Multiple parasitism promotes facultative host acceptance of cuckoo eggs and rejection of cuckoo chicks

Many hosts of brood-parasitic cuckoos reject foreign eggs from the nest. Yet where nests commonly receive more than one cuckoo egg, hosts might benefit by instead accepting parasite eggs. This is because cuckoos remove an egg from the nest before adding their own, and keeping cuckoo eggs in the nest reduces the odds that further host eggs are removed by subsequent cuckoos. This ‘clutch dilution effect’ has been proposed as a precondition for the evolution of cuckoo nestling eviction by hosts, but no previous studies have tested this in a host that rejects cuckoo nestlings. We tested the clutch dilution hypothesis in large-billed gerygones, Gerygone magnirostris, which are multiply parasitized by little bronze-cuckoos, Chalcites minutillus. Gerygones evict cuckoo nestlings but accept cuckoo eggs. Consistent with multiple parasitism favouring egg acceptance, we found gerygone egg survival was higher under scenarios of cuckoo egg acceptance than rejection. Yet gerygones were also flexible in their egg acceptance, with 61% abandoning clutches if they contained only cuckoo eggs. Our results thus confirm a clutch dilution effect in gerygones and suggest that multiple parasitism might favour a facultative response to cuckoo eggs, whereby hosts accept or reject cuckoo eggs depending on clutch composition.