We investigated the winter social organisation of a population of British Marsh Tits, using radio-tracking, colour-ringing, and analysis of spatial and social relationships. The observed social structure was defined by adults and juveniles occupying individual but partially-overlapping home-ranges, with typical groups of 2–3 birds (maximum of 9 birds) having a changeable composition. There was no evidence of exclusive territoriality among individuals or groups during the winter, and no evidence of stable social groups with a consistent membership. Instead, the major social units detected were male-female pairs, with mostly casual associations between other individuals. Individual home-ranges averaged 31 ha (10-63 ha, n = 9) and their core areas overlapped with each other by an average 14% (0-65%), indicating that Marsh Tits in this population had very large spatial habitat requirements during winter. Our results have relevance for Marsh Tit conservation, and also the understanding of the drivers and plasticity of social organisation in the family Paridae; we discuss our findings in the context of the discrete and basic flock system exhibited by other species within this group.