Exploring the multi-scale drivers of predation on artificial caterpillars by insectivorous birds in a Central European montane forest region

BackgroundInsectivorous birds provide a key ecosystem service as predators of folivorous insects. Changes in forest structural complexity and composition, currently underway in temperate forests, may affect the predation pressure exerted by birds on folivores, by altering the abundance of avian predators. However, studies examining this are few, as well as on how predation rates vary within forest stands. We aimed to address these gaps, by focusing on beech (Fagus sylvatica) trees in managed montane forests of the Black Forest region, in Central Europe. We measured abundance and diversity of birds using point counts on 135 1-ha plots in 2017-2022. To assess bird predation pressure on caterpillars, we placed 3153 artificial caterpillars on selected 24 plots for a week, in the spring of 2021, and inspected them for bird bite marks. We placed caterpillars on up to four focal tree locations in each plot, in both the canopy and the understorey of each tree. We also measured forest structure and composition both at the scale of plots and the vicinity of focal trees.ResultsWe found that higher shrub-layer cover and share of broadleaf trees at the plot scale were associated with higher overall abundance of foliage-gleaning insectivorous birds. However, predation rates on artificial prey appeared to be more closely related with the abundance of single species (particularly the Eurasian chaffinch, Fringilla coelebs), which in turn responded to other facets of forest structural complexity. Within plots, predation pressure was overall higher in the canopy than in the understorey, but this may reflect observer effects rather than true differences between vegetation layers. Predation rates also showed strong variation across trees, being higher in beech crowns with a monospecific neighbourhood, and in saplings surrounded by a mixed-species shrub layer.ConclusionsOur results suggest that specific bird species and particular facets of forest structural complexity contribute disproportionately to the predation pressure exerted by birds on caterpillars, and that fine-scale management decisions may also affect the role of birds as predators. However, further research is needed to confirm these patterns and overcome the limitations we identified.