Determinants of mating success in female tree swallows (Tachycineta bicolor)

Females from a wide variety of taxa display elaborate ornaments and aggressive behaviours that are similar to those expressed by males. Although recent empirical investigation has demonstrated that ornamental traits and behaviours of females may function by attracting mates or signalling competitive ability when competing against conspecifics for access to mates and resources, less is known about how such traits influence the mating success of free-living female birds. For my research, I experimentally examined how variation in plumage brightness and behaviour of female tree swallows (Tachycineta bicolor) influenced their mating success. Plumage brightness of females had no influence on investment in parental care or mating strategies of males, or the quality of social mate paired to the female. These results suggest that bright plumage is not a signal of attractiveness preferred by male tree swallows. In contrast, I report evidence that plumage brightness of female tree swallows is involved in agonistic interactions with conspecifics. Females whose plumage brightness was enhanced to signal high quality were less able to retain their nest site than females whose plumage brightness was reduced to signal low quality. This suggests that females displaying bright plumage may be challenged by conspecifics of high quality to test the quality signalled by bright plumage, and is supported by the finding that females displaying enhanced plumage brightness suffered social costs, such as delaying breeding and producing low-quality nestlings. Despite these costs, females in the enhanced plumage brightness treatment mated with extra-pair males that were higher quality than their social mate. Behaviour of females, manipulated by elevating testosterone (T), lowered the proportion of extra-pair offspring in the broods of T-treated females. Females manipulated so that the androgenic and estrogenic actions of T were blocked also produced fewer extra-pair... .