Early fledging mortality and the timing of juvenile dispersal in the marsch tit Parus palustris

Family flocks of Marsh Tits Parus palustris in southern Sweden kept together until 11-15 d after fledging and stayed within the former territory of the parents. Mortality in the family flocks was low in one year (1.2-3.6%) but higher in another (18.6%). This level of mortality is compared with post-dispersal mortality in other species of tits. The occurrence of a parent-offspring conflict with regard to when juveniles should disperse is called in question. The level of aggression from parents towards their young was very low. Experimental broods of increased and reduced size stayed equally long within the parental territory, thus refuting the hypothesis that parents force their young to emigrate to avoid local competition for food or the harassment from the begging young. Dispersal of young from a family flock took place over more than one day. Late dis- persers were significantly smalller than their nestmates. This supports the hypothesis that dominant individuals disperse first, while subdominants stay longer in the safety of the parental territory to increase their self-feeding ability,