Asymmetric information in mixed-species mobbing flocks: why are leader species special?

Abstract

Asymmetric information transfer can shape the social structure of animal groups. Birds in mixed-species flocks can gain antipredation benefits by relying on both personal and social information, ultimately increasing their survival. However, not all species in a flock contribute danger-related information equally, leading to complex patterns of information flow between species. Understanding these differences in information flow and usage among species is key to exploring the mechanisms underlying the formation of such social assemblages. In this study, we combined mixed-species flock surveys with playback experiments to investigate the asymmetric information transfer about danger between leader and follower species in the montane forests of Taiwan. The playback experiments included four treatments: predator calls (personal information), leader and follower mobbing calls (social information) and control. We predicted that leader species would be better at detecting threats than follower species and that the mobbing calls of leader species would elicit stronger antipredator responses in mixed-species flocks than those of follower species. As predicted, and more striking than anticipated, leader species were always the first to initiate mobbing calls in response to predator calls, suggesting strong asymmetry in predator detection and/or danger communication between the two species' roles. Moreover, our results showed that birds responded much more strongly to the playback of predator calls than to playback of mobbing calls. Contrary to our prediction, there was not a greater antipredator response to mobbing calls given by leader compared to follower species. Here, we propose that larger conspecific group sizes and associated kin-selected behaviour could drive leader species to develop superior antipredator signalling systems, potentially leading to asymmetric information transfer about predation risk between species. We also suggest that mutual antipredator benefits may be common in mixed mobbing flocks, promoting positive interactions among bird species, and ultimately strengthening the cohesion and formation of mixed flocks.