Patterns of nest predation contribute to polygyny in the Great Reed Warbler

According to the polygyny threshold model, females are compensated for the cost of sharing a territory with other females by breeding in territories o f higher quality than those of monogamously mated females. In the polygynou s Great Reed Warbler (Acrocephalus arundinaceus), the variation in territor y quality may be associated with nest site characteristics or food supply. In this study, we examined the probability of nest predation in Great Reed Warblers in relation to an indirect measure of territory quality (territory attractiveness rank as indexed by settlement order) and to several variabl es of nest site structure. Artificial nests with attached soft plasticine m odel eggs were placed in Great Reed Warbler territories of different attrac tiveness; Marks in the soft model eggs were compared to marks from possible predator species. Both inter- and intraspecific predators were found to pr ey upon eggs in artificial nests. Marsh Harriers (Circus aeruginosus) predo minantly preyed upon artificial nests located in territories of low attract iveness, in low-density reed beds, and early in the season. Bitterns (Botau rus stellaris) and Water Rails (Rallus aquaticus) showed a preference for a rtificial nests located inside the reeds. When all types of predators of ar tificial nests were pooled, the probability of nest predation was lower in more attractive territories. This pattern was corroborated in an analysis o f predation on real nests. In general, polygyny occurred in territories wit h a low rate of nest predation, whereas unmated males held territories wher e the risk of predation was high. The low rate of nest predation in attract ive territories on average compensated secondary females for the cost assoc iated with sharing a male (i.e., reduced male assistance when feeding nestl ings). The polygyny threshold model has been around for several decades, ho wever, only few studies have identified the factors that may compensate sec ondary females for the cost of sharing a mate. In the present study we were able to show that the patterns of nest predation can be an important terri tory quality parameter that compensate secondary females, and, hence, contr ibute to the occurrence of avian polygyny.