Avian brood parasitism is a model example of coevolution, yet measuring the rate at which coevolution takes place between a brood parasite and its host is difficult, as parasites rarely switch to completely naïve hosts (Nakamura et al. 1998). In Australia, the brood parasitic eastern koel (Eudynamis orientalis) recently switched to a new host, the red wattlebird (Anthochaera carunculata), providing a rare opportunity to observe coevolution in action (Brooker and Brooker 2005; Lenz et al. 2009). We compare the evolution of defences in the new host with those of traditional hosts at two sites with different durations of parasitism by the koel: Sydney (~37 years) and Canberra (~7 years). Specifically, we test 1) which hosts have evolved the ability to reject foreign eggs, and 2) whether koels have evolved mimicry of host eggs.

Model egg rejection experiments show that the traditional hosts are expert egg rejecters, whereas the new host has not evolved egg rejection at either site, indicating that 37 years of parasitism is insufficient time for egg rejection to evolve. However, wattlebirds exposed to high parasitism rates were more likely to abandon their nests in response to disturbance than those where parasitism was rare, which may be an early stage in the evolution of defences. We used objective measures of colour to determine if egg rejection by hosts has selected for egg mimicry in koels. Koel eggs from wattlebird nests in Sydney appeared significantly more similar to Sydney wattlebird eggs than to the eggs of their traditional hosts or Canberra wattlebirds. This result is puzzling considering the lack of rejection response shown by wattlebirds, but could be a counteradaptation to nest abandonment by wattlebirds. This study provides important information on how quickly hosts and parasites can adapt, which can be used in fields such as conservation and speciation.