Climate and food synchronize regional forest bird abundances

Analysis of synchrony in population fluctuations can help to identify factors that regulate populations and the scales at which these factors exert their influence. Using 15 years of data on the abundances of songbirds at four replicate forest sites in New Hampshire, USA, we addressed two main questions: (1) Are forest bird populations synchronous at the scale measured (tens of kilometers), and if so, (2) what environmental factors are responsible for the synchrony? Nine of the 10 bird species we examined exhibited significant spatial synchrony across the four sites. Within nesting and foraging species groups, tree nesters and foliage gleaners exhibited the highest spatial synchrony. Long-distance (Neotropical) migrants exhibited higher spatial synchrony than did short-distance migrants or year-round residents. Synchrony within and among six species of long-distance, migratory, insectivorous birds was correlated with synchronous fluctuations in the abundance of lepidopteran larvae, a primary food type during the breeding season, which in turn have been shown to be influenced by El Nino/La Nina global climate patterns. Abundances of year-round resident species were related to another large-scale climatic phenomenon, the North Atlantic Oscillation. Winter weather can have both direct (e.g., via temperature-mediated mortality) and indirect (e.g., via winter food availability) effects on year-round resident species. We do not believe that predation on adults or nests accounted for the observed synchrony. Dispersal among regional populations in this system may have played a role but is likely a product of the influence of regionally synchronous caterpillar fluctuations on bird reproduction. Long-term regional population trends may have contributed to the observed synchrony for some species, but we do not consider these trends to be primary factors. Our findings of population synchrony support the importance of food and climate in influencing forest bird abundances and have broad implications for potential responses of bird and insect populations to climate change.