Experimentally increased insectivorous bird density results in a reduction of caterpillar density and leaf damage to Pyrenean oak

The indirect effect of insectivorous breeding-bird density on Pyrenean oak (Quercus pyrenaica) through consumption of leaf-damaging caterpillars in a temperate deciduous forest was examined. Caterpillar abundance, caterpillar body mass and the resultant leaf damage were compared between a plot with nest-boxes and a plot without nest-boxes within the same forest. First, the installation of nest-boxes led to an increase in the breeding population of insectivorous birds. Second, the present study reports experimental evidence, under natural conditions, that a high breeding-bird density depresses the numbers of free-living defoliating caterpillars. Third, caterpillars reached significantly lower body masses in the nest-box plot than in the control plot. Finally, caterpillar damage to oak leaves was significantly less in the nest-box plot compared with the control plot. Therefore, the installation of nest-boxes is an effective means of decreasing insect damage to oak leaves. These results suggest that breeding-bird predation (the third trophic level) can be important in the regulation of interactions between oak and caterpillars (the first and second trophic levels, respectively).