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

Abstract

1999. Density-related predation by the Carolina chickadee, Poecile carolinensis, on the leaf-mining moth Cameraria hamadryadella at three spatial scales. -Oikos 87: 105-112.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

A perspective on the importance of within-tree variation in mortality risk for a leaf-mining insect

“…In general, mine size might also be a reliable cue for natural enemies to detect a suitable host or prey since it corresponds to the size of a larva. In the case with leaf-mining larvae, avian predators, except for chickadees, may not prefer small leaf-mining larvae because they are difficult to excavate and especially if alternate prey are available (Low and Connor, 2003;Connor et al, 1999).…”

“…We studied a population of the solitary oak leafminer, Cameraria hamadryadella (Lepidoptera: Gracillariidae) within the oak grove at the Orland E. White State Arboretum of Virginia in Boyce, VA, which has been researched previously (Connor and Cargain, 1994;Connor, 1991;Connor and Beck, 1993;Connor et al, 1999). Cameraria hamadryadella is a bivoltine leaf-mining moth that feeds on oaks (Quercus spp.)…”

Native Birds and Alien Insects: Spatial Density Dependence in Songbird Predation of Invading Oak Gallwasps

“…In addition to responding to variation in prey density between trees, birds may also respond to variation in prey density at smaller spatial scales, such as branches within trees, or shoots within branches [27], [28], [32]. However, such local variation in density is difficult or impossible to estimate for mobile prey.…”

“…Most studies on tit foraging have used rates of caterpillar frass fall as a surrogate for caterpillar density, but this measure is impossible to estimate for specific branches within the same tree. A major advantage of working with sessile prey (including galling insects) is that their extraction by birds leaves characteristic signs, allowing analysis of patterns at within-tree spatial scales [18], [28], [32]. At each of our 8 sites we therefore analysed bird foraging responses to variation in prey density at three spatial scales: shoots within branches, branches within trees, and trees within sites.…”

“…The persistence of any signal of density dependence provides an indication of the attractiveness of this resource to native birds. Our a priori expectation was that any signature of density dependence would decline as total gall abundance declines and birds recruit to alternative, more profitable foods [28], [33].…”