Functionally referential communication about danger in cooperatively breeding white-winged choughs

Effectively communicating information about danger can enhance the anti-predator benefits of group living. Many social mammals use functionally referential alarm calls to convey predator-specific information, such as predator type and threat urgency, enabling appropriate responses. However, relatively few bird species—particularly cooperative breeders—have been subjected to experimental assessment. White-winged choughs, Corcorax melanorhamphos, are obligate cooperative breeders living in groups year-round. They have a complex yet barely studied vocal repertoire. We carried out field observations, acoustic analyses and a playback experiment to test whether chough alarm calls are functionally referential. Additionally, we explored the occurrence of a possible visual display when responding to threats. Choughs produced three different alarm calls in response to different threats: terrestrial alarm calls for ground-based predators, aerial whistles for high-flying hawks and flee alarm calls for immediate, high-urgency threats. Blind scoring of video revealed that birds responded appropriately to playbacks of alarm calls alone, typically looking around to terrestrial alarms, looking up to aerial whistles and fleeing to flee alarms. Furthermore, birds were much more likely to display ‘bulging eyes’ in response to aerial whistles compared to terrestrial alarms, suggesting that ‘bulging eyes’ might be a visual signal associated with aerial threats. Our findings demonstrate that white-winged choughs, a highly social bird species, possess a functionally referential alarm-call system that conveys information about the type of threat and urgency. We propose that integrating information from multiple sensory modalities in alarm communication, such as auditory and visual signals/cues, may be common in social animals and warrants further investigation.