Social complexity arises from the formation of social relationships like social bonds and dominance hierarchies. In turn, these aspects may be affected by the degree of fission-fusion dynamics, i.e. changes in group size and composition over time. Whilst fission-fusion dynamics has been studied in mammals, birds have received comparably little attention, despite some species having equally complex social lives. Here, we investigated the influence of environmental factors on aspects of fission-fusion dynamics in a free-ranging population of carrion and hooded crows (Corvus corone ssp.) in the urban zoo of Vienna, Austria over a one-year period. We investigated i) the size and ii) spatio-temporal structure of the local flock, and iii) environmental influences on local flock and subgroup size. The local flock size varied considerably over the year, with fewest birds being present during the breeding season. The spatio-temporal structure of the local flock showed four distinct presence categories, of which the proportions changed significantly throughout the year. Environmental effects on both local flock and subgroup size were time of day, season, temperature and weather, with additional pronounced effects of the structure of the surroundings and age class on subgroup size. Our findings show environmental influences on party size at the local flock and subgroup level, as well as indications of structured party composition in respect to the four presence categories. These results suggest that environmental factors have significant effects on fission-fusion dynamics in free-ranging crows, thereby influencing social complexity.