Seasonal rainfall at long-term migratory staging sites is associated with altered carry-over effects in a Palearctic-African migratory bird

$\textbf{Background:}$ An understanding of year-round habitat use is essential for determining how carry-over effects shape population dynamics in long-distance migratory songbirds. The recent discovery of long-term migratory staging sites in many species, prior to arrival at final wintering sites, adds complexity to efforts to decipher non-breeding habitat use and connections between sites. We investigated whether habitat conditions during migratory staging carry over to influence great reed warbler ($\textit{Acrocephalus arundinaceus}$) body condition at final wintering sites in Zambia. We asked whether the presence/absence and strength of such carry-over effects were modified by contrasting rainfall conditions during 2 years. $\textbf{Results:}$ First, we found that individuals staging in a dry year had higher corticosterone (CORT$_f$) and stable nitrogen isotope values (suggesting higher aridity) than birds staging in a wet year, indicating that regional weather affected staging conditions. Second, we found that carry-over effects from staging habitat conditions (measured via carbon and nitrogen isotopes) to final winter site body condition (measured via scaled mass index and $\beta$-hydroxybutyrate) were only present in a dry year, suggesting that environmental factors have consequences for the strength of carry-over effects. Our results also suggest that wet conditions at final winter sites may buffer the effects of poor staging conditions, at least in the short term, since individuals that staged in a dry year had higher scaled mass indices in Zambia than individuals that staged in a wet year. $\textbf{Conclusions:}$ This study provides a first insight into the connections between long-term migratory staging sites and final wintering sites, and suggests that local environmental factors can modify the strength of carry-over effects for long-distance migratory birds.Gates Cambridge Trust, Natural Sciences and Engineering Research Council of Canada, Royal Society (Dorothy Hodgkin Fellowship), Biotechnology and Biological Sciences Research Council (David Phillips Fellowship, Grant ID: BB/J014109/1), DST-NRF Centre of Excellence at the FitzPatrick Institute, NERC (LSMSF Grant ID: EK206-16/12)This is the final version of the article. It first appeared from BioMed Central via http://dx.doi.org/10.1186/s12898-016-0096-