Resource limitation drives fission–fusion dynamics of group composition and size in a social bird

Fission–fusion (FF) societies are characterized by frequent changes in group cohesion, composition or size. While the ecological and social drivers of group size are well acknowledged, their effect on FF dynamics is largely unknown, partly due to lack of suitable data sets spanning multiple individuals and spatiotemporal scales. Theory predicts that constraints imposed by local resource availability on group size can yield FF dynamics; yet it has not been applied in contexts other than foraging. In addition, FF dynamics are relatively understudied in nonmammalian vertebrates, although prevalent across taxa. To fill these conceptual and taxonomic gaps we tested predictions arising from current FF theory by tracking multiple Eurasian jackdaws, Coloeus monedula, for two seasons. Using active RFID (radio frequency identification) tags we monitored individual presence/absence over several temporal scales in sites of different ecological functions (nesting, foraging and roosting), and simultaneously quantified local resource availability in these sites. Group size was correlated with resource limitations operating at different ecological contexts and times. Social network analysis revealed remarkably consistent FF patterns which occurred on a diel cycle and changed between seasons, involving changes in group size and composition. Social preferences had a significant influence on FF patterns only during the breeding season and only in nesting and foraging contexts, mainly due to male–female bonds. Our results highlight the importance of ecological context, temporal scale and social preference in avian FF societies and broaden the scope of current theory.