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Keywords: aggressionCardinalis cardinalis corticosterone female incubation maternal aggression nest defence northern cardinal testosterone Testosterone's (T) influence on male aggression has been well established in many vertebrate species, but the impact of T on female aggressive behaviour is poorly understood. Among birds, a link between T and female aggression is plausible, as females of many species exhibit a seasonal peak in T concentrations at the onset of breeding when social instability is greatest and they may have circulating T through much of the breeding season. However, investigations examining the relationship between T and female aggression are few and have yielded conflicting results, with experimentally or endogenously elevated T supporting aggressive behaviour in females of some species but not others, and T elevating with aggression at some points of the reproductive cycle but not others. We examined the relationship between endogenous levels of T and female aggression in the northern cardinal, Cardinalis cardinalis, a resident temperate species in which pairs exhibit prolonged territoriality and females have measurable levels of T year-round, including all stages of reproduction (incubation, nestling feeding, etc.). Using simulated nest intrusions, we assessed aggressive responses of incubating females to intrasexual 'intruders' at the nest and quantified T levels after each aggressive encounter. Displays of aggression towards 'intruders' varied among females; yet, individuals showing greater levels of aggression did not demonstrate higher levels of T. These results imply that T might not support maternal aggression in this species.

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