Adult male birds advance spring migratory phenology faster than females and juveniles across North America

Advances in spring migratory phenology comprise some of the most
well-documented evidence for the impacts of climate change on birds.
Nevertheless, surprisingly little research has investigated whether birds
are shifting their migratory phenology equally across sex and age
classes – a question critical to understanding the potential for
trophic mismatch. We used 60-years of bird banding data across North
America – comprising over 4 million captures in total – to investigate
both spring and fall migratory phenology for a total of 98 bird species
across sex and age classes, with the exact numbers of species for each
analysis depending on season-specific data availability. Consistent with
protandry, in spring (n=89 species), adult males were the first to arrive
and immature females were the last to arrive. In fall (n=98), there was
little difference between sexes, but adults tended to depart earlier than
juveniles. Over 60 years, adult males advanced their phenology the fastest
(-0.84 days per decade, 95CrI = -1.22 to -0.47, n=36), while adult and
immature females advanced at a slower pace, causing the gap in male and
female arrival times to widen over time. In the fall, there was no overall
trend in phenology by age or sex (n=57), driven in part by high
interspecific variation related to breeding and molt strategies. Our
results indicate consistent and predictable age- and sex-based differences
in the rates at which species’ springtime phenology is shifting. The
growing gap between male and female migratory arrival indicates sex-based
plasticity in adaptation to climate change that has strong potential to
negatively impact current and future population trends.