Brood prédation, habitat characteristics and nesting decisions in Acrocephalus scirpaceus and A. palustris

Reed habitats are characterised by a gradient of increasing vegetation density and decreasing prey abundance. We examined the effect of nest prédation on nest site selection and parental investment in relation to this gradient in two closely related warbler species. The marsh warbler Acrocephalus palustris and the reed warbler A. scirpaceus have to contend with this ecological gradient but the distribution along this gradient differs between them. To examine whether this affects mating strategies of the two species differently we looked at i) different vegetation features between depredated and non-depredated nests in both species, ii) female preference of safe nest sites for territory or male quality; and iii) male parental investment. In monogamous reed warblers there is no relationship between nest cover and prédation. Using start of egg-laying as an index for female preference, we only found a correlation with paternal feeding, but not with nest cover. This suggests potential female choice for male quality since male care was independent of food abundance and correlated with early egg-laying by females. Also, reed warbler males actively defended the nest to a controlled observer approach. In the opportunistically polygynous marsh warbler, there is a negative correlation between nest cover and brood prédation. Furthermore, female start of egg-laying correlates with nest cover (i.e. safe nest sites) whereas there is no relationship between male feeding and start of egg-laying. This suggests female preference for nest site quality. Paternal quality for female choice is proposed to be less extensive due to the lack of relation between start of egglaying and male feeding care, and because males generally did not actively defend the nest to a controlled observer approach.