Individual consistency of long-distance migration in a songbird: significant repeatability of autumn route, stopovers and wintering sites but not in timing of migration

Through new tracking techniques, data on timing and routes of migration in long-distance migrant birds are accumulating. However, studies of the consistency of migration of the same individuals between years are still rare in small-sized passerine birds. This type of information is important to understand decisions and migration abilities at the individual level, but also for life history theory, for understanding carry over effects between different annual cycle stages and for conservation. We analysed individual repeatability of migration between years in great reed warblers Acrocephalus arundinaceus; a medium-sized European songbird migrating to sub-Saharan Africa. In seven males, with geolocator data from 2-4 yr per bird, we found low to moderate (non significant) repeatability in timing of migration parameters (R 0.41), but high (and significant) repeatability for most spatial parameters, i.e. autumn route (R = 0.64) and stopover sites (R = 0.59-0.87) in Europe, and wintering sites (R = 0.77-0.99) in sub-Saharan Africa. This pattern of high spatial but low temporal within-individual repeatability of migration between years contrasts other tracking studies of migrating birds that generally have found consistency in timing but flexibility in routes. High spatial consistency of migration in the great reed warbler may be due to it being a specialist in wetlands, an unevenly distributed habitat, favouring a strategy of recurrence at previously visited sites. Low temporal repeatability may be caused by large between-year variation in carry-over effects from the breeding season, high flexibility in decision rules during migration or high sensitivity to environmental factors (weather, wind) during migration.