Because they regulate small mammal populations, mast seed crops have the potential to exert strong indirect effects on songbird populations. When more small mammals are present, nest predation rates increase, resulting in decreases in both nest productivity and the recruitment of breeding adults. White-footed mice and eastern chipmunks attack eggs within nests of ground-nesting and shrub-nesting birds. Our long-term monitoring data indicate that fledging success of veeries, red-eyed vireos, and woodthrushes is poor in years of high mouse abundance and good in years of low mouse abundance. Our expectation, therefore, was that population density of vulnerable songbirds would be inversely correlated with prior year's mouse density and with acorn production two years previously. In fact, we found that veery and woodthrush abundance was strongly reduced following a summer of abundant mice, but that these birds also were scarce following a summer of low mouse abundance. Only following intermediate mouse years did these birds thrive.
We are currently testing the hypothesis that low bird abundance following rodent population crashes was caused by aerial predators increasing their attack rates on fledgling and/or adult birds in the year of a rodent crash. Christmas Bird Count data for Sharp-shinned and Cooper's hawks demonstrate a positive correlation between rodent abundance and accipiter abundance the next year, supporting this hypothesis. Songbird predation by accipiters when rodents are scarce, in combination with high nest predation by mice and chipmunks when these rodents are abundant, may explain the unimodal relationship. Nonlinear relationships between predator and prey abundance and indirect interactions appear to characterize this system.