Host personalities and the evolution of behavioural adaptations in brood parasitic–host systems

Animals show consistent individual variation in behaviour over time and across different situations, a phenomenon commonly termed personality. Personality studies are important because, if personality traits extend their influence across a wide range of situations, they may help to explain the paradoxical occurrence of nonoptimal behaviours. A paradigmatic example of nonoptimal behaviour is the striking lack of behavioural defences of some hosts of obligate avian brood parasites despite the evident costs they incur. Here we propose that the interplay between personality features expressed by hosts outside the nest and the specialized behaviours expressed by hosts in/or near the nests to deter avian brood parasites (i.e. nest defence, egg and nestling recognition and ejection) may affect the average level of host behavioural defences exhibited at the population level as well as the evolutionary dynamics of brood parasite–host interactions. Differences in personality between hosts may relate to the observed variation in susceptibility to brood parasitism in a given population, as well as to the set of host defensive behaviours expressed in/or near the nests to prevent the costs of brood parasites. In addition, differences in parasite density might have different consequences for the fitness of different personality traits and might potentially explain intermediate tolerance to parasitism and even acceptance of parasitic eggs by hosts in populations exposed to low selection pressures by brood parasites. Future research should thus aim to identify how the different personality traits may relate to specialized host defences to explain the occurrence of apparent maladaptive host behaviours.