Interactions between avian brood parasites and their hosts are one of the most suitable model systems for studying coevolution. Brood parasitic cuckoos lay their eggs in the nests of other species, called hosts. In most cuckoo species, the cuckoo chick evicts or outcompetes the host’s own brood and becomes the sole occupant of the nest. Therefore, parasitism by cuckoos is highly costly to hosts, selecting for the evolution of host defences. In turn, host defences select for counter-adaptations in cuckoos, giving rise to a coevolutionary arms race. Many hosts have evolved the ability to recognize and reject mimetic cuckoo eggs, but most hosts fail to discriminate cuckoo chicks. Theory predicts that hosts should not evolve to recognize and reject cuckoo chicks via imprinting because of the risk of mistakenly imprinting on a cuckoo chick in their first brood and thereafter always rejecting their own chicks. However, recent studies have found that the large-billed gerygone (Gerygone magnirostris), a host of the little bronze-cuckoo (Chalcites minutillus) rejects the cuckoo nestlings and accepts non-mimetic cuckoo eggs. In my exit seminar, I will explore the mechanisms of chick rejection behaviour in large-billed gerygone.