Long-term decoupling of a local population trend of the european pied flycatcher ficedula hypoleuca from nest box abundance indicates the importance of old-growth forest

Most populations of the european pied flycatcher Ficedula hypoleuca are decreasing. Different reasons for the decline are discussed, including biotic interactions and climate change. It is evident that many studies have been conducted in populations depending completely on nest boxes, but the influence of this artificial support on population dynamics is not well understood. we investigated the population dynamics of the pied flycatcher in the Kottenforst, an old- growth forest in western Germany, using recent data as well as historical records. we also determined the proportion of pairs breeding in nest boxes vs. natural nesting places. Specifically, we quantitatively analysed forest structure around tree holes occupied by the pied flycatcher. we found a continuous increase in population size since its establishment in the 1960s, which contrasts with overall long-term population trends in europe as well as the regional trend. whereas importance of nest boxes decreased over recent years, the majority of pairs are occupying tree holes for breeding, which are abundant in the richly structured, open old-growth forest. This forest structure seems to be optimal for the pied flycatcher since it allows flying insects to be hunted close to the nest. finally, we discuss how forest structure and age as well as tree hole and insect availability may determine population trends of the pied flycatcher and highlight the importance of long-term studies.