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 c onsumers, the white-footed mouse (Peromyscus leucopus) and the eastern chip munk (Tamias striatus), on nest predation rates of forest songbirds. Mice a re numerically dominant at our study sites and were shown to be strong pred ators in other predator-prey interactions, such as those involving gypsy mo ths. Therefore, we hypothesized that removal of mice would result in decrea sed 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), wher eas chipmunks depreciated similar to 20%. Daily nest mortality rates in mou se removal treatments were less than half the rates in controls but were vi rtually identical between chipmunk removal and control treatments. Nonethel ess, 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 p redators decreased as the density of mice increased and overwhelmed increas ing mortality from mice to drive the overall dynamics of the system. Analys is 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 re vealed 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 abundanc e. In these areas, songbirds may be faced with higher overall nest predatio n dominated by non-mouse predators. Mice thus influence nest predation rate s through both direct and indirect pathways.