A.J. van Strien

Indexing European bird population trends using results of national monitoring schemes: a trial of a new method

Many European countries have annual breeding bird monitoring schemes based on nationwide samples; most are in northern and western Europe. We have developed a method to produce yearly population indices of bird species across countries by combining the results of existing national schemes. The method takes into account the differences in population sizes per country, as well as the differences in field methods, and the numbers of sites and years covered by the national schemes. In order to test the method, we collected raw data from a number of countries and applied an index method to produce scheme results per country. Data were collected for five farmland species (Lapwing Vanellus vanellus, Linnet Carduelis cannabina, Skylark Alauda arvensis, Whitethroat Sylvia communis and Yellowhammer Emberiza citrinella), from seven countries (UK, Netherlands, Denmark, Germany, Finland, Latvia and Estonia) for a 20-year period (1978–97). The trial demonstrated that it was possible to combine national indices to provide supra-national yearly totals and their standard errors; the results were similar to those produced when the raw data were used. Thus, yearly European indices can be produced by exchanging only limited amounts of information, that is the national yearly indices of each species or, preferably, the yearly population numbers and their standard errors. At a European scale, the populations of the five species selected have changed considerably. In western Europe (UK, Netherlands, Denmark and former West Germany combined), Linnet, Skylark and Yellowhammer have declined and Whitethroat has increased. Most changes occurred during the first ten-year period (1978–88). The changes in eastern Europe (the remaining countries) were less clear, in part because the statistical power of the national schemes is as yet limited.