Habitat-specific bird trends and their effect on the Farmland Bird Index

摘要

Farmland birds in Europe have declined in numbers from the last decades, as reflected in the Farmland Bird Index (FBI), a multi-species indicator of population trends adopted by the European Union. Some of these farmland bird species breed to a varying degree also in non-farmland habitats; however, the indicator is calculated using trends estimated from all birds counted of each species, irrespective of habitat. Consequently, when interpreting indicators of farmland birds, it is important to know the influence of including the trends of the populations in non-farmland habitats. We estimated national and regional bird-habitat relationships and population trends of 14 of the species included in the Swedish version of FBI. Bird data came from the Swedish Bird Survey (1996-2008) including 716 separate routes, and detailed habitat data came from the Swedish Board of Agriculture (1999-2008). The proportion of individuals that occurred in non-farmland habitats ranged from 2% to 96%. Local densities were generally highest in farmland habitats, but because only 7-8% of the surface of Sweden is farmland, even low local densities in non-farmland sum up to high national totals. With few exceptions, the temporal trends of species were independent of the amount of farmland on routes, suggesting similar trends within and outside farmland. Accordingly, incorporating data on farmland birds from non-farmland habitats had only a limited effect on the FBI. But given that habitat-specific trends may well occur, it is important that multi-species indices are checked for potential habitat-specific differences among the included species.