Small body size means a large surface area relative to volume, and a wren cannot store enough fat to burn through a long freezing night alone. Communal roosting solves this: birds pack into a nest box, a hole in a wall or a disused nest, reducing the exposed surface of each individual and sharing warmth. Roost counts in the dozens have been recorded in a single box, with far higher single-site counts reported in exceptional cold.
The behaviour is strictly seasonal and weather-driven. Through the breeding season the same males are territorial and intolerant, building multiple nests and singing to defend a patch, so the winter tolerance is a temporary suspension of normal social rules.
Wren populations reflect this fragility directly: numbers crash in severe winters and rebuild over several mild years, producing a boom-and-bust pattern in long-term monitoring that is one of the clearest weather signals in European bird census data.