Absolute Densities of Breeding Birds in Estonian Forests: A Synthesis

Abstract

This paper synthesizes all available quantitative information on total breeding densities of Estonian forest bird assemblages, as estimated in territory mapping studies since the 1950s. There have been two approaches, one based on multiple visits (typically 7–8), and the other of reduced effort, developed for long-term monitoring. Compared to these mapping methods, line (strip) transects give ca. 30% lower estimates; they also vary widely. After quality checking, a total of 90 mapping estimates were extracted. The estimates were then integrated into ‘typical densities’ by habitat types, after critically assessing data quality and bias. ‘Typical density’ is conceptualized as a range of common density values in actual topographic, soil, forest-age and landscape conditions, under minor recent thinning influence. It also considers 15–20% underestimation of the territory mapping, which was demonstrated using a case study on post-breeding nest searching of three thrush (Turdus spp.) species. The Estonian forest bird densities show monotonous increases along the stand age, following three alternative shapes related to site productivity: a major increase within 100 years in the most productive forests, a delayed increase (also post 100 years) in pine forests of medium productivity, and slow and slight increases at low-productivity sites. Such density estimates can serve as a basis to analyse deviations in particular conditions and be integrated into land-use scenario-modelling tools across landscapes. It is important to keep the territory mapping approaches to bird census in active use and development, since several issues in conservation and ecosystem functioning cannot be addressed by relative assessments.