Dispersal, Survivorship and Demography in the Co-Operatively-Breeding Bell Miner Manorina melanophrys

Abstract

Females are the dispersing sex in Bell Miners; males tend to be philopatric. The mean age of dispersal (8.0 ± 1.0 months) corresponded closely to the minimum age of first breeding (8.3 months) and may reflect intolerance by females of their sexually mature daughters which may compete for the parental care provided by males. The adult sex ratio was male-biased, probably because of greater dispersal-related mortality suffered by females. Juveniles up to the age of first breeding experienced extremely high mortality (93%). The life expectancy of buds increased dramatically once they reached breeding- age. The number of breeding positions varied little relative to the total number of buds in the social unit. In a social unit of Bell Miners studied over six years there were 6.3 unmated breeding-age buds per breeding vacancy in any month. The level of competitor pressure for breeding positions was more severe for males than females due to the skewed sex-ratio.