Can social interactions rescue "lost" behavioural defences in an allopatric cuckoo host?

Behavioural defences are a key adaptation across animals, but confirming decay under relaxed selection is challenging. If expression involves decision-making, then the defence may only be expressed when the right environmental cues are available (cryptic plasticity). Even if defences wane, they could be quickly restored once selection resumes as animals can learn by observing others (social memory). Here we tested cryptic plasticity and social memory under relaxed selection in a cuckoo host, the Common Reed Warbler, by manipulating perceived parasitism risk with social information in geographically-distinct areas differing in allopatry (100 to 1000 years). Despite predicting social information of a cuckoo vs. a control would elicit otherwise cryptic behavioural defences, only birds that mobbed model cuckoos prior to manipulation upregulated this defence, and only in the more recently allopatric population. Social information also had no effect on low-to-absent rates of foreign egg rejection, a second line of defence common to cuckoo host species. These results contrast with parasitised locations where similar methods provoke mobbing and increase egg rejection ten-fold. Our study therefore demonstrates that, even when manipulating information to account for reaction norms, behavioural defences may degrade rapidly under relaxed selection, and thus facilitate a geographic mosaic of coevolution.