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 dev eloped a method to produce yearly population indices of bird species across countries by combining the results of existing national schemes. The metho d takes into account the differences in population sizes per country, as we ll 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 arven sis, Whitethroat Sylvia communis and Yellowhammer Emberiza citrinella), fro m seven countries (UK, Netherlands, Denmark, Germany, Finland, Latvia and E stonia) for a 20-year period (1978-97). The trial demonstrated that it was possible to combine national indices to provide supra-national yearly total s and their standard errors; the results were similar to those produced whe n 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 t heir standard errors. At a European scale, the populations of the five spec ies selected have changed considerably. In western Europe (UK, Netherlands, Denmark and former West Germany combined), Linnet, Skylark and Yellowhamme r have declined and Whitethroat has increased. Most changes occurred during the first ten-year period (2978-88). The changes in eastern Europe (the re maining countries) were less clear, in part because the statistical power o f the national schemes is as yet limited.