Species traits explaining long-term abundance changes of breeding birds in farmland areas in Álava (N Spain)

In recent decades, the populations of many common bird species have greatly declined in Europe, particularly in farmland habitats. However, this decline has not affected every species, and to understand the mechanisms behind this interspecific variability, its relationship with several species-specific traits has been studied repeatedly. We used this approach to analyse long-term changes in the abundance of breeding passerines in farmland areas in northern Spain. Abundance changes were quantified in 2015 and 2016 by repeating the same line transects conducted in 1988/89 in three agricultural habitats located along a steep Atlantic-Mediterranean gradient in the province of Álava. We found that total bird abundance did not decline, as most species not closely linked to farmland habitats showed positive population changes, while the number of increasing and decreasing farmland specialists was similar. Specific traits explained a substantial proportion of the variability in abundance change, as warm-dwelling species and those occupying habitats with a complex vegetation structure showed more positive changes than those preferring colder conditions and more open habitats. These patterns can be explained by climate warming and by the increase in hedge and forest cover in the study area. Although we did not find a long-term decline in the overall abundance of the studied bird community nor in that of farmland specialists, open habitat species associated with a cool climate should be the focus of particular concern.