Extra-pair copulations, intra-specific brood parasitism, and quasi-parasitism in birds: a theoretical approach

Although 90 % of all bird species are monogamous, many species practice alternative reproductive strategies as extra-pair copulations, intra-specific brood parasitism, and quasi-parasitism. In territorial monogamous species, both partners hold and defend the territory from intruders. Often, the intruders are males and usually the local male banishes the intruders. Indeed, many studies focused on the response of the local male toward intruder males. However, the benefits and costs associated with the responses of the local male toward intruder females have been largely overlooked. Focusing mainly on alternative reproductive strategies, we developed a model to predict the aggression a monogamous male may demonstrate toward an intruder female during the pre-egg laying stage of his local female partner. This model demonstrates that the intensity of aggression that the local male shows toward an intruder female depends on the extra-pair copulations that his local female partner may perform. Further, the aggression also depends upon intra-specific brood parasitism and quasi-parasitism that might be carried out by the intruder female. Our approach suggests that when considering mating strategies, there is a need to assess how these three alternative reproductive strategies may affect the local male's aggression toward intruder females.