Social competition is ubiquitous in nature, but social environments are seldom static. Instead, they can change as conspecific challengers come and go. Social environments also shift over weeks and months as animals move through different life history stages that shape the relative costs and benefits of competition. Animals should respond to social competition with phenotypes that enhance their success, such as aggression. There is strong evidence in male vertebrates that aggression is mediated by the steroid hormone testosterone (T) over multiple timescales, but whether and how this applies to females is unclear. We have been systematically testing these ideas in female tree swallows (Tachycineta bicolor), an obligate secondary cavity-nesting songbird species in which aggression a) is beneficial for obtaining and defending nest sites, b) can interfere with parental care, and c) is at least partially mediated by T. In a series of studies, we showed that females rapidly respond with intense aggression to both experimentally-enhanced competition and simulated territorial intrusions. Population-average aggression decreases with date, and individuals who more plastically adjust aggression across breeding stages have higher fitness. Females exhibit greater T production capabilities during breeding stages with more frequent social challenges (territory-establishment) and reduced T levels during breeding stages with greater parental care demands (incubation, chick rearing). However, even during earlier breeding stages, females do not increase T levels following real or simulated contests. Together, these results shed new light on the relationship between T and female aggression across contexts and over varying timescales.