A honeybee colony regulates its own entrance behaviourally with propolis and clustering, but beekeepers help it along with a physical entrance reducer: a wooden or metal bar that narrows the gap bees pass through. A narrower entrance is easier for a small winter cluster of guard bees to defend against mice, wasps and robbing bees, but it also restricts the airflow that carries moisture-laden air out of the hive, so it must be balanced against ventilation — usually via a small top entrance or matting under the roof.
UK beekeepers often reduce the entrance to its smallest notch (around 20–40mm) by late November and keep it there until the first real spring flow, since a full-width summer entrance left open over winter is one of the most common causes of avoidable colony losses — from both chilling and rodent damage.
A 3D observation hive where you slide the entrance reducer, fit a mouse guard and toggle the moisture vent across the seasons to see how bee traffic, predator risk and condensation change together.
Entrance width trades off bee traffic and defensibility against ventilation; a sealed hive with no moisture vent condenses warm, humid cluster air on a cold roof and drips it back onto the bees.
Pick a season to load realistic temperature and recommended entrance width, then override the width slider and the mouse guard / vent toggles to test your own setup against the live risk readout.
Condensation dripping onto a winter cluster is a bigger colony killer than cold air alone — a well-ventilated but well-insulated roof matters as much as a snug entrance.