Aerial alarm calling by male fowl (Gallus gallus) reveals subtle new mechanisms of risk management

Alarm calling is a classic problem in evolutionary biology. Although a signaller may increase the likelihood of survival for group members, which typically include kin and mates, there are inherent risks associated with any behaviour that increases conspicuousness to predators. Callers can increase their indirect benefits by calling only in the presence of an appropriate audience and manage concomitant costs by judicious investment. Possible tactics for controlling costs include facultative variation in call structure and timing, as well as sensitivity to the environmental and social factors that predict personal vulnerability. We examined individual variation in the alarm-calling behaviour of male fowl in naturalistic social groups. Previous studies of cost management have focused on variation at the level of alarm call rate. We took advantage of recent advances in wireless sound recording and remote video monitoring to test for more subtle variation in signal structure and timing. These were then mapped onto individual mating success and moment-to-moment changes in environmental and social context. Results replicate the previous finding that alarm calling is sensitive to both social rank and recent mating success. In addition, we detected systematic variation in call structure as a function of personal vulnerability and proximity to a rival male. The frequency bandwidth of alarms was reliably influenced by degree of vigilance prior to calling, suggesting that this acoustic dimension reflects motivational state. Taken together, these results reveal several novel tactics for risk management, complementing those previously described at the level of gross variation in alarm-calling behaviour.