The spatial organization and mating system of Horsfield's bronze-cuckoos, Chalcites basalis

In theory, liberation from parental care in brood parasites should facilitate polygamous matings by both sexes. We used a combination of mtDNA and microsatellite analysis to reconstruct sibling groups of Horsfield's bronze-cuckoo chicks to infer the mating system of this species. By mapping the distribution of sibling groups we also inferred the breeding ranges and breeding duration of individual cuckoos. Genetic analysis revealed that individual female cuckoos laid distinctive egg types, enabling inclusion of data based on egg morphology. Pairs of Horsfield's bronze-cuckoos occupied exclusive breeding ranges encompassing clusters of host territories. There was a bimodal pattern in the timing and duration of breeding: early-arriving females laid eggs over a period of up to 5 weeks and were then replaced by late-arriving females, which continued laying periodically over at least 2 months. In other brood-parasitic birds polygamy is widespread. By contrast, Horsfield's bronze-cuckoo females were genetically monogamous, and males were either monogamous or sequentially monogamous. Polygamy may be constrained in Horsfield's bronze-cuckoos by the exclusive home ranges of females.