Density-Related Predation by the Carolina Chickadee, Poecile carolinensis, on the Leaf-Mining Moth, Cameraria hamadryadella at Three Spatial Scales

Abstract

The impact of predation by the Carolina chickadee. Poecile carolinensis, on populations of the leaf-mining moth Cameraria hamadryadella, was inversely spatially density-dependent at each of three spatial scales: among woodlands, among trees. and among leaves. P. carolinensis exhibited an aggregative response to the density of C. hamadryadella at the scale of woodlands, but did not preferentially forage in trees or on leaves with high densities of leaf mines. P. carolinensis exhibits no numerical response to the abundance of C. hamadryadella The functional response of P. carolinensis suggests that per capita foraging efficiency is actually lower in high-density populations of C. hamadryadella. This may arise because of predator confusion and because of mechanical limitations to foraging behavior. Human subjects, used as surrogates for chickadees, took more time to discover and discovered fewer leaf-miner larvae on leaves with higher proportions of the leaf-area mined. Chickadees hang upside down to attack leaf-miner larvae and this posture may limit the duration of foraging bouts. Either predator confusion or mechanical limitation, alone or in combination, could account for the observed inversely density-dependent impact of predation at the leaf scale. In outbreak populations of C. hamadryadella where P. carolinensis is a common predator, the inversely density-dependent attack by P. carolinensis on C hamadryadella complements the strongly density-dependent mortality caused by intra-specific competition. Therefore, the pattern of predation by P. carolinensis contributes more to suppression of C. hamadryadella than would occur if P carolinensis foraged in a density-dependent manner. Birds in the family Paridae appear to be the dominant avian predators of leaf-mining insects. We suggest that their altered leg musculature permits them to use a hanging upside down posture more frequently and pre-adapts them for foraging on leaf-mining insects feeding on broad-leafed plants.