ANALYSIS OF TRAPPING FIGURES AT METTNAU STATION, S GERMANY, WITH RESPECT TO THE POPULATION DEVELOPMENT IN SMALL BIRDS - A 20 YEARS SUMMARY 34 REPORT OF THE MRI-PROGRAM

1) The trapping station ''Mettnau'' in S. Germany as part of the ''Mettnau-Reit-Illmitz-Program ( MRI-Program)'' of the Vogelwarte Radolfzell has been in operation for 20 years since 1972. During this period, 119 132 individual birds (first traps) of 35 species of small birds were captured. The station uses mist nets in which passage migrants are trapped during the autumn migratory period. The annual trapping period lasts from the end of June to the beginning of November. Trapping and working methods are strictly standardized. In this report, the trapping figures have been analyzed with respect to the development of the respective (mainly Central European) source populations of the passage migrants. 2) Regression analyses of the annual trapping totals yielded negative correlations between dates and numbers for 30 of the 35 species. The trends were statistically significant in 14 species. Among the 5 positive results only one was significant. Significant negative trends in the trapping totals are primarily found in long-distance (trans-Sahara) migrants. These were followed by partial migrants with high portions of residents. The smallest declines occurred in species that winter mainly in the Mediterranean area. 3) In comparison with the preceeding analyses (BERTHOLD et al. 1986) in which data up to 1983 were treated, the negative picture of the annual trapping totals and thus that of the development of the respective populations has deteriorated considerably in the meantime. Up to 1983 the annual trapping totals of the Mettnau station had produced negative coefficients in 69 % of the species. By 1991 this value increased to 85 %. None of the species show a trend turn towards positive development. The annual trapping totals of all species which continuously decrease have declined significantly from the first to the second decade of the period of investigation by 15 %. The decreases in the trapping totals of the trans-Sahara migrants are significant since 1980. 4) The extremely negative picture from these trapping figures reflect population changes which are in general agreement with the current ''Red lists'' of the avifauna of Central Europe. In a number of species like Savi's warbler, aquatic warbler, woodwarbler and wryneck individuals are no longer even trapped annually at the Mettnau station and the gaps will certainly increase. 5) Finally, differences in the development of various populations, some sources of error within the trapping figures and some possible causes for population decline on the basis of great reed and sedge warbler data (decrease in the proportions of adult birds) are briefly discussed. It is emphasized that in future population studies investigations of the causes of population declines should have a high priority to further monitor these trends.