PREDATOR-MEDIATED INTERACTIONS BETWEEN AND WITHIN GUILDS OF NESTING SONGBIRDS - EXPERIMENTAL AND OBSERVATIONAL EVIDENCE

Apparent competition (i.e., a mutually negative indirect interaction b etween prey species through shared predation) arises when predator abu ndance or foraging effort increases with total, prey availability. We review and formalize several patch-use models from which we derive pre dictions for how the degree of coupling (from the predators' perspecti ve) between nesting guilds (defined as species nesting within a vegeta tion stratum) affects the outcome of shared predation. We then determi ne which model best applies to nest predation on woodland songbirds an d artificial nests by a natural population of raccoons. Using artifici al nests, we showed that increasing the density of nests placed either in shrubs or on the ground increased overall predation (i.e., proport ion of nests) on both types. We also tested for apparent competition b etween American robin and wood thrush, two coexisting woodland songbir ds that commonly nest within the shrub stratum. Nest predation increas ed for wood thrushes but not robins as the combined density of robin a nd thrush nests within two individual substrate types, Lonicera and Rh amnus, increased. Thus, we documented apparent competition both within and among nesting guilds. We discuss the possible relevance of this i nteraction in determining species diversity, particularly in the light of increasing generalist nest predators through anthropogenically dri ven changes in human-altered landscapes.