Background/Question/Methods This study quantifies top-down impacts on caterpillar density in a temperate forest ecosystem. During late spring, many forest birds rear their young on a diet dominated by caterpillars. At the same time, spiders are known to use caterpillars as prey, making these two predators a substantial source of mortality for these larval lepidopterans. Here, we use bird exclosures to quantify the impact of bird predation on caterpillar density across three tree species and relate this impact to spider density.

Results/Conclusions Tree branches with exclosures exhibited 47 % higher caterpillar density than those without exclosures. We can confidently attribute this difference in caterpillar densities to bird predation because this pattern was observed for the most abundant, palatable caterpillar species (Melanolophia canadaria; Geometridae), but was not observed for the most abundant, unpalatable caterpillar species (Lymantria dispar; Lymantriidae). Furthermore, we show that birds exhibit positively density-dependent predation on caterpillars, as the effect of bird predation was greatest where caterpillar densities were highest. Spiders did not decrease caterpillar densities, but rather accumulated on branches with high caterpillar densities. The positive association between caterpillar and spider densities differed across the three tree species.