Songbird populations in fluctuating environments: Predator responses to pulsed resources

In response to mast production of acorns, populations of small mammals, such as the white-footed mouse and eastern chipmunk, greatly increase and, in turn, initiate cascades of direct and indirect effects that permeate throughout forest communities. To examine the strength of the effects stemming from acorn crops, we conducted studies in oak-dominated deciduous forest of the Hudson Valley, New York, documenting the connections between predator abundance (small mammals and raptors) and the abundance and reproductive success of five woodland songbirds. Nest predation rates increased at high rodent densities for Veeries, Red-eyed Vireos, and Wood Thrushes. Analysis of Breeding Bird Survey (BBS) data from surrounding counties demonstrated a unimodal relationship between rodent density (year t) and songbird abundance (year t + 1), rather than a monotonic decline. Regional Christmas Bird Count data for Sharp-shinned and Cooper's Hawks showed a positive correlation between rodent density and accipiter abundance the following winter, suggesting that hawks exhibit a numerical response to rodents. Taken together, we suggest that predation by accipiters when rodents are scarce, in combination with high nest predation when rodents are abundant, may explain the unimodal relationship between rodents and songbirds. Nonlinear relationships between predator and prey abundance and indirect interactions appear to characterize this system.