Experimentally induced sex differences in parental care: an effect of certainty of paternity?

Abstract

Abstract. In biparental mating systems males are predicted to adjust their parental care in relation to their certainty of paternity. In this study, certainty of paternity in the European starling, Sturnus vulgaris, was manipulated by temporarily placing fertile females in cages next to decoy males within sight of their mates. Manipulations 2 weeks prior to egg laying provided a control, whilst watches on unmanipulated monogamous and polygynous pairs provided further comparison. Males did not respond to the manipulation by intensifying their mate guarding, but they did increase their song rates, which possibly reflected attempts to attract alternative females. No differences were observed between groups in any measures of mate guarding following the manipulation. There were significant sex differences in chick feeding rates within experimental pairs compared with controls owing to a reduction in male visits. Total visit rates and chick weights did not differ between experimental and control pairs. Visit rates in control pairs matched those of unmanipulated monogamous pairs, whereas those in experimental pairs did not differ from polygynous pairs. Low male visit rates were accompanied by a higher frequency of non-foraging activity around the colony, which was itself positively related to male song rates during the manipulations. It is suggested that this reflected a trade-off for males between chick feeding and time spent on additional mating opportunities, which would have been dependent on male 'quality' as indicated by song rate. Such a trade-off may have been crucial in producing the sex differences in chick feeding caused by the manipulation.