Breeding site and host selection by Horsfield's bronze-cuckoos, Chalcites basalis

Cuckoos are faced with a series of reproductive decisions unique to the brood-parasitic lifestyle. Choice of the appropriate host to rear their young requires decision making at three levels. First, selection of a breeding site may take into account host densities in addition to environmental considerations. Second, once they have selected a breeding site, female cuckoos must ensure that they choose the nests of an appropriate host species to rear their young. Third, cuckoos may also choose among individuals of the host species in relation to the likelihood that the host will successfully rear their young. By observation and experiment, we investigated the factors that influenced annual parasitism rates and the mechanisms of host choice in Horsfield's bronze-cuckoos, Chalcites basalis. Parasitism rates varied from 0% to 37% annually, and were influenced by host density and spring rainfall. Despite the availability of several suitable hosts with similar nest sites within the same habitat, over 99% of Horsfield's bronze-cuckoo eggs were laid in superb fairy-wren, Malurus cyaneus, nests, lending strong support to the Host Preference Hypothesis for host choice. Patterns of parasitism were nonrandom with respect to host female age and identity, but we found no evidence that cuckoos preferentially parasitized those individuals that were most likely to successfully rear their young.