Data and code from: Migration speed, timing, and long-term shifts in age structure in North American passerines during fall migration

Migration is challenging for birds, particularly juveniles, who experience increased mortality. Conservation efforts that increase juvenile survival could support the recovery of depleted populations. Understanding where and when juvenile abundances peak during migration would help identify priority conservation locations. However, it is unknown whether the age structure of migrant communities shifts spatiotemporally during migration. To investigate, we analyzed age-related differences in migration speed and timing using 6,567,747 bird banding encounters, as variability in these traits could drive demographic shifts. We found adults generally migrate faster than juveniles, with timing differences tied to adult molt strategy. Our analyses reveal previously undocumented shifts in community demographics: the proportion of juveniles decreases at northerly latitudes and increases at southerly latitudes throughout migration. Over the past 53 years, the juvenile proportions have increased in the north and decreased in the south. Integrating these findings with stopover abundance data can inform targeted conservation efforts.