Seasonal and Spatial Patterns of Bird Communities in a Highly Disturbed Atlantic Riparian Corridor

Land use changes pose major threats to ecosystems, particularly affecting vulnerable habitats, such as riparian forests. These transitional habitats play a crucial role in supporting biodiversity, particularly avian communities. Despite their recognised importance, studies on the land use effects on bird communities in the riparian corridors of southern Europe remain scarce. Here, we aimed to investigate the seasonal variation of the effects of land use on avian communities in an Atlantic riparian area in northern Portugal and whether bird assemblages can be used as bioindicators of riparian ecosystems&rsquo: quality. To achieve this, we conducted bird surveys during three periods of the birds&rsquo: life cycle: post-nuptial migration, wintering and breeding. Bird species were grouped into assemblages reflecting diet, foraging stratum, phenology and preferred habitat affinities. To analyse the effect of land use, we modelled the abundance of the respective bird assemblages with the land use gradients obtained through principal component analysis. A total of 62 bird species were identified (73% observed during post-breeding migration, 77% in winter and 68% during breeding). Among these, 45 species (73%) were residents, while 17 species (27%) were short- or long-distance migrants. All bird assemblages showed seasonal differences in species richness, with the exception of granivores, forest species, resident, ground- and understorey foragers, and in abundance, with the exception of invertivores, farmland birds and tree foragers. The predicted abundances of farmland birds, ground-feeding birds and granivores often showed positive associations with gradients reflecting anthropogenic land uses (e.g., farmlands and acacia stands) and negative relationships with natural land uses (e.g., deciduous riparian forests, pine and oakwood). Conversely, invertivores&rsquo: and tree foragers&rsquo: abundances were positively related to natural land uses and negatively related to anthropogenic ones. Furthermore, we highlight the negative effects of exotic tree species on the bird community and the effects caused by adjacent land uses on riparian habitats. Our results are consistent with studies showing that the grouping of birds by functional characteristics can serve as an indicator of disturbance in riparian corridors.