Behav Ecol Sociobiol (1981) 9:59~53 Behavioral Ecology and Sociobiology 9 Springer-Verlag 1981 Social Hierarchy Among Siblings in Broods of the Oystercatcher Haematopus ostralegus Uriel N. Safriel* Edward Grey Institute of Field Ornithology, South Parks Road, Oxford OX1 3PS, England Received November 17, 1980 / Accepted April 3, 1981 Summary. On Skokholm Island, Wales, the young of the Oystercatcher (Haematopus ostralegus) are ful- ly precocial, yet are totally dependent on parents for food. Though all young (commonly three) usually hatch within one day, small hatching intervals do exist, and first-hatched nestlings are usually slightly heavier at hatching than those hatched later. First- hatched young and/or initially heavier young become socially dominant over their siblings; a stable, linear, non-aggressive social hierarchy among siblings devel- ops, which determines the partition of food among them and results in unequal distribution when food is scarce. Food shortage impairs the growth rate of subordinate young. They are rather hungry and rest- less and consequently predation on them is higher than on dominant young. It is proposed that Oystercatchers achieve effective brood-reduction by the parents' control of differential egg weight, by the eggs' hatching order, and by the young's acceptance of a non-aggressive social hierar- chy. The latter is an adaptation associated with the unusual phenomenon of fully precocial wader young being exclusively fed by parents. Introduction Sibling interrelations are often expressed in social hierarchies and/or size hierarchies and frequently lead to brood reduction. Most cases of social hierarchies within broods develop against a background of asyn- chronous hatching, and among semialtricial (Mey- burg 1974) and semiprecocial (Procter 1975) species, i.e. among birds whose young are confined to a small space where they are fed by parents. In these cases, * Present address: Department of Zoology, The Hebrew Universi- ty of Jerusalem, Jerusalem, Israel hierarchy is maintained by explicit and non-ritual- ized aggression, otherwise rare among birds. A case is reported here of non-aggressive social hierarchy among fully precocial siblings that are fed by parents, but which hatch fairly synchronously. The social hier- archy of Oystercatcher (Haematopus ostralegus) sib- lings and its manifestation in brood-reduction is dis- cussed first; its adaptive value is then examined. Materials and Methods Observations were made on Skokholm Island in Wales during 1965 and 1966 (Safriel 1967). The eggs (usually three) hatch within one day, and the nestlings leave the nest a few hours after hatching and are dependent for food on their parents until fledging. During the first month they move between hiding places in their parents' territory and expose themselves only to be fed. Except at a very early age when they require considerable brooding, the young move, hide and feed mostly singly. Their behaviour is there- fore very different from that of many fully precocial birds, in which the family acts as a tight group. In 1965, I erected canvas pens (10 x 5 x0.5 m) around four nests in incubation. The parents became accustomed to flying over the obstacle, and once the young hatched the parents fed and brooded them. Observations on the individually marked young were made from a hide. In 1966 observations on 13 unconfined broods were carried out with the aid of four portable hides. They were pitched at different parts of a given territory and were either used alternatively or were relocated to follow the movements of the individually marked young. The young were weighed as often as possible, and their fledging success was assessed. Results Dominance Hierarchy in Confined Broods The weight of confined broods lagged behind that of unconfined ones (Fig. 1 a), the former showing ap- parent signs of permanent hunger. All siblings rushed together towards the feeding parent, but the heaviest chick was faster in reaching the parent and in snatch- ing food. As time went on, the distribution of food items among siblings became markedly uneven, most 0340-5443/81/0009/0059/$01.00