Intrasexual and intersexual territorial aggressive responses are repeatable and correlated in blue tits of both sexes

Aggressive behaviours are commonly expressed in the competition for limited resources and influence the survival and reproduction of individuals. However, because of a predominant focus on male–male competition and aggression, the importance of female aggression remains much less understood. In this study, we quantified both female and male aggressiveness in breeding blue tits, Cyanistes caeruleus, in response to same- and opposite-sex territorial intruders. We asked whether behavioural responses to same-sex and opposite-sex intruders were different. Using taxidermic mounts and song playback, we simulated territorial intrusions by female or male conspecifics during the nest-building and egg-laying stages of breeding blue tit pairs. First, females showed overall stronger aggressive responses than males. Furthermore, females were similarly aggressive towards female and male intruders, while males were less aggressive towards females than males. These observations demonstrate a sex-specific territorial response. Second, we found repeatable variation in the aggressive responses of both females and males. Remarkably, aggressive responses towards mounts of different sexes were strongly positively correlated. This result suggests that the aggressive responses of individuals in intrasexual and intersexual conflicts reflect the same underlying behavioural trait. We discuss possible functional explanations for our findings. Our findings highlight the importance of female aggression in a biparental territorial species, warranting further investigations into the ecological costs and benefits of individual differences in aggressiveness in both females and males.