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

Abstract

Animals may update their assessment of predation risk according to how apotential predator approaches them. For example, the predator’s head and gazeorientation (direction of attention) may reveal its intentions, and faster-approachingpredators are likely to represent greater risk. We examined the reactions ofhadeda ibises Bostrychia hagedash. These large birds demonstrate a wide repertoireof responses to being approached (e.g. continuing to forage, slow walking, rapidescape walking, flight and alarm calling). Birds were approached tangentially 112times by a human who either had the head and eyes directed towards (65approaches) or directed away from (47 approaches) the birds to test the hypothesisthat the direction of the observer’s attention informs alert distance (AD) and flightinitiation distance (FID) in these birds. Direction of attention had a significanteffect on AD and FID as well as the likelihood of taking flight and alarm calling byhadedas, with birds appearing to associate attention directed towards them as anindication of increased risk. Hadedas were able to differentiate between thedirection of attention of an approaching human, whether or not there weremultiple other humans in the near vicinity. We also examined whether theobserver’s approach speed altered the birds’ responses. Approach speed affectedthe birds’ FID, suggesting that they perceive greater danger in a faster-approachingintruder compared with a slower-walking one. These results support thepredictions of optimal escape theory and emphasize the high resolution of antipredatoryawareness in these birds. The marked success of hadeda ibises in urbanenvironments may be due to their ability to become habituated to human presenceand to modify their antipredator behaviour in response to subtle cues. These maybe common traits of bird species that successfully adapt to urban environments.