Brood reduction is a within-brood partial mortality due to sibling rivalry, and, in some species, the death of at least one sibling in the brood is almost guaranteed (obligate brood-reducers). Imperial Cormorants (Phalacrocorax atriceps) usually lay three-egg clutches that hatch asynchronously over 4–5 days. This species exhibits obligate brood reduction, and last-laid (marginal) eggs serve as insurance against early failure of elder (core) members. Within-brood sibling asymmetries were manipulated to analyze their effects on breeding success, brood reduction, parental body condition and chick growth. Two types of symmetrical broods containing three similar-sized chicks at the beginning of chick-rearing (3-days old) and close to the peak in brood reduction (8-days old) were generated to contrast the natural asymmetrical brood. Breeding success and parental condition were unrelated to sibling asymmetry levels. Asymptotic mass of fledglings from 8-day old broods was lower than those for natural and 3-day old broods. Our results suggest that hatching asynchrony favors early brood reduction and improves fledging condition. Regardless of asymmetry levels, Imperial Cormorants were obligate reducers, and the insurance value provided by the marginal chick was negligible. Therefore, the insurance value of the marginal offspring appears to serve mainly at the egg stage.
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Vol. 40 • No. 3