The interactions between avian interspecific brood parasites and their hosts provide tractable and informative systems for investigating coevolution. Generally, these investigations have emphasized the egg and chick stages of the coevolutionary arms race; however, recent studies demonstrate that coevolution operates at all stages of the host nesting cycle and emphasize the importance of reciprocal adaptations prior to deposition of the parasite egg in the host nest: the ‘frontline’ of the arms race. Here we review the diversity of adaptations at the frontline and its implications for our understanding of brood parasite–host relationships. Coevolution at the frontline can fundamentally shape the life histories, morphologies, physiologies and behaviours of both brood parasites and their hosts, and influences the trajectories and outcomes of their subsequent coevolutionary interactions. We advocate the incorporation of frontline interactions in empirical and theoretical investigations of brood parasite–host arms races to provide a more holistic understanding of the coevolutionary processes in these systems.