"Aggressive competition between sibling chicks differs between blue-footed and brown boobies. In a brood of two blue-foot chicks, the senior one (four days older) maintains dominance over the three-month nestling period by daily low-level attacking, ensuring privileged growth and survival for itself by limiting junior's begging and feeding. Both blue-foot siblings often survive and fledge, but when parents provide insufficient food, senior increases its aggression and junior starves to death or gets killed by adult neighbors while seeking adoption in their nests. In contrast, brown booby parents bring less food to their broods and their senior chicks are unconditionally intolerant and violent; they attack their sibling at every opportunity and toss it out of the nest, to certain death. The behavioral roles of booby family members - including conditional sharing, siblicide, and parental passivity - are explained by Hamilton's theory of inclusive fitness and kin selection, which also applies to humans"--