun exposure: In hives with thinner walls, the cluster may shift towards the south-facing wall of the hive, which is warmed by sunlight.

Dynamics of the cluster during wintering

The winter cluster is not static. Bees constantly move from the surface to the interior, swapping the roles of insulators and heaters. The entire cluster moves across the combs following the consumed stores. The standard direction of movement is from bottom to top, and after reaching the top bars of the frames – from front to back of the hive.

The continuity of stores is of critical importance. Bees can move onto adjacent frames only when the external temperature rises above 0 °C. During frost, if bees encounter empty cells on the frame they are sitting on, they can starve to death, even with full frames right next to them.

Practical guide to preparing the brood nest (How, What and Why)

The correct arrangement of frames in autumn determines the success of wintering. Below are the steps necessary for optimal cluster formation:

Step 1: Autumn inspection and frame selection (August/September)

• What: Old (black), damaged and excessively light (virgin) frames that retain heat poorly must be removed from the brood nest.
• Why: Bees overwinter best on dark brown frames, which ensure the thermal stability of the cluster.

Step 2: Adjusting nest size to colony strength

• How: Retain as many frames as the bees cover densely. For an average colony this is typically 7–8 frames, for a strong one 9–10.
• Why: An oversized nest leads to chilling of the side frames, condensation of water vapour and moulding of the stores.

Step 3: Distribution of stores (Nest arrangement) There are two main models of store arrangement:

1. Bilateral arrangement (beard pattern): The fullest frames (3.5–4 kg of honey) are placed at the edges, and lighter frames (approx. 2 kg) in the centre, opposite the entrance. This creates a natural „cradle” for the cluster in the middle.
2. Unilateral arrangement (angular pattern): The heaviest frame goes to one of the walls (e.g. the south wall), and the subsequent frames have progressively less honey.

• Critical rule: Every frame left in the brood nest must contain a minimum of 2 kg of honey.

Step 4: Ensuring access to pollen

• What: Frames with pollen should not be located in the very centre of the future cluster, but next to frames containing carbohydrate stores.
• Why: Pure pollen in the centre of the cluster can act as an insulator dividing the cluster into two parts, leading to a weakening of the colony.

Step 5: Final feeding and supplementing deficiencies

• When: Feeding must be completed by mid-September (at the latest by 10–20 September), so that the young winter bees are not worn out by processing the syrup.
• Standard: Each frame occupied by bees should have 2–2.5 kg of stores (18–25 kg in total per colony).

Summary for the practitioner

The gradual formation of the winter cluster is an autonomous process of the bee colony; however, the beekeeper’s role is to create the optimal „stage” for this phenomenon. The key is a tight brood nest, abundant and high-quality stores (free from honeydew honey, which causes dysentery) and ensuring calm. Let us remember that mistakes made when arranging frames in September are almost impossible to correct in January or February without exposing the colony to the risk of death from chilling.