SURVIVAL RATE OF ADULT BARN SWALLOWS HIRUNDO RUSTICA IN RELATION TO SEXUAL SELECTION AND REPRODUCTION

Abstract

Sexual selection often affects individuals of the two sexes differently by imposing severe survival costs on individuals of the chosen sex, usually males. These costs may be energetic, physiological, mechanical, or ecological, and they can give rise to sex differences in survival. We studied survival rates of male and female adult Barn Swallows Hirundo rustica in a Danish population during 1984–1998, for the first time using capture–mark–recapture techniques to estimate the consequences of sexual selection for sex differences in survival. Males had slightly, but not significantly, higher survival than females. Females had annual survival estimates that were significantly more variable than those for males. Male annual survival rates were not positively correlated with those of females. Hence, different factors were hypothesized to affect male and female survival rate. Consistent with this prediction we found that annual male survival decreased with increasing annual mean length of the outermost tail feathers, which is a secondary sexual character subject to intense directional sexual selection. Heavier females were more likely to survive. Furthermore, females with long tails survived better than short-tailed females. Since tail length in females is an indicator of female condition, both body mass and tail length relationships suggest that females survive better in years when they are in superior condition. Thus, sexual selection as reflected by differences in tail length of males and females contributes in different ways to survival of male and female Barn Swallows.