Wind conditions shape co-migration fidelity in trans-Saharan birds at a Mediterranean stopover site.

Weather is a key driver of bird migration, particularly near ecological barriers, where migratory species often converge before crossing. While its effects on individual timing are well documented, how weather reshapes the temporal assemblage of migratory bird communities remains unexplored. Using 18 years of standardised ringing data from 24 trans-Saharan species during pre-breeding migration, this study investigated whether weather conditions along the Mediterranean Sea influence annual network modularity, that is, the yearly interspecific clusters of co-occurrence at a stopover site, and measured the 'co-migration fidelity', defined as the tendency of species to co-occur in the same cluster over the years. Specifically, we examined the 'modularity' associations with tailwinds, temperature, and the North Atlantic Oscillation (NAO) index. Modularity increased significantly over time, revealing non-random species clusters whose arrival timing aligns under unfavourable winds. This suggests that less supportive winds may lead distinct species to migrate within the same narrow temporal window of favourable conditions, heightening co-migration fidelity. Conversely, modularity decreased sharply in years with stronger tailwinds, while temperature and NAO showed negligible effects. These findings provide novel evidence that shared environmental constraints during the sea crossing can shape the formation of migratory communities by prompting species' temporal convergence at a stopover.