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

Marjorie C Sorensen,Claire N Spottiswoode,Graham D Fairhurst,Jason Newton,Elizabeth Yohannes,Susanne Jenni-Eiermann

doi:10.1186/s12898-016-0096-6

Abstract

BackgroundAn 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 (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.ResultsFirst, we found that individuals staging in a dry year had higher corticosterone (CORTf) 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 β-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.ConclusionsThis 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12898-016-0096-6) contains supplementary material, which is available to authorized users.

Highlights

An understanding of year-round habitat use is essential for determining how carry-over effects shape population dynamics in long-distance migratory songbirds
Studying how conditions at multiple sites cumulatively affect year-round individual success is important for Palearctic-African species conservation, management, and effective reserve design, as sub-Saharan wintering migrants are in a state of severe population decline [15, 16]. To begin addressing these shortcomings we studied the great reed warbler (Acrocephalus arundinaceus), a 30 g long-distance Palearctic-African migratory songbird that winters widely across sub-Saharan Africa, in southern Zambia
We found that winter feather corticosterone (CORTf), BUTY, and TRIG did not differ between years; the great reed warbler population in Zambia in 2012 had a lower scaled mass index than the population in 2011 (t test t′56.3 = 2.4, p = 0.02)

Summary

Introduction

An understanding of year-round habitat use is essential for determining how carry-over effects shape population dynamics in long-distance migratory songbirds. Sorensen et al BMC Ecol (2016) 16:41 identifying such carry-over effects (defined as processes in one season influencing the success of an individual in the following season) between migratory staging sites has been challenging owing to difficulties in tracking smallbodied migrants across multiple sites throughout the annual cycle [3]. Longterm staging sites complicate our understanding of yearround habitat use for migratory birds because variation in habitat quality among staging sites may have important consequences for population dynamics via carryover effects on individual survival and condition

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