![intigeo light logger intigeo light logger](https://media.springernature.com/lw685/springer-static/image/art%3A10.1186%2Fs40657-021-00245-w/MediaObjects/40657_2021_245_Fig1_HTML.png)
Currently, the specific mechanisms driving adaptive population changes are poorly understood. For long-distance migrants, this is complicated by changes that may vary geographically in degree or direction 9, or include interactions with non-climate-related effects such as direct habitat alteration by humans 10. However, weak or inadequate phenological responses are commonly observed 6, as are general declines in migratory populations 7, 8, implying limitations to the capacity for adaptation. In general, the timing of bird migration and breeding are advancing 3, 4, as expected with earlier availability of seasonal resources for reproduction 5. For example, climate change is altering the timing of annual cycles in animals and plants worldwide 1, 2. The current pace and scale of environmental change tests the capacity of organisms to adapt. We demonstrate that plastic responses can drive population-level changes in timing of long-distance migration, but also that behavioral and environmental constraints en route may yet limit adaptive responses to global change. Moreover, changes in breeding-site phenology varied across Alaska, but were not reflected in within-population differences in advancement of migratory departure. However, in individuals tracked for the entire migration (50 total tracks of 36 individuals), earlier departure did not lead to earlier arrival or breeding in Alaska, due to prolonged stopovers in Asia. Using direct observations of bar-tailed godwits ( Limosa lapponica) departing New Zealand on a 16,000-km journey to Alaska, we show that migration advanced by six days during 2008–2020, and that within-individual advancement was sufficient to explain this population-level change. Although often attributed to phenotypic plasticity, there is no clear demonstration of long-term population advancement in avian migration through individual plasticity. Globally, bird migration is occurring earlier in the year, consistent with climate-related changes in breeding resources.