Decomposing an elevational gradient in predation by insectivorous birds

Insectivorous birds have ecologically important effects on prey abundance, behavior, and evolution, and through top-down control birds indirectly reduce herbivory and promote plant growth. While several studies sought to characterize biogeographic patterns in top-down control by birds, variation in bird predation along elevational gradients is not well characterized both in terms of its commonness and the mechanisms underlying such variation. Here we characterized variation in bird predation along a 700m montane elevation gradient using artificial clay caterpillars, assessing the roles of variation in aridity, other elevational effects not associated with aridity (e.g., most notably growing season length) and bird abundance and diversity. Multivariate models revealed attack rates increasing with aridity (when controlling for the effects of elevation) and elevation (when controlling for aridity). Because aridity declines with elevation, elevational patterns were not detectable in a univariate analysis. Bird abundance (but not diversity) decreased with elevation (but not aridity) and did not provide an explanation for our results, suggesting the underlying mechanisms were behaviorally based. We speculate that the declining abundance of insect prey with elevation and aridity leads to increased bird foraging efforts and thus the likelihood of attacking clay caterpillars. If widespread, these dynamics have important consequences for both the interpretation of predation bioassays generally and for our understanding of the multivariate drivers of variation top-down control by predators and predation risks experienced by prey.