BREEDING SUCCESS AND REPRODUCTIVE STRATEGIES OF 2 ACROCEPHALUS WARBLERS

Several hypotheses have been proposed in the literature to account for the different responses birds show to varying degrees of nest predati on. These are discussed using data on Marsh and Reed Warbler (Acroceph alus palustris, A. scirpaceus) which differ greatly in nest predation even when they share the same habitat. Breeding success was distinctly higher in Marsh than in Reed Warblers per breeding attempt (averaging 68.1% and 44.9% respectively). Different levels of egg predation and Cuckoo (Cuculus canorus) parasitism were the main reasons. During an e gg laying period of about 14 weeks Reed Warblers produce many replacem ent clutches and second broods resulting in an average annual reproduc tive output of 3.8 fledged young per female. The single brooded Marsh Warbler with a laying period of 7 weeks produces 3.3 fledglings. Diffe rences in structure and seasonality of the species' respective habitat s, i.e. reedbeds in Reed Warbler and herbaceous vegetation in Marsh Wa rbler, contribute to the differences in breeding success and the lengt h of breeding periods open to the two species. Marsh Warblers respond to limited breeding opportunities due to their narrow breeding season with higher clutch size, and by avoiding nest losses. They conceal nes ts better, breed in lower densities, and efficiently reject Cuckoo egg s. Ecological constraints may restrain the Reed Warbler to its compara bly unsafe nest sites. In other open-nesting and ecologically similar species pairs, a similar higher breeding success also occurs in those species that stay for a shorter period on the breeding grounds. It app ears that high breeding success is an important prerequisite for speci es breeding in ephemeral habitats.