Hatching asynchrony, sibling aggression and cannibalism in the Bearded Vulture Gypaetus barbatus
We report data on laying and hatching asynchrony in the Bearded Vulture Gypaetus barbatus, and provide the first detailed description of the provisioning of nestlings, sibling aggression and cannibalism. The results suggest that brood reduction was mediated through the age and size differences between siblings, which resulted in the superior competitive ability of the older chick. Thus, brood reduction occurred through starvation facilitated directly by the older chick's aggression. The results support the hypothesis that the second egg probably functions as insurance in case the first egg does not hatch. The insurance-egg hypothesis is supported by the following facts: 1) in three of six breeding attempts, the second egg produced a chick when the first egg failed to hatch or the first chick died young. At least two of these B chicks fledged; 2) in the Bearded Vulture most breeding failures occur during the hatching period and thus the insurance value of last-hatched eggs would be especially important in this species; 3) clutch replacement, an alternative to laying an insurance egg, is relatively uncommon in this species and 4) the laying interval (5–7 days) and the hatching asynchrony (5–8 days) of this species are the longest recorded in any raptor, suggesting that they might represent an adaptive mechanism facilitating the rapid loss of the second chick if the first one hatches.
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