Monitoring common and scarce breeding birds in the Netherlands: applying a post-hoc stratification and weighting procedure to obtain less biased population trends

Abstract

The main objective of the Dutch Breeding Bird Monitoring Program (BMP) is to assess changes in population sizes of common and scarce breeding birds. Despite the large number of study plots, trends might be biased because plots are not equally distributed over the country. In this paper we present a post-hoc stratification and weighting procedure to correct for this non-random sampling. Indices and trends are first calculated for a number of speciesspecific strata (combinations of region, main habitat type and bird density class). Thereafter, the indices per stratum are weighted by population sizes (derived from an independently collected set of atlas data) and sampling effort per stratum. The procedure has a small but substantial effect on national trends, trends generally becoming less conservative. We believe that for the majority of breeding birds this procedure results in a substantial improvement of trends, and we will therefore continue the BMP in forthcoming years.