To compensate or not to compensate: testing the negotiation model in the context of nest defense

Theory of parental care in biparental species predicts that a decrease in 1 mate’s parental effort should trigger a partial increase of care by the other mate. Previous studies investigating compensatory behavior used nestling provisioning as the measure of parental effort. However, nest defense is also a costly component of parental care because defenders risk injury or death caused by predators. Here for the first time, we test the compensation hypothesis in the context of nest defense. We experimentally widowed (by temporarily removing the other mate) female or male great reed warblers Acrocephalus arundinaceus and faced them with a predator near the nest (the stuffed Eurasian sparrowhawk Accipiter nisus). Female responses were independent of their partner’s presence or absence. In contrast, lone males did not compensate for the absence of their mates; they even behaved more aggressively when their partner was present, contradicting the partial compensation hypothesis. We discuss potential determinants of between-species variation in sex-specific compensatory behavior. We predict that a lack of compensation might be found in species with different renesting and remating potentials between males and females, for example, where males are unwilling/unable to raise the brood when unassisted by females and therefore, avoid an investment that cannot increase their fitness.