Abstract

Coevolutionary arms races, where adaptations in one party select for counter-adaptations in another and vice-versa, are fundamental to interactions between organisms and their predators, pathogens and parasites. Avian brood parasites and their hosts have emerged as model systems for studying such reciprocal coevolutionary processes. For example, hosts have evolved rejection of foreign eggs in response to brood parasitism from cuckoos, and cuckoos have evolved host-egg mimicry as a counter-response. However, there are other host defences such as protecting the nest from being parasitised in the first place. Our recent research shows that mobbing of cuckoos can be socially transmitted and varied strategically according to local parasitism risk. This front line of defence is effective in reducing parasitism and has likely selected for mimicry of hawks by cuckoos. The emerging view is that cuckoo hosts use a ‘defence in depth strategy’, whereby they deploy sequential lines of defence in a coevolutionary arms race with corresponding offensive lines of the brood parasite. This highlights the need for more holistic research into the coevolutionary consequences when multiple adaptations and counter-adaptations evolve in concert.