Differential Mortality of Male and Female Offspring in Experimentally Manipulated Broods of the Rook

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Abstract
Rock fledglings found abandoned on the ground in 2 rookeries at Trondheim, Norway, comprised 22 males and 7 females, indicating a differential mortality among the young, according to sex. The mean bodyweight of abandoned young was lower than that of young of a control sample, and their tarsi were also shorter, indicating that they had starved throughout much of the nestling period. The existence of a differential mortality between male and female young was verified in an experiment in which broods were manipulated to contain 4 males, or 4 females. For these experimentally enlarged broods a polarization of the body weight among the young of the brood, i.e., the parents did not alter their reproductive effort according to the composition of their brood. The rook shows sexual dimorphism in size, the adult males being about 15% heavier than the adult females. The rearing of a male offspring to independence may therefore involve a greater parental cost than the rearing of a female offspring. For the rook this difference in cost seems to be offset in the population as a whole by an increase in the mortality rate of the male offspring. The observations made are thus consistent with Fisher''s theory on the evolution of the sex ratio.