Experimental demonstration of the insurance value of extra eggs in an obligately siblicidal seabird

A variety of organisms regularly produce more offspring than they raise. De spite the apparent energetic waste of such a reproductive tactic, overprodu ction may be favored by natural selection in some cases. One such case is w hen surplus offspring can serve as replacements, or insurance, for failed s iblings. We tested the Insurance Egg Hypothesis (IEH) as an explanation for the overproduction of offspring in an obligately siblicidal seabird, the N azca booby (Sula granti), which fledges a maximum of one nestling regardles s of its clutch size. We manipulated clutch sizes within the range of natur al variation encountered in this species (one-two eggs). The IEH predicts t hat parents with two-egg clutches should have higher reproductive success t han those with one-egg clutches because the second egg can provide a nestli ng when the first egg fails to hatch, or when the first chick dies young. C onsistent with the IEH, natural one-egg clutches that were enlarged to two eggs produced more hatchlings and fledglings than control one-egg clutches did, and natural two-egg clutches that were reduced to one egg produced few er hatchlings and fledglings than control two-egg clutches did. We also eva luated aspects of the Individual Optimization Hypothesis, which proposes th at individual optimal clutch sizes differ, as an explanation for clutch siz e variation in this Species In Nazca boobies, selection driven by replaceme nt value appears to favor clutches larger than one even though final brood size is invariably one. One-egg clutches may be produced by parents experie ncing some proximate limitation, such as a lack of food.