Abstract:

The rapid deterioration of the environment may now noticeably affect the relatively numer- ous passerine birds. Their numbers in Western Europe decreased rapidly, in some species by 70-80% during the past 30 years. Most breeding populations in the Carpathian Basin are not connected to the Western European ones, and the migrant birds also come from elsewhere. These populations may well demonstrate a different dynamics. We carried out our research at two different sites. The database of Ócsa (47°15'N, 19°15'E) goes back 10 years, and consists of data from 33000 individuals of 65 passerine spe- cies. The database of Fen6kpuszta (46°44'N, 17°18'E) was collected over 8 years and contains data from 44000 birds of 53 species. Capture efforts at the two sites were consistent between years. Over this period, the number and relative abundance of warbler species in the reedbed at Ócsa decreased, whereas the number of forest species increased Many species did not show any significant population trends. The yearly values of diversity and evenness did not differ significantly overthe years, whereas the overlap values decreased compared to the first year. The data from Fenékpuszta did not show any significant trends. The values of diversity and evenness are similar over the years, and lower compared to that of Ócsa. From the two sites, Fenékpuszta can be considered as an unchanging reedbed, while the vegetation in Ócsa changed rapidly. The decrease in the proportion of reedbed species and the increase in the proportion of forest species was possibly a consequence of habitat degradation, and was not the sign of a population change. This was also supported by the fact that under the stable conditions of the reedbed in Fenékpuszta the proportion of species did not change.