NATURAL selection is thought to favour differences in resource use (resource partitioning) among coexisting species because overlap is assumed to incur fitness costs(1-5). However, studies of resource partitioning in animal systems in the wild have ignored or had difficulty demonstrating fitness costs of overlap, particularly at the level of individuals, Here I report manipulative and observational tests demonstrating individual-level fitness costs of resource overlap. Based on an extensive data set (2,400 experimental nests and 1,408 natural nests), I show that seven coexisting bird species differ in nesting microhabitats, and that overlap in use of nest sites increases nest predation rates (fitness costs), Moreover, predation risk is greater for individuals within species that use nest sites that overlap with coexisting species. Such intraspecific variation can allow natural selection to shape species differences and patterns of species coexistence.