Ecological conditions influence sentinel decisions

Ecological conditions can influence decisions relating to antipredator behaviour through impacts on the likelihood of detecting predators and the ability to hear vocalizations. Previous studies of antipredator behaviour have tended to focus on foragers, whose vigilance behaviour may be confounded by the type of food they are eating, and on receivers in vocal communication networks. We examined the impact of habitat and wind conditions on the behaviour of sentinels, individuals that suspend their own foraging to adopt a raised position to scan for danger while groupmates continue feeding, and that produce a variety of calls used by foragers to adjust their antipredator behaviour. Sentinels of the pied babbler, Turdoides bicolor, a cooperatively breeding bird, started guarding sooner and guarded for longer in long grass compared to more open habitats, and also initiated sentinel bouts sooner in high wind, probably because of the increased predation risk in such circumstances. Sentinels also selected positions that were both lower and closer to the foraging group when it was windy, potentially improving transmission of vocal signals that are valuable to foragers. Our results demonstrate that sentinel behaviour can be influenced by extrinsic factors, as well as the intrinsic factors previously shown, and suggest that ecological variation may affect decisions bearing both selfish and cooperative benefits.