A social lifestyle is abundant in nature. The social interactions between animals form the basis for complex, highly dynamic structures and may determine group-level processes such as group cohesion. Social behavior is influenced by local environmental conditions, varies over time and has diverse, context-dependent functions. In my thesis I have investigated the spatial, temporal, contextual and structural aspects of social complexity in three model species. 

First, I explored effects of increasing population density on aggression and contact behavior within and between sexes in water striders (Aquarius paludum). The current, local density regime affected male mating behavior as males sought more frequent contacts to available mates in denser groups. Frequencies of male-to-male conflicts and the duration of male harassment behavior were unaffected by local density, however. Overall, males in high-density groups may experience intense scramble competition over reproductively active females. Current environmental conditions crucially affect the species’ mating system; yet, also the previously experienced environment should be considered. 

Second, in a flock of free-flying jackdaws (Corvus monedula) I studied seasonal variation in patterns of positive interactions between pair mates and linked the pair bond to group level dynamics. Jackdaw groups are dynamic social units into which mutualistic pair bonds are embedded. Both sexes invested into the bond with different social behaviors and at different times of the year; yet, these are likely the proximate mechanisms employed by males and females to perpetuate a successful bond and secure annual reproductive output. Third, I determined the factors regulating dyadic and polyadic conflict resolution in jackdaws and investigated patterns of social support between the sexes. Conflict aggressors receiving active, aggressive support had high chances of winning encounters and were probably at low risk of receiving counter-aggression. Females cooperated very closely with their mates during conflicts. In doing so, they likely secured male investment into offspring provisioning and care, whereas males might seek conflicts strategically to maintain or improve their social status. Conflicts and interventions hence constitute a vital aspect of jackdaws’ social system. 

Finally, I used a captive house sparrow (Passer domesticus) flock to determine the destabilizing effects of perturbations of group composition on social structure and behavior. The group’s dominance hierarchy destabilized after a second perturbation and did not re-establish itself quickly. Yet, irrespective of experimental treatment, birds fed regularly and interacted based on their initially determined dominance rank. Only females joined, rather than supplanted, feeding conspecifics more frequently following the second perturbation treatment, thereby shifting to a non-aggressive social foraging strategy. Thus, sparrows in this study might have tolerated and compensated for structural instability to some degree. 

In conclusion, environmental factors, like population density, may alter the social structure of animal groups, creating a potential for both conflict and cooperation. Cooperation is constantly threatened by the selfish interests of individuals, leading to intra-group conflicts, and group members must resolve these conflicts efficiently. Enduring social stability is likely required for maintaining higher-order structures, such as social alliances or linear hierarchies, but relatively simple mechanisms for mitigating conflicts may exist in fluctuating fission-fusion groups.