Feathers in the spotless starling nests: a sexually selected trait?

The carrying of feathers to adorn the nest has been recently described as a female behaviour that indicates quality in a passerine bird — the spotless starling (Sturnus unicolor) —, but the consequences that the variability of this trait may have on breeding success are still unknown. The feather-carrying behaviour is a sexual behaviour that is performed in response to a male display: the carrying of green plants. In this paper we explore whether foreign feathers affect male investment on chick rearing or reinforce the pair mating bonds. The experimental addition of feathers to nests caused an increase in clutch size and a reduction of nestling mortality, although it did not affect feeding rates or the removal of faecal sacs by males or females. Nest feathers did not increase the frequency with which females laid a second clutch in the same nest and with the same male. Thus, our results do not support the sexual selection hypothesis for the evolution of the carrying feathers behaviour. An alternative hypothesis that deserves to be addressed in specifically designed studies is that foreign feathers may be taken to nests as a sort of chemical warfare against nest parasites. The sexual selection and the chemical defence hypotheses are, however, not mutually exclusive and future research should evaluate whether the pre-existence of a behavioural trait arising by natural selection may drive the evolution of a signalling trait.