Migration speed, timing, and long‐term shifts in age structure in North American passerines during fall migration

Migration is challenging for birds, especially juveniles, who experience high mortality rates during migration. The challenge is exacerbated in the Anthropocene, contributing to widespread population declines. Conservation efforts focused on increasing juvenile survival could bolster population recovery. Understanding how age structure of the migrant community shifts throughout migration could inform conservation efforts and future questions of migration ecology. However, it is unknown whether the age structure of the migrant community shifts spatially or temporally during migration. To answer these questions, we first analyzed age‐related differences in migration speed and timing of departure during fall migration using 6 567 747 banding encounters, as variability in these components of migration could generate shifts in community demographics. We found widespread differences in migration speed (km d −1 ) with adults being faster than juveniles in most species, and departure timing differences tied to adult molt. Our analyses revealed shifts in community demographics, with the proportion of juveniles within the community decreasing at northerly latitudes throughout migration. We also determined that demographics have shifted over 53 years, with the proportion of juveniles increasing in the north, and decreasing in the south. Our findings contribute to our knowledge of migration ecology, and our understanding of community shifts over time.