Rapid environmental change disproportionately affects migratory animals worldwide. Partial migration, where some individuals migrate and others remain resident, is widespread and especially well-studied in birds. Partial migration rates depend on conditions that shape the relative advantages of migratory and resident strategies. Environmental changes are both increasing migration hazards and reducing barriers to overwintering, through trends including milder winters and increasing food sources. I conducted a global meta-analysis to address whether more partial migrants are remaining resident over time since the mid-20th century. A systematic Web of Science search yielded 1,764 potentially relevant published articles, which I reviewed to identify cases containing data that could be extracted on trends (minimum 15 years) in partial migration rates.
Results/Conclusions
I found 31 cases of long-term trend data (range 19-37 years) in proportions of avian populations that pursued resident vs. migratory strategies. The unweighted mean increase in winter residency fraction was 1.09% year-1 (95% CI [0.15, 2.03]). Sixteen additional avian studies reported long-term population trends of winter residents in individual species, without data on breeding population sizes. The mean increase in overwintering populations of these partial migrants was 13.8% year-1 (95% CI [7.4,20.2]). Authors’ attribution of causes included climate changes, increased food availability, reduced ice cover on water bodies, and new water bodies created by impoundment. Nearly all studies took place in Europe, with the remainder in the US. Among volant mammals, a single study of a large U.S. bat colony detected a 22-year trend of 0.12% annual increase in winter residency. Preliminary review of non-volant mammal cases indicates that some partially migratory ungulates are also increasing winter residency over time. However, causes are complex and can include intensive management and barriers to movement as well as directional environmental changes. These results and the paucity of published findings pinpoint the need for more comprehensive study. Global environmental changes appear to be reducing animal migration in at least some regions and taxa through declines not only in populations, but also in partial migration rates.