Subadult movements contribute to population level migratory connectivity

Migratory connectivity describes the degree to which migratory populations are mixed across space and time. Unlike adults, subadult birds often exhibit distinct migratory patterns and continually refine their migratory behaviour and destinations as they mature. Consequently, the influence of subadult movements on overall migratory connectivity might be different from that of adults. However, current studies on migratory connectivity often overlook population age structures, predominantly focusing on adults. In this study, we investigated the role of subadult movements in shaping population level connectivity by using satellite tracking data from 214 black-necked cranes, Grus nigricollis, in western China. We first assessed the variances in spatial separation in different age cohorts using the continuous temporal Mantel correlation coefficient with data from 17 juveniles tracked in the same year for 3 consecutive years. We then calculated the continuous temporal migratory connectivity for the entire population (comprising various age groups) from 15 September to 15 November and compared the result to that of the family group (consisting of juveniles and adults only). Our results revealed a positive correlation between temporal variation in spatial separation and age after the juveniles separated from the adults, suggesting that subadults may have fine-tuned their migration paths. Moreover, the migratory connectivity of the all-age cohort was moderate (below 0.6) in the winter season, and notably lower than that of the family group during the autumn period. Given the considerable impact of subadults on migratory connectivity, we recommend using data collected from birds across all age categories to improve the precision of population level migratory connectivity estimates.