Food limitation explains most clutch size variation in the Nazca booby

1. Natural selection is expected to optimize clutch size, but intrapopulati on variation is maintained in many bird species. The Nazca booby provides a simple system in which to investigate clutch size evolution because clutch size and brood size are decoupled due to obligate siblicide. The indirect effect of brood size on clutch size evolution can therefore be eliminated. 2. In Nazca boobies, second eggs provide insurance against the failure of t he first egg or early death of the first hatchling, but approximately half of all females lay only one egg. We tested the hypothesis that one-egg clut ches result from food limitation by providing female Nazca boobies with sup plemental food. 3. A higher proportion of supplemented females produced two-egg clutches th an did control females. Supplemented females produced larger second-laid eg gs than did control females, but not first-laid eggs. Laying date and layin g interval were not affected. 4. Comparisons of clutch size and egg volume between years indicated that t he supplemental feeding experiment was not conducted in a year with a poor natural food supply. Thus supplemented females produced larger clutch sizes despite apparently normal natural food levels. 5. This experiment nearly completes our understanding of clutch size variat ion in the Nazca booby, and indicates that food limitation and the costs of egg-laying should be considered carefully in studies of clutch size evolut ion.