Trends in weather conditions favor generalist over specialist species in rear-edge alpine bird communities

Using the community of six passerine species breeding in the alpine zone of the Cantabrian mountains (NW Iberian Peninsula), we sought to document the changes in bird abundance across the elevational gradient during the last decade, evaluate the relationship between bird abundance and local climatic conditions (i.e. weather conditions), and discuss the mechanisms by which these conditions might be mediating the observed abundance trends in a global warming context. We estimated bird abundance at the home range level using point count transects and distance sampling during the breeding season, and fitted generalized linear mixed models to describe their temporal trends across the elevational gradient. We used a structural equation modeling approach to estimate the direct, indirect and total effects of weather and temporal variables, while considering the correlations and causal relationships among them and with the elevational gradient.