In the struggle between predators and prey, a split-second can separate the quick from the dead. Alarm calls warning of immediate danger must, therefore, send rapid messages, yet animals often signal more urgent danger using repeated notes. This is paradoxical because more notes take more time to deliver. New Holland honeyeaters solve this problem elegantly. They front-load information about urgency into the first note of their alarm calls so that listeners can respond in the blink of an eye. But they also follow with more notes to back up the message and to signal for how long to remain hidden.

In a new paper, Jessica McLachan, a recent PhD student in the Department, and Robert Magrath, ANU, report a novel solution to a paradox of alarm signal design, specifically how to communicate both rapidly and reliably about danger. In brief, they show that wild birds use two complementary mechanisms – the acoustic structure and number of alarm notes – to communicate detailed information about immediate danger, enabling listeners to assess the degree of danger and determine whether to flee to cover and for how long to remain hidden. Remarkably, the birds can respond to alarm calls in the blink of an eye. 

Alarm calls warn others about danger and they need to communicate about threats both quickly and reliably, creating a conundrum for signal design. Many species signal more urgent danger by producing more notes in their alarm calls, which makes it more likely that listeners will detect the call. But this is puzzling in the context of immediate danger and fast-moving predators: it takes longer to hear more notes and listeners will, therefore, take longer to respond; yet research on alarm calls generally overlooks this problem. 

In this study, McLachlan and Magrath found that honeyeaters produce more notes in response to more dangerous predators, but that they also adjust the frequency (pitch) of the first note in the call according to the degree of danger. Playback experiments showed that listeners pay attention to both of these aspects of alarm calls: honeyeaters were more likely to flee when played only the first note from more urgent alarms, but they remained in cover for longer, and were more likely to respond, after hearing an alarm call artificially composed of more notes. The signaling system of honeyeater alarm calls thus resolves the paradox by facilitating rapid responses through changes in the acoustic structure of notes while enhancing signal detection and reliability through an increase in the number of notes when faced with greater danger.

Rapid responses to danger can be critical for survival, yet we know very little about how quickly species process social information coming from anti-predator signals. The researchers used a mobile video system to record the precise timing of honeyeater responses to playbacks, enabling us to address this gap in the literature on reaction times. These recordings showed that honeyeaters respond remarkably quickly to alarm calls. They reacted in 100 ms and took flight in around a quarter of a second. But honeyeaters did not simply react quickly to any sound played nearby, as they took an average of about 500 ms to respond to control calls played at the same amplitude, suggesting that their rapid response is fine-tuned to signals of danger. These results thus provide insight into the astonishing speed of fear in wild birds.

Surprisingly, the issue of rapid communication has not already been addressed in alarm calls, given its importance in predator avoidance and the implications for signal design. This study provides the first clear evidence of a mechanism by which alarm calls can transmit information remarkably quickly to listeners. These results make an important contribution to our understanding of animal speed, socially-mediated anti-predator defenses and the efficacy of social information transmission, while solving the problem of fast yet fail-safe communication in signal design.