Avoiding predators in a fluctuating environment: responses of the wood warbler to pulsed resources

Deciduous forests are characterized by the production of seed crops that may vary dramatically among years. In response to these pulsed resources, rodent populations grow rapidly, which may have crucial consequences for entire forest communities, including songbirds. It has been hypothesized that in response to these rodent outbreaks, the wood warbler Phylloscopus sibilatrix may exhibit nomadic behavior to avoid nest predation. We used data from the Polish Common Breeding Bird Survey and search query time series from Google Trends as indices of rodent numbers to investigate whether wood warbler respond to rodent outbreaks at a broad spatial scale. Additionally, we investigated whether population fluctuations of Eurasian Jay Garrulus glandarius, the important predator of wood warbler nests, negatively correlated with wood warbler densities and how rodent outbreaks may have affected the outcome of interactions between those species. Results suggested that in years with low rodent abundance, wood warblers avoided settling in areas with high densities of jays. However, when rodent abundance increased in response to masting, wood warblers switched settling strategy and exhibited nomadic behavior. Moreover, this phenomenon acts at a broad geographical scale, which is a unique feature for European forest-dwelling insectivorous. We proposed the hypothesis that wood warblers perceive different predators as unequal and exhibit a risk sensitive antipredator behavior in their habitat selection process. Such a sophisticated mechanism of avoiding predators would suggest that wood warblers are able to acquire information about predation risk, for example, by assessing rodent abundance and use this information to adjust settlement decisions appropriately.