Timing of reed management affects habitat use and breeding success in Great Reed Warblers: A field experiment on agricultural drainage canals

Abstract

Freshwater wetlands and their biodiversity have suffered substantial losses globally, mainly due to the drainage of wetlands for agriculture. Recent studies suggest that small drainage and/or irrigation canals established in lowland areas can function both as partial replacement of wetlands lost and as biodiversity hotspots in agricultural landscapes. However, small canals are usually not protected or managed for biodiversity. We studied whether and how the timing of reed management (removal in winter, in spring, no removal) on small canals affects nest site selection and breeding success in Great Reed Warblers, Acrocephalus arundinaceus, a flagship species of small canals in an intensive agricultural landscape in a five-year quasi-experiment. We found significantly more nests in winter-managed than in non-managed canal sections and hardly any in spring-managed sections. The number of nests also increased with the proportion of reed removed and with reed density. Breeding success measured as the probability of fledging chicks was positively influenced by reed density only and was not affected by management type, nesting height, water depth and water level fluctuation. Reed density had a central role as it mediated the effect of management both on nest site selection and breeding success, likely through better concealment of the nests from predators. Our results suggest that winter management should be favored as much as possible in the conservation of Great Reed Warblers and other reed-nesting passerines, whereas spring management should be avoided as it results in a complete lack of reed or poorly grown reed unsuitable for nest-building by the start of the breeding season. Our study draws attention to the importance of proper management of small canals that can serve as local hotspots of biodiversity in heavily human-modified landscapes, and which thus deserve more conservation attention than they currently have.