Birds selecting a nest site have to find the best compromise between the risk of encountering predation, the availability of food near to the nest and microclimatic requirements. As the optimal solution of this problem will vary with changes in predator abundance, we ask whether birds are capable of assessing such changes and of adjusting their nest-site choice accordingly. The reproductive success of dusky warblers Phylloscopus fuscatus, breeding in a mosaic of bushland and tundra habitat in the Russian Far East, varied greatly depending on the abundance of a nest predator, the Siberian chipmunk Tamias sibiricus. Using artificial nests we analysed which strategies dusky warblers should follow to avoid nest predation by chipmunks. We then compared the nest sites which dusky warblers actually had chosen in years with very high chipmunk densities (1998 and 1999) with those chosen in 1997, when chipmunks were almost absent from the study area. We found that safe nest-sites were preferred over those offering other advantages (microclimate, proximity to food) when the risk of predation was high, and we could not detect any confounding factor that might alternatively have caused these striking between-year differences. Our study suggests that even a short-lived passerine may be capable of choosing its nest site according to the actual predation risk. We show that such behavioural plasticity can lead to a paradoxical situation where better-protected nest-sites (selected in years and areas with high risk), on average, suffer greater predation than sites offering low safety. Thus, behavioural plasticity, if undetected, may result in serious misinterpretation of nest-predation patterns. A review of the literature suggests that adaptive plasticity in nest placement may be more widespread than is currently recognized.
