Nest defence in an altricial bird with uniparental care: the influence of offspring age, brood size, stage of the breeding season and predator type

Abstract

This study examines factors affecting nest defence in Aquatic Warblers Acrocephalus paludicola, a passerine species with uniparental (female) care. Stuffed predators, the Hen Harrier Circus cyaneus and the Polecat Mustela putorius, were presented at nests at various stages of the breeding cycle throughout the breeding season. The intensity of nest defence increased as the breeding cycle progressed and the offspring value hypothesis explained this pattern better than the vulnerability hypothesis. The number of nestlings and time of the breeding season (date) did not affect the level of nest defence. The results suggest that the nest defence behaviour of Aquatic Warblers was predator-specific. As expected, females' defence behaviour towards the mammalian predator was more risk-prone than that towards the avian one: the Polecat was approached more closely with a greater number of flights. Also the vocalisations given towards the two predators were different: in the presence of the polecat, compared with the harrier, loud, conspicuous "trr" calls were more frequent.