How Bees Decide to Swarm

What causes a bee colony to swarm?

Swarming is a colony's primary method of reproduction — the way a single colony becomes two. It is triggered when the colony outgrows its ability to distribute the queen's pheromone effectively throughout the nest.

The queen continuously produces mandibular pheromone, which spreads through the colony via worker-to-worker contact. In a small colony, every worker receives this signal regularly. As the colony grows in spring, the population of bees eventually exceeds what the pheromone can reach consistently. Workers on the colony's periphery — at the hive entrance, on the outermost frames, or at the top of a brood box — receive weaker, less frequent doses. This reduction in chemical signal is one of the primary cues that prompts swarm preparations.

Congestion of the brood nest compounds this effect. When the brood area is fully packed with eggs, larvae, and capped cells, and food storage fills frames above, the queen has limited space to lay. Reduced laying space and crowded combs increase the density of bees in any given area, lowering the per-bee pheromone dose further.

Genetics also plays a role. Some strains — older British black bee colonies, certain Buckfast lineages — are more prone to swarming than others. Colonies with a history of swarming are more likely to swarm again.

The trigger is not a single moment but an accumulating set of conditions. A colony building swarm cells has already been in pre-swarm mode for days: workers reduce feeding of the old queen to slim her down for flight, scout bees begin exploring potential nest sites, and comb construction slows as resources are redirected.

In a typical UK season, these conditions converge in May and June.

How do bees make the collective decision to swarm — who decides?

No single bee decides to swarm. The decision emerges from the behaviour of many individual workers, each responding to local conditions. It is a collective process, and the mechanisms behind it are among the most studied in animal behaviour.

The process begins with scout bees. Before the swarm leaves, a subset of experienced foragers starts searching for potential new nest sites — tree cavities, gaps in walls, chimney stacks. When a scout finds a promising site, she returns to the hive and performs a waggle dance describing its location and quality. The duration and vigour of the dance signals her confidence in the site.

Other scouts attend these dances, fly out to inspect the advertised site, and return to either repeat the dance if they agree with the assessment or perform their own dance for a different site. Over hours or days, dances for poorer sites gradually stop as scouts who danced for them visit the leading site and switch. Dances for the best site grow in frequency and attract more scouts.

The threshold for departure is reached when a quorum — roughly 15–20 scouts from the same nest site — are all actively dancing for the same location simultaneously. Once this quorum is met, the swarm departs. Bees activate each other through a specific buzzing-run behaviour, and within minutes the air fills with departing bees.

This quorum process produces robust decisions. Computer models of the scout behaviour show it selects for genuinely better sites rather than random convergence. A larger, better-insulated cavity generates stronger, longer-lasting dances and accumulates quorum faster.

When in the year do British hives most commonly swarm?

In the UK, the main swarming period runs from late April to the end of July, with peak swarming in May and June. This timing aligns with the spring buildup of colony population and the main nectar flows from early-flowering species like hawthorn, oil seed rape, and horse chestnut.

May is the most active swarming month across most of England. In the south, swarming can begin in mid-April in warm years. In Scotland and northern England, the swarming season starts later — typically May — and ends earlier, reflecting the shorter warm season.

The combination of conditions that drives swarming — large population, full brood box, reducing space, long warm days — typically coincides with the first main nectar flow of the year. Workers returning with nectar find storage space limited, adding to the congestion that promotes swarm behaviour.

Secondary swarms, or casts, can follow the primary swarm at intervals of several days. In a colony that issues multiple casts, this can stretch swarming activity into July or even early August. However, late-season swarms face a harder establishment challenge: less time to build up stores before winter.

Beekeepers in the UK time their swarm prevention inspections accordingly. The standard recommendation is to inspect every seven to nine days during May and June. A queen cell becomes a swarm risk once it is capped — roughly eight days after the egg was laid — so a weekly inspection should catch cells before they pass that threshold.

Weather significantly affects actual swarm departure even when preparations are complete. Prolonged rain or cold in May and June can hold a prepared colony in place for a week or more. When the weather breaks, multiple prepared colonies may swarm within the same few days.

What happens inside the hive in the days before a swarm leaves?

The pre-swarm period involves measurable behavioural changes at every level of the colony, starting several days before departure.

Scout bees begin exploring outside the colony in a more systematic way, investigating tree hollows, eaves, and wall cavities within a kilometre or two of the hive. This scouting activity starts well before the swarm is ready to leave — sometimes 5 to 10 days in advance.

Workers reduce the amount of food fed to the queen. A swarm queen must fly, so she is progressively slimmed down from her peak laying weight. Her egg-laying rate drops noticeably in the days before swarming. This is a practical preparation: an overweight, fully laying queen cannot sustain a flight of any distance.

Comb building slows or stops. Workers that had been drawing new foundation redirect their wax production energy toward stores, and clusters of bees at the hive entrance — called 'bearding' — increase. This bearding is partly thermoregulation in warm weather but also reflects the larger population with less productive work to do.

Queen cells along the lower frame edges are constructed and become capped. On the day of swarming, a final trigger — thought to involve specific piping sounds and vibrations — activates departure. Workers gorge on honey stores before leaving, filling their honey stomachs with enough fuel to sustain flight and help establish the new colony's first comb.

The swarm exits the hive entrance rapidly, filling the air within minutes. The flying mass initially clusters nearby — on a fence post, a tree branch, a wall — within 50 metres of the original hive, while scout bees continue finalising the nest site decision.

How do scout bees find and vote on a new nest site?

Scout bees use their standard foraging capability to locate potential nest sites, but the evaluation process is far more systematic than food foraging. A scout investigating a cavity checks multiple features: volume (approximately 40 litres is optimal), entrance size and height from the ground, direction the entrance faces, dryness, and freedom from draughts and infestation.

After inspecting a site, the scout returns to the swarm cluster and dances on its surface. Her waggle dance communicates direction and distance using the same encoding used for flower patches — angle relative to the sun for direction, duration for distance. The enthusiasm and duration of the dance correlates with her assessment of the site quality. A scout excited by a prime cavity dances for longer and more vigorously than one lukewarm about a marginal site.

The process is competitive but not adversarial. Scouts for different sites dance simultaneously on the cluster. Watching scouts attend each other's dances, then fly out to inspect the advertised sites, then return with their own assessment. Gradually, dances for inferior sites decay — the scouts stop dancing and may switch allegiance to the leading site — while dances for the best site grow in frequency and attract new scouts.

Quorum sensing ends the deliberation. When approximately 15–20 scouts are all dancing for the same site simultaneously, this threshold appears to trigger the warming-up and departure behaviour in the rest of the swarm. The quorum mechanism prevents premature departure on contested information and allows better sites to accumulate more support.

The whole process can take as little as a few hours for a clear winner, or two to three days in poor weather or when multiple sites are evenly matched.