Experimentally increased egg production constrains future reproduction of female house wrens

Theory predicts that investment in current offspring should negatively influence a parent’s ability to invest in future offspring. Despite extensive interest in documenting reproductive costs in birds, evidence for fitness-related costs of egg production to breeding females is scarce. In this study, we used an egg-removal experiment on the house wren, Troglodytes aedon, to test the hypothesis that producing eggs is reproductively costly. By removing eggs from the nest as they were laid, we induced females to produce more eggs than normal, although experimental and control females incubated clutches of similar size. Females producing extra eggs paid steep fitness costs for their increased effort; relative to controls, females with increased egg-laying demands were less likely to reproduce again within the same breeding season, and those that did took longer to do so, had a smaller clutch size and produced fewer fledglings from their subsequent broods. Although females in each treatment group were equally likely to return to breed the following summer, experimental females produced fewer eggs than control females in their next breeding season. This is the first demonstration that increased effort in egg production reduces the future reproductive output of passerine birds.