Brood Parasites as Predators: Farming and Mafia Strategies

Abstract

Avian brood parasites may depredate unparasitized host nests in advanced stage (farming strategy) or those of hosts that have rejected the parasitic offspring (mafia strategy). Both predatory strategies induce host renesting, but the benefit of the former is to increase availability of host nests for future parasitism, while the latter imply extra fitness costs to rejecters and, thus, induce hosts to accept parasitism in replacement clutches. Despite clear expected benefits of these strategies, they have been reported only rarely. Mafia has been demonstrated in only two parasitic species, but not all available results confirm it. Results from recent studies indicate that magpies (Pica pica) reject real great spotted cuckoo (Clamator glandarius) eggs less frequently and that cuckoos punished rejecter magpies less frequently now than about 25 years ago. We suggest that costs of retaliatory behaviours together with phenotypic plasticity of magpie and great spotted cuckoo’s defences and counter-defences are the crucial points explaining these differences. Reported geographical and temporal variation in parasitism rates suggests that parasitism by great spotted cuckoos would depend on environmental variables, intensity of host defences or propensity to compliance of magpies. This highly variable risk of parasitism, together with the facultative virulence of cuckoos against rejecters, makes plasticity of antiparasitic defences adaptive in magpies. Understanding the mechanisms underlying the mafia remains elusive. Future studies should concentrate on examining how often and in which context brood parasites prey upon host nests in different host populations and in different brood parasite–host systems.