BROOD DIVISION IN BIRDS IN RELATION TO OFFSPRING SIZE - SIBLING RIVALRY AND PARENTAL CONTROL

In some altricial birds with biparental care, it is the female, and in others the male, that provides more food to the smallest offspring wi thin the brood. Many hypotheses have been proposed to account for such puzzling patterns of parental care. A parsimonious explanation is tha t no difference exists between the parents in priority of care but tha t differences arise simply from sibling rivalry, with dominant chicks trying to position themselves closest to the parent that provides most care (the sibling rivalry hypothesis). A refinement of the idea is th at parents use the way they approach the chicks to counter selfish off spring and in this way control allocation of care (the parental approa ching hypothesis). A comparison across species suggested that female c are of the smallest chick within a brood is the ancestral and most com mon pattern. However, strong variation exists within single population s. In one species, the American robin, Turdus migratorius, the sibling rivalry hypothesis and the parental approaching hypothesis were both supported because in broods where males provided more care than female s, the largest chick was predominantly fed by the male whereas the sma llest chick was predominantly fed by the female. When the male provide d less care than the female, an opposite result was found. The same pa tterns of allocation of care also seemed to exist when chicks were qui te immobile just after having left the nest and when their positions w ere experimentally controlled, suggesting parental control. (C) 1997 T he Association for the Study of Animal Behaviour.