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In 1978 and 1979 we studied brood reduction in Black-legged Kittiwakes (Rissa tridactyla) on St. Paul Island, Alaska. In two-egg clutches, first-laid eggs were largest; they were incubated before second eggs were laid and consequently hatched first. Chicks from first-laid eggs begged more frequently, fed more frequently, grew faster, and were more aggressive than their sibs. These sibling differences frequently facilitated the ejection of younger siblings from the nest by first-hatched young. Lower feeding and growth rates and higher aggression levels occurred in broods in which the second chicks subsequently died than in broods in which both chicks fledged. Chick loss was most frequent during inclement weather. These data are consistent with the hypothesis that brood reduction is related to the amount of food that chicks receive.

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