Twenty-two years of brood parasitism data do not support the mafia hypothesis in an accepter host of the Brown-headed Cowbird (Molothrus ater)

Abstract

Most host species of the obligate brood parasitic Brown-headed Cowbird (<em>Molothrus ater</em>) accept foreign eggs and offspring despite the associated reproductive costs. One theoretical explanation for this host tolerance is the potential for mafia-like behavior by the brood parasite, which involves the retaliatory destruction of a host’s eggs and/or nestlings in response to that host’s rejection of the parasitic progeny. Here, we use a long-term dataset of an accepter cowbird host, the Dark-eyed Junco (<em>Junco hyemalis carolinensis</em>), where researchers opportunistically removed parasitic cowbird eggs from the clutch, to test critical assumptions and predictions of the mafia hypothesis in this cowbird-host system. For the mafia hypothesis to be supported, at least some junco nestlings should survive in cowbird parasitized nests. In turn, if cowbirds parasitizing juncos engage in mafia behavior, then nests where the parasitic egg was removed should have lower nest success than nests where the cowbird egg remained. We found no support for the mafia hypothesis, because fledgling success was higher in nests where the cowbird egg was removed compared to nests where the cowbird egg remained. If mafia behavior is present in this cowbird population, as suggested by anecdotal observations, it is likely inconsistent over time and space or localized to a small number of individual female parasites rather than a population-wide phenomenon. Further studies are needed to assess how widespread cowbird mafia behavior is across host species and populations and what its implications and impacts may be on host fitness.