Perch proximity does not predict the probability of cuckoo parasitism in a woodland host

Abstract

Obligate avian brood parasites typically impose great costs on their hosts by replacing the host’s offspring with their own. Multiple theories have been put forward regarding which factors influence the probability that a host nest will be parasitized. One of these, the perch proximity hypothesis, predicts that nests that are close to trees are more likely to be parasitized, as trees provide a vantage point for the parasite to observe nest building by the host. Substantial support for this hypothesis has been found in several species of brood parasites that occur in a range of open habitats, such as reed beds, marshlands and grasslands, where vantage points are likely to be scarce. However, it is unclear whether this prediction applies to species that occupy woodland habitats. Although brood parasites may be more likely to exploit nests with suitable vantage points nearby, it is possible that a high density of vegetation might obscure the nest, thereby reducing the likelihood of parasitism. Here we used a long-term data set of superb fairy-wren, Malurus cyaneus, nests to test this hypothesis in the brood-parasitic Horsfield’s bronze-cuckoo, Chalcites basalis. Contrary to the prediction of the perch proximity hypothesis, parasitism risk did not increase with closer proximity of a perch to the nest; instead, we show that the risk of parasitism may decrease as tree density increases. These results suggest that exploration of the host activity hypothesis and nest exposure hypothesis may be fruitful for future work in this system.