Thicker eggshells are not predicted by host egg ejection behaviour in four species of Australian cuckoo

Defences of hosts against brood parasitic cuckoos include detection and ejection of cuckoo eggs from the nest. Ejection behaviour often involves puncturing the cuckoo egg, which is predicted to drive the evolution of thicker eggshells in cuckoos that parasitise such hosts. Here we test this prediction in four Australian cuckoo species and their hosts, using Hall-effect magnetic-inference to directly estimate eggshell thickness in parasitised clutches. In Australia, hosts that build cup-shaped nests are generally adept at ejecting cuckoo eggs, whereas hosts that build dome-shaped nests mostly accept foreign eggs. We analysed two datasets: a small sample of hosts with known egg ejection rates and a broader sample of hosts where egg ejection behaviour was inferred based on nest type (dome or cup). Contrary to predictions, cuckoos that exploit dome-nesting hosts (acceptor hosts) had significantly thicker eggshells relative to their hosts than cuckoos that exploit cup-nesting hosts (ejector ..., Cuckoo and host species. The parasitic cuckoo species selected for use in this study were chosen based on previous knowledge of their host selection and the egg ejection behaviour of those hosts. We selected four Australian cuckoo species based on the known egg ejection rates of their primary hosts from our earlier studies. Two congeneric species, Horsfield’s bronze-cuckoos (Chalcites basalis) and shining bronze-cuckoos (C. lucidus), exploit hosts that build dome-shaped nests in which detection of foreign eggs is constrained by poor visibility in the dark interior. Hosts of these two cuckoo species rarely eject either naturally-laid cuckoo eggs or experimental, non-mimetic plastic model eggs, of similar size to their own (Table 1). The two other cuckoo species in the study, the pallid cuckoo (Cuculus pallidus) and the Pacific koel (Eudynamis orientalis), exploit hosts that build cup-shaped nests with good visibility, and these hosts routinely eject both naturally-laid cuckoo eggs and ex..., Microsoft Excel