Risk taking in natural predation risk gradients: support for risk allocation from breeding pied flycatchers

Predation risk is vital in foraging decisions because activity involves some degree of risk. In a natural setting, predation risk shows temporal variation, which has been largely neglected in antipredator studies. We tested a prediction of the risk allocation hypothesis in which allocation to antipredator behaviours will depend on temporal variation in perceived risk. Individuals are predicted to allocate heightened antipredator behaviours to brief infrequent periods of high risk, but with increasing frequency of high-risk periods, individuals will invest less in these behaviours. We tested this prediction using pied flycatchers, Ficedula hypoleuca, breeding at different distances from sparrowhawk, Accipiter nisus, nests. Sparrowhawk encounters are assumed to be frequent near hawk nests and decrease with distance away from them. Parent risk taking was measured as the time taken to resume nestling feeding after a period of increased risk. We found that the time to resume nestling feeding was associated with distance from sparrowhawk nests, supporting the paradox prediction of the risk allocation hypothesis. Flycatcher parents breeding near nesting sparrowhawks displayed weak antipredator allocation to a focal high-risk encounter. In contrast, flycatcher parents nesting further away increased their allocation to antipredator behaviour and showed long delays to resume nestling feeding. Our results highlight a natural setting in which risk allocation can be applied. Via encounter rates, central-place-foraging predators create perceived predation risk gradients that may lead to predictable antipredator responses in nature, especially when the prey is sedentary.

► We test prey risk taking in natural predator encounter gradients. ► Prey show increased risk taking with increased predator encounter rate. ► Results support the paradox prediction of the risk allocation hypothesis. ► Central-place-foraging predators appear to create predictability in prey antipredator responses. ► Spatial gradients in prey behaviour may be found in natural settings.