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.