THE FUNCTION OF HATCHING ASYNCHRONY IN THE BLUE FOOTED BOOBY

The blue-footed booby (Sula nebouxii) commonly hatches two eggs 4 days apart; then the senior (first-hatched) chick aggressively dominates t he other and sometimes kills it. Two hypotheses explaining the functio n of the hatching interval were tested by creating broods with synchro nous hatching: the facultative brood reduction hypothesis of Lack (195 4) and the sibling rivalry reduction hypothesis of Hahn (1981). The re sults contradicted most predictions of both hypotheses: synchronous br oods formed an aggressive hierarchy similarly to asynchronous broods ( controls), and subordinate chicks grew poorly (Fig. 1) and died freque ntly, similarly to junior chicks in control broods. However, compared with synchronous broods, asynchronous broods showed less aggression (F ig. 2), diminished food allocation to subordinate chicks (Fig. 3) and less total food consumption (30% fewer feeds at age 0-10 days). These behavioral comparisons took into account the different ages of chicks in different treatments. The results suggest that natural asynchrony m akes brood reduction more efficient and decreases the costs of sibling aggression to parents, in terms of their future survival or fecundity , as proposed by Mock and Ploger (1987). Further, in exaggeratedly asy nchronous broods (8-day hatch interval) junior chicks suffered more ag gression (Fig. 4) and grew more slowly than junior chicks in control b roods. This result supports the hypothesis of optimal hatch asynchrony of Mock and Ploger (1987).