Brood parasites are organisms that use the strategy of brood parasitism found among birds, fish or insects, involving the manipulation and use of host individuals either of the same (intraspecific brood-parasitism) or different species (interspecific brood-parasitism) to raise the young of the brood-parasite. This relieves the parasitic parent from the investment of rearing young or building nests, enabling them to spend more time foraging, producing offspring etc. Additionally, the risk of egg loss to raiders such as raccoons is mitigated, by having distributed the eggs amongst a number of different nests.
It has often been questioned why the majority of the hosts of brood parasites care for the nestlings of their parasites. Not only do these brood parasites usually differ significantly in size and appearance, but it is highly probable that they reduce the reproductive success of their hosts. So what possible benefits are gained from providing this parental care? The "Mafia hypothesis" evolved through studies in an attempt to answer this question. This hypothesis revolves around host manipulations induced by behaviors of the brood parasite. Upon the detection and rejection of a brood parasite's egg, the host's nest is depredated upon, its nest destroyed and nestlings injured or killed. This threatening response is indirectly enhancing selective pressures favoring aggressive parasite behavior that may result in positive feedback between Mafia-like parasite and compliant host behaviors.
Under the Mafia Hypothesis, parasite birds often return to the nests where they deposited their eggs with the eggs of the host and they would destroy the nest and kill the offspring of the host if they rejected the parasites eggs. “They were made an offer they could not refuse.” Since the destruction of their nest and offspring results in a drastic decay in fitness for the host, they have no other option than spending their resources caring for the parasites.
