Multi-year change and annual variation in fall migratory timing of songbirds captured across a 19-year time span in northeastern Pennsylvania, USA

Although there is abundant evidence that timing of spring migration in landbirds is influenced by annual variation in temperature and longer-term warming due to climate change, much less is known about how these factors influence fall migration, especially in North America. Similarly, less is known about the influence of annual variation in temperature and precipitation and the combination of intrinsic factors (age, sex, wintering location, diet, number of broods produced in a breeding season) on the timing of fall migration. We used 13 years of capture data, spanning a 19-year period (2006–2024) to look for species-specific evidence of multi-year change in fall migratory timing while also looking to see if precipitation amount or temperature experienced by birds using our site changed. Finally, we looked for evidence that the above extrinsic and intrinsic factors influenced annual variation in migratory timing. We found evidence that (1) 46% (6 of 13) of the species exhibited either delays (4 species) or advances (2 species) in migratory timing, (2) average temperature increased by 1.4 °C during the fall migratory period, (3) sex and wintering location influenced timing with females (2.3 days) and species that winter in Central or South America (14.3 days) passing through our site before males and species wintering in North America, and (4) in warmer and/or wetter years birds advanced timing. Our results increase understanding of how intrinsic and extrinsic factors influence annual timing of fall migration. Further, our findings add to the growing evidence that climate change has altered fall migratory timing and that differences exist in how species and populations have responded. More work is necessary to determine whether changes in timing of fall migration are due to developmental plasticity, selection on heritable variation, or some combination.