Combining direct and indirect tracking techniques to assess the impact of sub-Saharan conditions on cross-continental songbird migration

Understanding how events throughout the annual cycle interact to influence individual fitness and hence population dynamics is crucial to optimize conservation strategies for migratory birds. Despite major advancements in technology, direct tracking devices for passerine songbirds are still limited by the need for recapturing tagged individuals. Stable-isotope analysis of bird tissue has the opportunity to supply information on a larger number of individuals, although it is limited by the indirect knowledge of the geographical position of the birds. In this study, we provide a first attempt to combine annual spatio-temporal data achieved from direct tracking with information on local environmental conditions by using stable isotopes (d13C and d15N). We use this approach to investigate if environmental conditions at the wintering area in southern Africa influence timing of spring migration and carry-over to affect breeding performance in a long-distance migratory passerine bird, the Red-backed Shrike (Lanius collurio). We found that individuals wintering in relatively moist conditions (depleted in d13C) and at lower latitudes (closer to the breeding grounds) departed later on spring migration than individuals in more xeric habitats. However, the effect of non-breeding area conditions and latitude were not found at subsequent migration stages and late departing individuals spent fewer days on migration towards the breeding grounds. Although conditions in the non-breeding range have previously been suggested to influence population fluctuations in Redbacked Shrikes, we found no evidence of a carry-over effect on breeding performance. This study highlights the potential of combining different approaches when investigating seasonal interactions in migratory animals.