Exogenous testosterone in female spotless starlings reduces their rate of extrapair offspring

The effects of exogenous testosterone on the breeding biology of avian females are largely unknown. One unanswered question is whether testosterone has a role in female mating strategies affecting, for example, the rate of extrapair offspring (EPO) obtained. We experimentally increased circulating testosterone in females of a free-ranging population of the spotless starling, Sturnus unicolor, and measured rates of mixed paternity in their broods. Females treated with exogenous testosterone had a significantly reduced percentage of extrapair offspring in relation to controls and this difference persisted in the years after treatment. In addition, female age was negatively, and the number of nestlings was positively, related to EPO. To our knowledge, this is the first study on females in which EPO has been analysed in relation to an experimental testosterone manipulation. We suggest that females treated with testosterone are less attractive to males seeking extrapair mates and that there are delayed effects of testosterone on female mating that persist for years.