Spatio-temporal insights into intraspecific variation in barn swallow migration: population- and age-specific migration patterns

Understanding spatio-temporal patterns of migratory species is crucial for developing effective flyway-based conservation strategies. Variation in migratory behaviour among individuals from different populations or ages can reveal key insights into the flexibility of migration timing and spatial patterns. The barn swallow (Hirundo rustica), with its broad geographical range spanning from Argentina to eastern China, is an ideal species for investigating the constraints and trade-offs in migratory behaviour due to comprehensive tracking across different populations. In this study, we used a unique dataset of geolocator-tracked barn swallows from three breeding populations—two in northern Italy and one in southern Switzerland—including repeated migrations from the same individuals and juveniles undertaking their first long-distance migration. We analysed the similarity of migration routes across populations and age groups, as well as the effect of age on migration timing. Our findings revealed no significant difference in migration routes between populations or age groups. However, juveniles showed a notably delayed pre-breeding migration timing (26 days) compared to older individuals. These results suggest that barn swallows from different breeding populations within the same flyway exhibit consistent migratory routes, while age emerges as an important intrinsic factor affecting migration timing, and potentially fitness, even in a short-lived species. This study represents the first step of a global-scale analysis on barn swallow migration, conducted within the framework of the COST Action CA22117 EUFLYNET. The main goal of the analysis is to assess migration strategies and constraints on annual cycle scheduling across all barn swallow populations tracked so far.