Effects of forest heterogeneity on the efficiency of caterpillar control service provided by birds in temperate oak forests

Controlling herbivore insects by insectivorous birds is a major ecosystem service, nevertheless little is known about how local habitat features and forest management influence the efficiency of this service and about how the pest control service birds provide can be maintained and improved. We conducted an experiment in temperate oak forests in the Mátra Mountains, northern Hungary to measure bird predation rate of artificial caterpillars resembling winter moth (Operophtera brumata L.) larvae, to evaluate the relationships among insectivorous bird communities, caterpillar populations and leaf damage caused by caterpillars and to assess the effect of forest heterogeneity on these processes. We found, that structurally heterogeneous forests maintained a significantly higher abundance of insectivorous birds. Especially the tree size heterogeneity increased bird abundance. The rate of bird predation was positively related to the abundance of insectivorous birds as well as to caterpillar abundance, which indicates that birds were able to respond to caterpillar density. We were not able to demonstrate a direct negative effect of bird predation on caterpillar abundance and a positive effect of caterpillar abundance on leaf damage. Structurally heterogeneous forests, however, suffered from less leaf damage than did homogeneous forests, which result may indicates that the higher activity of insectivorous birds in heterogeneous stands resulted in lower activity of insect herbivores. Thus, we concluded that forest management can contribute to the mitigation of insect damages by maintaining the suitability of forest stands to the insectivorous bird communities through the maintenance of high stand heterogeneity and the presence of some key elements (e.g. retention tree groups, tree diversity, shrub layer).