Territory size of a migratory songbird in response to caterpillar density and foliage structure

The food value theory of territoriality predicts an inverse relationship between territory size and food density. However, several non-mutually exclusive hypotheses can explain this relationship. We studied the relationship between foliage density, food density, competitor density, and territory size of the Red-eyed Vireo (Vireo olivaceus), a canopy-dwelling, insectivorous songbird, in the Monongahela National Forest, West Virginia, USA, 1995-1998. First, we established the relationship between territory size and the canopy-tree foliage density. We then utilized the relationship between foliage density and foliage arthropod density (in particular, Lepidoptera larvae) to evaluate whether an inverse correlation between food density and territory size existed and to examine the proximate cues that Red-eyed Vireos used to adjust territory size. Foliage density in a habitat patch was used by Red-eyed Vireos as a proximate cue to set the size of a three-dimensional territory volume that ultimately corresponded to the average caterpillar density on those trees during the nestling stage. Territory volumes were smaller where foliage density was greater. Two measures of competitor density were not inversely correlated with territory size. These results support the structural-cues hypothesis for a species that forages in the structurally complex foliage of deciduous forests, and they are consistent with the food-value theory of territoriality. Caterpillar density varied widely over the course of the breeding season, among locations and among tree species, reaching a low during the nestling stage of the breeding cycle. Yet, the total territory volume utilized by Redeyed Vireos was inversely related to the density of caterpillars during this stage (i.e., when they would be needed as a food source to provision young), consistent with the food-value theory. Food availability at the time of territory selection was not a reliable cue for future food resources because of spatial and temporal variability in caterpillar density. Instead, a stable structural cue, foliage density, was highly predictive of caterpillar density during the nestling stage of an average, or typical, year. This study suggests that Red-eyed Vireos can reliably secure food resources for nestlings at the time of territory establishment, using foliage density as a cue.