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

Abstract

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 ectoparasitic mites to determine whether conditions experienced during growth have differential effects depending on sex. A higher fledgling mass was significantly related to a higher average mass at the adult stage in both sexes. Apparently, male and female fledglings did not differ in mean mass or condition but this was due to significant interactions between fledgling sex and environmental factors which affected the expression of both traits. Male fledglings were heavier and in better condition than females in nestboxes of low quality and also when the nest had a high abundance of mites. Sex-related effects of stressful environments on fledgling mass and condition were opposite to those previously reported for tarsus length in the same population. Differences in the susceptibility of traits to environmental stress are discussed. They might be due to differential growth allocation depending on sex, to selection pressures differing between the sexes, to compensatory gains in mass by males after tarsus growth has been almost completed, or because males have a greater competitive ability, especially under stressful conditions promoting sibling competition.