Parental investment with a superior alien in the brood.

When a parent's parentage differs across breeding attempts, established theory predicts that the parent should invest more in a brood when perceived parentage is high. We present a model of parental investment in which offspring unrelated to the parent have a competitive advantage over the parent's own offspring and take a larger share of investment. We show that this can weaken or, if the competitive advantage is great, reverse the predicted relationship between perceived parentage and parental investment. A moderate competitive advantage of extra-pair young over within-pair young could partly explain the lack of any clear relationship between paternal care and paternity in many studies, and could easily arise if females choose extra-pair partners for good genes. Our results are also relevant to interspecific avian brood parasitism. As parasites reared together with host offspring are often superior competitors, their hosts could benefit from increasing investment in response to suspected parasitism.