The first stages of coevolution between a brood parasite and its new host: are naïve hosts defenceless?

Little is known about the early stages of the coevolutionary interactions between brood parasites and their hosts or how rapidly hosts evolve defences. We investigated the success of breeding attempts by the brood-parasitic Pacific Koel (Eudynamys orientalis) and its newest host, the Red Wattlebird (Anthochaera carunculata), at two sites where wattlebirds have been exposed to different durations of parasitism: Sydney (38–86 years) and Canberra (8–33 years). Parasitism was more frequent in Sydney and nest predation increased in Sydney after the arrival of migratory koels, yet the nest success rate of wattlebirds was not significantly lower than in Canberra. Wattlebirds rarely reject foreign eggs, but their low clutch size, earlier breeding and abandonment of koel eggs laid before the host’s laying period might explain the low impact of koels. Parasitised nests fledged wattlebird and koel young at equal rates, but fledged fewer wattlebird young than unparasitised nests. The koel’s later breeding and poor timing of egg laying may have contributed to the low success of koel eggs. These results suggest that host switching may have a negative impact on the reproductive success of both the parasite and the new host, and naïve hosts may avoid parasitism using generalised defences that require no egg or parasite recognition.