Choice of foraging area with respect to predation risk in redshanks: The effects of weather and predator activity

Animals often trade off risk of predation and risk of starvation when making foraging decisions. Redshanks, Tringa totanus, wintering in an area of high sparrowhawk, Accipiter nisus, predation were studied to determine how simultaneous variations in energy requirements and the risk of predation determine choice of foraging area. In particular, we focus on the effects of weather on this trade-off. Redshanks could either feed on a saltmarsh where both food availability and predation risk are high, or on safer, but less productive, inter-tidal flats. On the saltmarsh, redshanks could choose to feed in profitable but high risk areas close to cover or in lower risk creeks. In general, low temperatures resulted in birds using the saltmarsh habitat more, foraging closer to cover, and making less use of safe creek micro-habitats. This is consistent with low temperatures increasing redshanks' metabolic costs, thus increasing their preparedness to accept higher predation risk for greater foraging returns. At low temperatures sparrowhawks were more successful at catching redshanks, probably as a consequence of adoption of more risky foraging options by the redshanks, but rate of attack was probably not strongly affected by weather. Redshanks responded to precipitation by foraging in areas that reduced their predation risk. When precipitation was high, redshanks also became more prone to false alarm flights. These effects suggest that precipitation may increase predation risk by making the approach of a predator harder to detect. Redshanks foraged further from cover in high winds, suggesting that wind affected predation risk more than starvation risk. Redshanks responded strongly to sparrowhawk activity: use of the saltmarsh was less and redshanks foraged further from cover during days with a high attack frequency.