The four triggers

Four pressures push a colony toward swarming, and naming them tells you what to check on every spring visit.

• Congestion comes first: a brood nest and box that are simply too full, bees packed shoulder to shoulder with nowhere left to walk
• Backfilling follows close behind, incoming nectar stored in cells the queen would otherwise use for eggs, which squeezes her laying space even as the population keeps growing
• a surplus of young nurse bees relative to open brood needing care is the third trigger, idle hands with nothing to nurse
• the fourth is the queen herself: an aging or failing queen produces weaker suppressing pheromone, and workers read that as a signal to prepare a replacement

Genetics can make any one line more prone to all four, but congestion, backfilling, nurse surplus, and queen age are the levers you can actually manage.