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 nestboxes 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).