Animals use signals to facilitate the fundamental behaviours required for survival and reproduction. In social species, where many individuals interact and have to coordinate numerous behaviours, specialised signals are likely to arise. There are numerous costs and benefits to living in a group, and communication signals that minimise the costs and maximise the benefits of group-living are likely to be adaptive. In this thesis, I use a combination of behavioural observations, playback experiments and acoustic analyses to explore how vocal signals facilitate group-living in a cooperatively-breeding bird species, the pied babbler (Turdoides bicolor). Pied babblers forage together throughout the day, using their bills to find small invertebrates in the substrate, and emitting characteristic vocalisations whilst doing so. In chapter 3, I show that the acoustic structure of these chuck calls changes when a forager comes across certain food items and that the resulting `elevated chuck calls' attract other group-members to the foraging site. Calls are: not always given in situations suitable for sharing, sex-specific in structure, and (although dominant males gave elevated calls more often than expected by chance) males were less likely to be approached than females when giving these calls. These data suggest that elevated chuck calling is not an active signal to promote food sharing, but rather, I suggest, an asymmetry in calling among group members reflects the variation in the costs incurred by calling. Despite the fact that nutritional requirements and other incentives are likely to vary between group members, groups rarely split. In chapter 4, I explore the mechanisms that keep the group together whilst foraging and moving around the territory. I found that dominant individuals are more likely to initiate leading events and more likely to be followed by all other group members, initiating a `successful' lead that keeps the group together. The calls used whilst leading off do not appear to contain a dominant signature, and individuals that call and lead further are more likely to be followed, regardless of their dominance status. The most common patterns in leading and following are likely to reflect the most stable strategy for pied babblers, where dominant individuals hold the highest incentive to lead and subordinates pay thelowest costs of synchronizing movements with others. However, calling provides the opportunity for subordinates to successfully lead in situations where their own personal incentives are high, such as on days prior to dispersal. Pied babblers give a variety of loud calls in various contexts and these can be performed as either solos or group choruses. In chapter 5, I classify eight distinct call types, two of which are unique to males and one unique to females. Three additional calls types were used significantly more by the dominant male of the group, and another most often by dominant females. I discuss the possible functions and implications of the different calling patterns in this species. Group chorusing always occurs if another group is present, but also occurs in some intra-group contexts. In chapter 6, I investigate the function of group choruses, first looking at the patterns in which they occur, the difference in investment between individuals, and the significance of sex-specific call types. In common with other studies, my results suggest that choruses serve multiple functions, both in territory defence but also potentially acting as vocal billboards for the dominant pair to advertise their presence. In this way, choruses may aid in maintaining intra-group dominance hierarchies, and allow dominant females especially to deter opposite-sex competitors in order to retain their breeding position. Groups must defend their territory in order to retain access to resources such as food, breeding sites and sleeping sites, and all group members benefit from this behaviour. In pied babblers territorial signalling involves movement to territory boundaries and then long periods of group chorusing in combination with vigorous posturing displays. In chapter 7, I explore the seasonal patterns in territory defence and show that a reduction in territorial encounters and the strength of response to intruders in the nonbreeding season may be due to an energetic constraint, rather than being driven by breeding behaviours. Taken together, these results suggest that complex groups, where requirements and incentives are likely to be heterogeneous, can function successfully as a group by using signals to mediate the costs and benefits of group-living