Evolutionary significance of antiparasite, antipredator and learning phenotypes of avian nest defence

Avian nest defence, which is expected to serve both antiparasite and antipredator functions, may benefit or be detrimental to birds, although selective forces that potentially operate on nest defence have not been quantified as a whole. Together with fitness values, we analysed two traits of nest defence, intensity and plasticity, in two distantly related passerine species, yellow warbler (Setophaga petechia) in North America and reed warbler (Acrocephalus scirpaceus) in Europe, both favourite host species for brood parasites. Breeders that escaped parasitism were the most vocal among reed warblers, whereas there was no specific defence phenotype that predicted prevention of parasitism in yellow warblers. Breeders that escaped nest predation were, in both species, those with the most distractive response at the first exposure to a nest-threatening event, such as the experimental predation or parasitism simulated at the nest. However, increasing defence intensity benefited yellow warblers but was detrimental to reed warblers, because intense defence responses attracted predators. Adaptiveness of nest defence was revealed by nest defence phenotypes when examined in concert with the seasonal fitness (i.e. measures of reproductive success). Results revealed selective forces favoured yellow warblers with strong defence phenotypes. Opposite forces were instead revealed among reed warblers whose favoured phenotypes were strong, yet less flexible, defenders.