The socio-ecology of fear: Nonlethal predator effects on the social composition of wild bird flocks

Nonlethal predator effects arise when individuals of a prey species adjust their behaviour due to the presence of predators. Such effects can affect individual fitness and population dynamics as strongly as lethal effects. To date, non-lethal predator effects have been studied in the context of individual behaviour, but they have considerable potential to be an important influence on the social structure and social behaviour of their prey. In this experimental study, we used model sparrowhawks to launch attacks on flocks of wild great tits and blue tits whilst monitoring their social dynamics. We show that nonlethal attacks caused instantaneous turn-over and mixing of group composition within foraging flocks. A single experimental ‘attack’ caused the amount of turn-over expected over three hours (0.9—3.3 hours) of undisturbed foraging. This suggests that nonlethal predator effects can greatly alter social structuring within populations, with implications for individual social behaviour, as well as longer-term consequences for pair formation and emergent effects determined by social structure. We propose that the emergent non-lethal effects of predators on prey - the ‘socio-ecology of fear’ - offers a new perspective on key drivers of social behaviour in wild populations.