Whereas geographic variation provides fundamental information for addressing a range of evolutionary questions, attempt to characterise intraspecific variation in social organization of highly social birds has been scant. As a result, our knowledge on evolutionary and ecological lability of sociality is lacking for most species. The long-tailed tit (Aegithalos caudatus) is a facultative cooperative breeder in which breeding groups are formed through failed breeders joining offspring care of other breeding pairs as helpers. In this study, I characterise the regional variation of group size using a database of spot census across the Japanese archipelago, which, for this particular species, has a temporal resolution suitable for discerning the size of groups encountered during the survey. After the latitudinal change in phenology was controlled for, the group size variation negatively scaled with the variation in probability of sighting the species, indicating that smaller group sizes are resulting from more fragmented social organization and perhaps reflecting a higher proportion of breeders sustaining their own breeding. This pattern is against a mere effect of higher density promoting temporary aggregation. The revealed geographic variation was not readily explained by factors known to affect individual decisions for group formation. A thorough search of influential environmental conditions would yield further insights into a broad-scale variability of cooperative breeding in the long-tailed tit and perhaps other species with a similar social system in which failed breeders take a helper role.