Changes in the spatial patterns of avian migrations: Evidence, mechanisms and causes

AbstractAimsUnderstanding the suite of environmental changes impacting migration and disentangling these from the dependencies between migratory stages is fundamental to understanding the drivers and mechanisms causing spatial shifts to migratory bird distributions. In this review, our objectives are to: (i) provide evidence of spatial change at all migratory stages, (ii) identify the key mechanisms driving change, (iii) discuss how anthropogenic change influences migratory patterns, (iv) highlight interdependencies between migratory stages and (v) offer a framework for understanding migratory pattern changes.LocationMigratory land birds globally.MethodsWe defined a literature search to find examples of spatial changes in avian migrations and their underlying mechanisms. From the top 1000 search results, sorted by relevance, we retained 81 papers published between 1991 and 2020.ResultsSpatial shifts in migratory bird distributions occur throughout the annual cycle, but patterns vary across taxa and migratory stages. Since avian migrants occupy multiple sites across the annual cycle, environmental change has particularly complex repercussions for their distributions. Multiple environmental changes can affect distributions at each migratory stage due to fitness asymmetries between migratory strategies and individuals responding flexibly to environmental changes within and across generations. Spatial changes at one stage (e.g. breeding) can also trigger further indirect spatial changes at other stages (e.g. stopovers) due to dependencies between stages.ConclusionsTo understand the drivers and mechanisms underpinning spatial changes, comprehensive year‐round distributional data are crucial for observing change across multiple migratory stages and disentangling competing drivers. Biologging technology could elucidate the mechanisms causing changes to individual‐level strategies and uncover fitness asymmetries between strategies. Ultimately, gaining a clearer understanding of spatial changes will aid in predicting distributional responses to anthropogenic change in migratory birds. This is paramount for mitigating the effects of anthropogenic change and assessing future population viability within this vulnerable and extensive taxonomic group.