Timing shapes flyway selection in juvenile white storks at the European migratory divide

Migratory divides are junctures where populations of the same species following different migratory routes intersect. In white storks (Ciconia ciconia), migration strategies differ between the western and eastern flyways, yet the factors influencing flyway selection remain unclear. In this study, we used GPS tracking data from 151 juvenile storks to examine what determines flyway selection of juvenile, first-time migrants. Generalized linear mixed models revealed that timing-related variables were the primary factors correlated to flyway choice. Eastern flyway storks had significantly later fledging dates, shorter pre-migratory periods, and a narrower migration onset window compared to Western flyway storks. While temporal co-occurrence was observed in the overlap areas, western storks generally used these areas earlier. Wind and sex were not found to affect the juvenile flyway choice. Our results show that western storks may be experiencing favorable conditions of reduced time pressure and shorter overall migration routes. These migratory differences could still potentially contribute to the concurrent shift in population dynamics at the migratory divide toward the Western flyway, by inducing carry-over effects or affecting the adults' reproduction success. We suggest that understanding the dynamics of the migratory divide is important for maintaining behavioral diversity and preventing the loss of migratory routes.