Ef. Conner et al., Density-related predation by the Carolina chickadee, Poecile carolinensis,on the leaf-mining moth, Cameraria hamadryadella at three spatial scales, OIKOS, 87(1), 1999, pp. 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 woodlan
ds, 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 abundanc
e of C. hamadryadella.
The functional response of P. carolinensis suggests that per capita foragin
g efficiency is actually lower in high-density populations of C. hamadryade
lla. This may arise because of predator confusion and because of mechanical
limitations to foraging behavior. Human subjects, used as surrogates for c
hickadees, took more time to discover and discovered fewer leaf-miner larva
e 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 d
uration of foraging bouts. Either predator confusion or mechanical limitati
on, alone or in combination, could account for the observed inversely densi
ty-dependent impact of predation at the leaf scale. In outbreak populations
of C. hamadryadella where P. carolinensis is a common predator, the invers
ely density-dependent attack by P. carolinensis on C. hamadryadella complem
ents the strongly density-dependent mortality caused by intra-specific comp
etition. Therefore, the pattern of predation by P. carolinensis contributes
more to suppression of C. hamadryadella than would occur if P. carolinensi
s foraged in a density-dependent manner.
Birds in the family Paridae appear to be the dominant avian predators of le
af-mining insects. We suggest that their altered leg musculature permits th
em to use a hanging "upside down" posture more frequently and pre-adapts th
em for foraging on leaf-mining insects feeding on broad-leafed plants.