Acoustic communication and collective decision-making in social animal groups

Abstract

Many species live in complex societies in which individuals must coordinate their behaviour to maintain cohesion and gain the benefits of social living, both within and between species. Communication plays a central role in enabling rapid coordination and reducing consensus costs when group members differ in their needs and information. In this PhD thesis, I examine how social animals use communication to coordinate behaviour, particularly in antipredator contexts. The thesis includes introductory and concluding chapters (Chapters 1 and 7), a conceptual review (Chapter 2), and four empirical studies on social birds (Chapters 3-6). In Chapter 2, I develop a conceptual framework describing how acoustic signals and movement cues are used to coordinate behaviour in social animal groups. By integrating information masking, discrimination ability, and encoding limitations, the framework predicts that behaviours involving the timing of events or the expression of specific actions should rely more on acoustic signals, whereas decisions involving complex choices should generally rely on movement cues. This framework also highlights key future directions, including multimodal communication and decision-making in mixed-species groups. Guided by this framework, I conducted three playback experiments on white-winged choughs (Corcorax melanorhamphos), a highly social cooperative breeder. In Chapter 3, I show that adult choughs possess a functionally referential alarm-call system that conveys threat type and urgency. Choughs produce three distinct alarm calls: terrestrial alarms for ground-based predators, aerial whistles for high-flying hawks, and flee alarms for high-urgency threats. Birds responded appropriately to alarm-call playbacks, typically looking around to terrestrial alarms, looking upwards to aerial whistles, and fleeing to flee alarms. In Chapter 4, I show that young choughs develop appropriate alarm-call responses slowly, remaining poor at interpreting alarm calls for several months after fledging. Most juveniles responded to all alarm types by simply looking around, even when exposed to high-urgency flee alarms, suggesting that extended social protection and vocal complexity may shape learning in this species. In Chapter 5, I show that choughs integrate multiple components of social alarm information to coordinate collective antipredator behaviour. They assess alarm-call type to decide what action to take, and caller number to modulate both the intensity of their response and the likelihood of propagating alarm information to others. This decision rule helps reduce false alarms and unnecessary information cascades that can negatively affect group living. Together, these findings demonstrate how acoustic communication enables white-winged choughs to remain both safe and cohesive. In Chapter 6, I extend these insights beyond single-species societies to mixed-species systems. I conducted playback experiments to examine danger information transfer in mixed-species bird flocks in Taiwan. I find strong asymmetries in predator detection and/or danger communication between leader and follower species, with leader species always initiating mobbing calls. However, mobbing calls from leaders did not elicit stronger antipredator responses than those from followers. I propose that larger conspecific group sizes and kin-selected behaviours may favour the evolution of superior antipredator signalling in leader species, leading to asymmetric information transfer about danger between species. These results further suggest that mutual antipredator benefits are common in mixed flocks, promoting positive interspecific interactions and enhancing flock cohesion. Together, these findings suggest that sociality not only increases animals' reliance on social information, but also favours communication systems that allow information about danger to be encoded, assessed, and shared in ways that support effective antipredator behaviour.