Experimentally increased insectivorous bird density results in a reductionof 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 eviden ce, under natural conditions, that a high breeding-bird density depresses t he numbers of free-living defoliating caterpillars. Third, caterpillars rea ched significantly lower body masses in the nest-box plot than in the contr ol plot. Finally, caterpillar damage to oak leaves was significantly less i n the nest-box plot compared with the control plot. Therefore, the installa tion of nest-boxes is an effective means of decreasing insect damage to oak leaves. These results suggest that breeding-bird predation (the third trop hic level) can be important in the regulation of interactions between oak a nd caterpillars (the first and second trophic levels, respectively).