Long-term variation in white-tailed deer abundance shapes landscape-scale population dynamics of forest-breeding birds

Abstract

Over-browsing by white-tailed deer impedes forest regeneration, alters floristic and structural attributes of stands, and reduces biodiversity. Although numerous exclosure experiments have demonstrated negative impacts of deer on songbirds at small spatial scales, the effects of deer on avian communities at large spatial and temporal scales remains poorly understood. We integrated data from the Breeding Bird Survey with historic information on deer abundance in a multi-species hierarchical model to assess spatio-temporal relationships between deer and bird abundances across the state of Pennsylvania between 1981 and 2015. Our results provide evidence of strong links between deer and both avian functional groups and individual species, depending upon the extent each relied upon particular forest strata. Specifically, species that forage or nest near the ground were more negatively affected by deer than canopy-associated species. Likewise, mature-forest species were more sensitive to deer than successional-forest species or habitat generalists. With only a few exceptions, species-level responses to deer were consistent with group-level effects. Given the diverse suite of spatio-temporal stressors impacting migratory songbirds throughout their annual cycle, it was not surprising that long-term population trends of focal species were not significantly associated with deer abundance. Our findings, however, provide evidence that deer play an important role in songbird community structure, as mediated by the reliance of individual species upon particular forest strata and, as such, should be considered within the context of forest bird conservation.