Batten down the thatches: front-line defences in an apparently defenceless cuckoo host

Avian brood parasites lay their eggs in the nests of other species, imposing high costs on their hosts. In theory, this should select for the evolution of defences against parasitism in hosts, yet eviction of parasite eggs or chicks is absent in many host species. One such host is the yellow-rumped thornbill, Acanthiza chrysorrhoa, the primary host of the shining bronze-cuckoo, Chalcites lucidus, in Australia. Here we tested whether the lack of egg and chick rejection in yellow-rumped thornbills has led to the evolution of alternative defences against brood parasitism. We provide evidence that this host has evolved two types of frontline defences. First, yellow-rumped thornbills responded to the presence of a cuckoo mount near the nest with mobbing behaviour and by occupying their dome-shaped nests, potentially as a means of blocking the small entrance hole. Second, we show that brood parasitism imposes directional selection for early breeding on the yellow-rumped thornbill and that yellow-rumped thornbills show a concomitant shift in their breeding phenology, breeding earlier than both congeneric and sympatric species. Our results highlight the importance of studying apparently defenceless hosts in order to identify alternative defence strategies.