Who are you looking at? Hadeda ibises use direction of gaze, head orientation and approach speed in their risk assessment of a potential predator

Animals may update their assessment of predation risk according to how a potential predator approaches them. For example, the predator's head and gaze orientation (direction of attention) may reveal its intentions, and faster-approaching predators are likely to represent greater risk. We examined the reactions of hadeda ibises Bostrychia hagedash. These large birds demonstrate a wide repertoire of responses to being approached (e. g. continuing to forage, slow walking, rapid escape walking, flight and alarm calling). Birds were approached tangentially 112 times by a human who either had the head and eyes directed towards (65 approaches) or directed away from (47 approaches) the birds to test the hypothesis that the direction of the observer's attention informs alert distance (AD) and flight initiation distance (FID) in these birds. Direction of attention had a significant effect on AD and FID as well as the likelihood of taking flight and alarm calling by hadedas, with birds appearing to associate attention directed towards them as an indication of increased risk. Hadedas were able to differentiate between the direction of attention of an approaching human, whether or not there were multiple other humans in the near vicinity. We also examined whether the observer's approach speed altered the birds' responses. Approach speed affected the birds' FID, suggesting that they perceive greater danger in a faster-approaching intruder compared with a slower-walking one. These results support the predictions of optimal escape theory and emphasize the high resolution of antipredatory awareness in these birds. The marked success of hadeda ibises in urban environments may be due to their ability to become habituated to human presence and to modify their antipredator behaviour in response to subtle cues. These may be common traits of bird species that successfully adapt to urban environments.