Making heads turn: association between head movements during vigilance and perceived predation risk in brown-headed cowbird flocks

Models of antipredator vigilance show that animals should be more vigilant when in smaller groups, when neighbours are farther away, and when they occur at the edge rather than at the centre of a group. However, models fail to specify how animals are expected to look out for threats while actually scanning. We suggest that head movements during scanning may represent a way that animals can control the value of scanning while in groups. Head movements can be used to increase visual coverage (visual search) as well as bring features of the environment to more sensitive parts of the eyes (visual fixation). We conducted a seminatural experiment with brown-headed cowbirds, Molothrus ater, in which we manipulated group size and neighbour distance, and recorded vigilance for peripheral and central individuals. During scanning, the rate of head movements by peripheral individuals increased, probably to enhance visual coverage due to their higher perceived risk of predation. However, individuals that were farther apart and in smaller groups used a different scanning strategy by reducing their rate of head movements (e.g. increased the averaged time they spent holding a given head position steady), probably to fixate their gaze longer on neighbours. Visual fixation may be useful for obtaining social information about predation threats from the behaviour of groupmates. We show that (1) a vigilance metric associated with head movement behaviour, which is attuned to the visual system of a species, can provide novel information about vigilance in groups, and that (2) birds have different head movement strategies, probably associated with different visual targets, when the risk of predation is higher.