Food limitation has been thought to explain the evolution of single—chick broods in pelagic seabirds. We compared the meal sizes, feeding rates, and time budgets of Masked Boobies (Sula dactylatra), typical pelagic seabirds, which raise one chick per breeding attempt, with those of sympatric Blue—footed Boobies (S. nebouxii), which often raise two or more chicks to fledging. Breeding adult Masked Boobies forage farther from land than do Blue—footed Boobies. If longer transit times caused Masked Boobies to deliver less food per day than did Blue—footed Boobies, and thus limited brood size, we expected Blue—footed Boobies, but not Masked Boobies, to be able to provision a two—chick brood. Analysis of chick meal size and adult foraging effort (time spent foraging) indicated that Blue—footed parents could typically feed two, but not three, chicks; brood size thus matches feeding capacity in this species. Masked Boobies, however, appear to have sufficient excess foraging capacity to raise an additional chick, yet do not. The different brood sizes of these species are frequently cited to support the idea that foraging biology determines brood size in seabirds. In contrast, we conclude that Masked and Blue—footed Boobies have similar capacities to provision two—chick broods; some other factor limits Masked Boobies, but not Blue—footed Boobies, to fledging single chicks. Obligate siblicide clearly limits brood size at the proximate level in Masked, but not Blue—footed Boobies. A role for sibling competition also seems likely at the ultimate level, either through an interaction with delivery frequency or by driving genetic parent—offspring conflict over brood size.