Species-specific responses of woodland birds to stand-level habitat characteristics: The dual importance of forest structure and floristics

Several species of woodland birds have recently declined in Britain and there is a need to understand better how species are distributed across a wide spectrum of habitat types in order to develop appropriate conservation management strategies. To assess habitat associations of 28 woodland species, a large-scale study was carried out across Great Britain. A total of 2668 stands embedded in 825 woodland sites were surveyed. The number of species recorded in each stand and the occurrence of each species was examined in relation to the unique (or marginal) effects of gross structural characteristics of stands (growth stage and understorey structure), plant species composition (floristics) expressed in terms of dominant tree, shrub layer and field layer species and both sets of variables considered together whilst controlling for the spatial distribution of sites and site identity. The number of species recorded was independently related to growth stage, understorey structure, tree composition and field layer type. Eighteen species showed at least one significant relationship in models containing structure variables only. Thirteen species showed at least one significant relationship in models containing floristic variables only. In models containing both structure and floristic variables; 14 species showed at least one significant relationship with a structure variable, 9 species with at least one floristic variable and 8 species with both at least one structure and at least one floristic variable. Different bird species showed individualistic patterns of association with habitat variables. Results suggest that structure and floristics have complex and inter-related effects on bird distribution across woodland stands. Some of the apparent relationships with structure were a consequence of between-stand differences in floristics and vice versa whilst in two cases relationships with structure variables became apparent after controlling for the masking effects of floristic variables. Both vegetation structure and floristics appear to be important determinants of composition in bird assemblages in British woodland, though gross structural factors have previously received most emphasis. The existence of species-specific responses to habitat variation suggests that habitat heterogeneity, embracing structure and floristics, should be a key element in conservation planning within forests especially in the light of uncertainty about future regional species pools due to climate change. We discuss how responses of birds to habitat variation at the level of stands can form a basis for developing large-scale conservation strategies within managed forests.