Changes in bird migration phenology over one century: a perspective from the Neotropical non-breeding grounds

Changes in the migration phenology of birds linked to global change are extensively documented. Longitudinal studies from temperate breeding grounds have mostly shown earlier arrivals in the spring and a variety of patterns during fall migration ^1,2 , yet, no studies have addressed whether and how migration phenology has changed using data from the tropical non-breeding grounds. Understanding whether changes in migratory phenology are also evident in non-breeding sites is essential to determine the underlying causes of patterns documented in breeding areas. Using data from historical scientific collections and modern repositories of community-science records, we assessed changes in migration phenology of 12 Nearctic-Neotropical long-distance migrant birds in Colombia over one century. We also explored whether shared breeding climatic niches explained variation in changes in phenological patterns observed among species. We detected changes in the timing of both spring and fall migration, with birds generally departing Colombia c. 3 days earlier during the spring and arriving c. 3 days later during the fall, but patterns differed among species in ways not attributable to breeding climatic niches. Our results suggest that birds use cues to time their migration at their non-breeding grounds which are most likely different to those they use on their breeding grounds. To better understand the effects of global change on biodiversity, exploring the underlying drivers of phenological changes with further research integrating more long-term datasets available through scientific collections and community science platforms should be a priority.