Spring migration passage advances are counteracted by range shifts

Quantitatively assessing species’ responses to climate change is central to contemporary ecology, amid concern that these responses may be insufficient to track accelerating environmental change. In birds, advancing the timing of spring migration is a key mechanism for matching warming conditions, and is commonly inferred from long-term series of passage dates recorded at fixed observatories. While such data are abundant, they may be biased when used as proxies for arrival at breeding grounds. As many migratory species shift their breeding ranges poleward, returning individuals increasingly head for higher latitudes where spring—and hence optimal arrival—occurs later. Tracking this timing, they must migrate correspondingly later, causing species-level passage records to occur at relatively later dates for range-shifting species compared to background trends in local arrival to breeding grounds. Consequently, long-term passage trends may simultaneously reflect advances in arrival phenology and changes in breeding destinations. We re-examined published migration trends of 33 bird species from four Nordic observatories, relating them to range-shift velocities derived from Finnish breeding-bird surveys. Species with faster northward shifts showed apparent delays in early-phase passage, but only among short-distance migrants (SDM). The difference between observed trends in passage dates and expected arrival could be several days—sometimes even reversing the trend. No corresponding effects were detected in mid- or late phases of migration or among long-distance migrants (LDM). Our findings reveal the need to account for shifting breeding destinations when studying passage-date time series. This is essential for accurately assessing avian responses to climate change.