摘要Coevolutionary arms races between brood parasites and hosts provide tractable systems for under-standing antagonistic coevolution in nature;however,little is known about the fate of frontline antipar-asite defenses when the host"wins"the coevolutionary arms race.By recreating bygone species inter-actions,using artificial parasitism experiments,lingering defensive behaviors that evolved in the context of parasitism can be understood and may even be used to identify the unknown agent of para-sitism past.Here we present the first study of this type by evaluating lingering"frontline"nest defenses that have evolved to prevent egg laying in a former brood parasite host.The Australian reed warbler Acrocephalus australis is currently not parasitized but is known to exhibit fine-tuned egg dis-crimination—a defensive behavior indicative of a past brood parasite-host arms race and common in closely related parasitized species.Here,using 3D-printed models of adult brood parasites,we exam-ined whether the Australian reed warbler also exhibits frontline defenses to adult brood parasites,and whether we could use these defenses to identify the warbler's"ghost of parasitism past."Our findings provide evidence that the Australian reed warbler readily engages in frontline defenses that are consid-ered adaptive specifically in the context of brood parasitism.However,individuals were unable to dis-criminate between adults of different brood parasite species at their nest.Overall,our results demon-strate that despite a relaxation in selection,defenses against brood parasitism can be maintained across multiple stages of the host's nesting cycle,and further suggest that,in accordance with previous findings,that learning may be important for fine-tuning frontline defense.