Response of bird communities to silvicultural thinning of Mediterranean maquis

1. Woodland management should consider biodiversity conservation world-wide. Landowners in some European Mediterranean regions receive subsidies to thin dense maquis. This practice eliminates most shrubs and saplings while the tallest trees are pruned to produce more open woodland stands. We investigated the impact of this practice on the conservation status of bird communities. 2. We designed a large-scale 'natural experiment' that included 21 paired thinned and unthinned maquis stands in central Spain. Every stand was sampled by means of five point counts, each consisting of a 50-m radius plot, in two consecutive years and in winter and spring. The vegetation structure was characterized after bird censuses in 10-m radius plots that coincided with the centres of the bird point counts. Data analyses were based on repeated-measures ANOVAS. 3. Thinning was responsible for a significant increase in species richness, but did not have any effect on total bird density. The average body mass of species in thinned stands was significantly larger than in unthinned, more densely vegetated, stands. The density of ground searchers was indistinguishable in thinned and unthinned stands, whereas the density of foliage gleaners was higher in unthinned stands. The winter density of granivorous species was marginally higher in thinned stands, whereas insectivorous and frugivorous species were marginally more abundant in unthinned stands. 4. Thinned areas were occupied by higher densities of bird species whose European conservation status is of high concern. Winter density of gamebirds was also higher in thinned stands. 5. Synthesis and applications. This is the first time that a large-scale experimental manipulation of habitat structure and vegetation volume has demonstrated the predicted allometric effect of habitat structural complexity on the average body mass of a bird community. Thinning of dense Mediterranean woodland enhanced habitat heterogeneity and suitability for several bird species and increased species richness. It was also beneficial for species of conservation concern and for non-threatened gamebirds. However, some unthinned patches should be preserved to provide refuge for the few species that are impacted by thinning.