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Sex differences in migration timing influence the network structure of avian assemblages at a stopover site

Shifts in migration phenology aimed at minimizing possible mismatches between the timing of breeding and seasonal peaks in the availability of food resources are a well- documented phenomenon in birds. As the extent of phenological adjustments are likely to differ between species, populations, sexes and age classes, these shifts may exert a strong influence on the potential interferences among co-migrants during their journey to breeding grounds. This can be particularly true for species showing asynchronous migration driven by sex-related mechanisms (i.e., protandry), for which not only the decision on when, but also with whom to move might represent a way to minimize the costs associated with migrations. Here, we used an extensive dataset from 16 years of standardized bird monitoring at the spring stopover area on the island of Ponza (Italy), to study how long-term changes in the phenology of species with protandric migration patterns affect the rewiring of potential interferences in avian assemblages. For each year, we reconstructed the temporal co-occurrence networks of females and males of 17 sexually dimorphic species known to exhibit phenological shifts in their migratory behaviour and measured network-specific properties related to connectivity and cohesion. We also provided a measure of co-migration fidelity, defined as the degree to which females and males tend to follow a consistent order of association over time. Co-occurrence networks in males showed higher cohesion and lower connectivity than their female counterparts and a more clustered pattern in terms of co-migration fidelity, with species wintering in sub-Saharan Africa having a more pronounced tendency to aggregate in semi-independent groups of species characterized by common timing of migration with different foraging strategies. The observed differences in the rewiring of associations over time showed an increasing clustering of species having common responses to changing migration phenology in males, but not in females. This finding suggests a sex-decoupled effect of changing migration phenology on the onset of potential interferences in migratory birds, possibly influencing the global migration economy of landbirds in the Palearctic-African migration system.