UC Irvine

Rissa tridactyla were studied on St. Paul Island, Alaska. In 2-egg clutches, 1st-laid eggs were largest; they were incubated before 2nd eggs were laid and consequently hatched first. Chicks from 1st-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 1st-hatched young. Lower feeding and growth rates and higher aggression levels occurred in broods in which the 2nd chicks subsequently died than in broods in which both chicks fledged. Chick loss was most frequent during inclement weather. Data are consistent with the hypothesis that brood reduction is related to amount of food that chicks receive.-from Authors