Experimental removal of strong and weak predators: Mice and chipmunks preying on songbird nests

We examined the effects of separate removal experiments of two generalist consumers, the white-footed mouse (Peromyscus leucopus) and the eastern chipmunk (Tamias striatus), on nest predation rates of forest songbirds. Mice are numerically dominant at our study sites and were shown to be strong predators in other predator-prey interactions, such as those involving gypsy moths. Therefore, we hypothesized that removal of mice would result in decreased levels of nest predation relative to control treatments with a complete predator assemblage, but that the removal of chipmunks would not result in decreased nest predation. Both hypotheses were supported. Mice depredated >60% of artificial nests in control plots (mouse populations intact), whereas chipmunks depredated ∼20%. Daily nest mortality rates in mouse removal treatments were less than half the rates in controls but were virtually identical between chipmunk removal and control treatments. Nonetheless, when we examined predation rates across plots in which the density of mice varied naturally, total daily mortality rates declined as the density of mice increased. This pattern occurred because mortality from non-mouse predators decreased as the density of mice increased and overwhelmed increasing mortality from mice to drive the overall dynamics of the system. Analysis of the relationships between the density of mice and predation rates by mice as a function of the abundance of natural food in their environment revealed probable reasons for these conflicting results. We suggest that high local densities of mice deplete resources for larger, non-mouse predators, which preferentially occupy areas of few mice and high local food abundance. In these areas, songbirds may be faced with higher overall nest predation dominated by non-mouse predators. Mice thus influence nest predation rates through both direct and indirect pathways.