GRAZING EFFECTS ON BETWEEN-YEAR VARIATION OF FARMLAND BIRD COMMUNITIES

Abstract

Livestock grazing is the dominant land use in the remaining seminatural grasslands in Europe. Abandonment of grasslands and, conversely, intensified grazing by livestock have been suggested as possible causes for the widespread population declines of many farmland birds, although the direct impact of grazing on farmland birds is poorly known. Here, we use a comprehensive, long-term data set (20 pastures surveyed over five years) to test the effects of changes in grazing intensities in seminatural dry pastures on between-year variation of the farmland bird community, functional groups of species, and individual species. Bird communities in all 20 seminatural pastures showed a low degree of temporal variability (Kendall's coefficient of concordance on ranked abundances: mean W = 0.72, range = 0.58–0.89). Community variability was not significantly related to site area, grazing pressure, vegetation structure, or adjacent habitat composition. However, analyses of functional groups of species categorized according to body mass and breeding diet showed that different species subsets had differential responses to between-year changes in grazing pressure (as reflected by changes in grass height). Local extinction and recolonization of ground-feeding insectivorous bird species were affected by yearly changes in grazing pressure, but there was no effect of grazing on ground-feeding species that fed on a mixed diet or on species that foraged in trees and shrubs. In general, large insectivores (>30 g) preferred moderately grazed pastures, and small insectivores (≤30 g) preferred pastures with intensive grazing pressure. We propose that current intensive grazing should be relaxed (i.e., by reducing the number of stock per hectare or by within-season rotational grazing) so that with a given stock size, larger areas of seminatural dry pastures could be grazed. This would decrease the rate of habitat loss and conserve a larger part of the farmland bird community breeding in this habitat.