Turdoides

Abstract: (1) Some aspects of the demography of the jungle babbler are described. Compared to the temperate passerines on which most studies of population dynamics have so far been conducted, jungle babblers showed little seasonal or annual changes in population size. The proximal mechanism by which this stability was achieved was the emigration of birds in their second summer or older. The survival of post-juvenile birds remaining with their natal group was high, about 90%O per annum, but this also applied to breeding birds, and hence the opportunity to attempt breeding was so limited that only a quarter of the birds reaching maturity ever managed to do so. (2) Despite the rate of interchange observed between groups, the mean degree of relatedness of all group members to the breeding pair was probably about 0 3, higher than that between half-siblings. Under theories of kin selection (Hamilton 1964; Eberhard 1975) the selection pressure for altruistic behaviour is proportional to the degree of relatedness of the donor and the recipient, and hence babbler groups apparently present an excellent situation for the evolution of altruistic behaviour. (3) The quality of habitat occupied varied on a larger scale than is typical of most temperate situations, and this resulted in wide differences in the reproductive success of different groups. Once the influence of territorial quality was taken into account there was no significant correlation between group size and breeding success, and there was some indication that larger size may have been correlated with lower fledging success in the spring. This appears to contradict the prediction of kin selection than non-breeding members of the group should make a positive contribution to the reproduction of the closely related breeding pair. (4) Dispersal is more important to females, which probably do not breed in their natal groups, than it is to males. Both sexes have the option of joining'in the establishment of a new group, but remaining in the natal group may constitute a more attractive strategy for males which can eventually inherit the parental breeding territory. The differences in reproductive success between different territory types must influence the strategies of males, and may account in part for differences in the size of groups.