Support for the nest mimicry hypothesis in Yellow-rumped Thornbills Acanthiza chrysorrhoa

The nest of the Yellow-rumped Thornbill Acanthiza chrysorrhoa (endemic to Australia) consists of a lower domed nest where the young are cared for and an upper cup-shaped structure, referred to as the false nest. Numerous hypotheses have been proposed to explain the primary function of the false nest, but their predictions remain largely untested. Here, we test predictions of the nest mimicry hypothesis (with artificial nests) whereby the empty false nest acts as a decoy, deflecting visual nest predators and/or brood parasites away from the true nest below, thereby increasing nesting success. Clear detection of the false nest by adversaries is an intrinsic aspect of nest mimicry; thus we also test whether conspicuous nests (natural) are favoured by natural selection. Specifically, (1) we compare predation level at artificial domed nests with and without a false nest; and (2) we analyse associations between nest site concealment and nesting outcome in natural nests in a wild population. Supporting the nest mimicry hypothesis, predation level was significantly lower for artificial domed nests with false nests, and for natural nests with lower levels of concealment.