We found that two avian predators exerted different selection on the morphology of galls induced on goldenrod, Solidago altissima, by the terphritid fly, Eurosta solidaginis. Galls may contain: Eurosta solidaginis, its parasitoid Eurytoma gigantea, and multiple Mordellistena convicta, an inquiline beetle, which can consume plant tissue and other larvae that they encounter in the galls. Since gall morphology is partially determined by Eurosta genotype when avian predators kill Eurosta they can exert selection on gall morphology. The objective of this study was to determine how the differing foraging strategies of these two bird species exerts selection on gall morphology. Predation on goldenrod galls from areas with high bird predation (forest) and from areas of low predation (prairie) was measured on specially designed stands placed throughout the forest habitat around Duluth, Minnesota. Predation on galls on transects of naturally occurring patches of goldenrod was also measured.
Downy woodpeckers (Picoides pubescens) and black capped chickadees (Parus atricapilla) use different cues to locate prey within the gall. Downy woodpeckers were significantly more efficient than black capped chickadees at locating and excavating larvae from galls. Parasitic wasps and inquiline beetles were avoided by both birds. Black capped chickadees attacked galls that were significantly larger than the mean gall size containing Eurosta. In contrast, downy woodpeckers attacked galls that were near the mean size that contained fly larvae. Eurosta galls have become a model system for the study of morphological evolution, and this is the first study to demonstrate that two bird species exert differing selection on gall morphology.