Environmental factors and sexual differences in mass and condition of nestling pied flycatchers Ficedula hypoleuca

The mass and body condition of pied flycatchers aged 13 days were analyzed in relation to nest quality of the natal nestbox and abundance of ectoparas itic mites to determine whether conditions experienced during growth have d ifferential effects depending on sex. A higher fledgling mass was significa ntly related to a higher average mass at the adult stage in both sexes. App arently, male and female fledglings did not differ in mean mass or conditio n but this was due to significant interactions between fledgling sex and en vironmental factors which affected the expression of both traits. Male fled glings were heavier and in better condition than females in nestboxes of lo w quality and also when the nest had a high abundance of mites. Sex-related effects of stressful environments on fledgling mass and condition were opp osite to those previously reported for tarsus length in the same population . Differences in the susceptibility of traits to environmental stress are d iscussed. They might be due to differential growth allocation depending on sex, to selection pressures differing between the sexes, to compensatory ga ins in mass by males after tarsus growth has been almost completed, or beca use males have a greater competitive ability, especially under stressful co nditions promoting sibling competition.