A rapid shift in migration schedule of an Austral shorebird is linked to a changing southern climate oscillation

Global climate change is reshaping the schedules and routes of migratory birds, which can disrupt annual cycles, ecological interactions and the fitness trade‐offs of migrating or staying resident. In particular, climate change has led to advances in arrival at the breeding grounds and to shifts in the timing of post‐breeding migration, whereby both later and earlier departures have been reported. Here, we studied the migration phenology of the Banded Dotterel Anarhynchus bicinctus , a small shorebird breeding exclusively in New Zealand but exhibiting highly variable seasonal movements that range from year‐round residency to trans‐oceanic longitudinal migration to (mainly) Australia. We used 12 years of citizen science data to estimate population‐level arrival and departure dates of migrants at their Australian non‐breeding grounds. We assessed temporal and spatial variation in migration phenology and explored potential links to climate change. Since 2014, Banded Dotterel post‐breeding arrival to Australia has been delayed by an estimated 4 days annually but prebreeding departure dates back to New Zealand showed no change – collectively reducing the species' non‐breeding residency duration in Australia by 25% (44 days) over the study period. The timing of arrival in Australia was positively associated with the Southern Annular Mode at the end of the New Zealand breeding season, suggesting that the southern contraction of the westerly wind belt circling Antarctica is postponing the species' (preparation for) post‐breeding migration. These findings suggest a rapid change in migration behaviour in response to climate change in a Southern Hemisphere species with a unique east–west migratory route.