Females Are More Aggressive Than Males towards Same- and Opposite-Sex Intruders in the Blue Tit (Cyanistes caeruleus)

SIMPLE SUMMARY: Animals use aggressive behaviour to gain access to resources such as food and mates, and protect their offspring. Individuals vary in the way they express aggression during a conflict. However, our understanding of female aggression is limited. In this study, we investigated male and female aggression towards opponents of the same and opposite sex in the blue tit (Cyanistes caeruleus). Aggressive encounters were simulated during the egg-laying phase of the female using blue tit taxidermy mounts. For each sex, we identified behaviours used during aggressive encounters. We compared the intensity of same- and opposite-sex aggression and examined if individual aggression was consistent in both situations. Finally, we compared the intensity of aggression between males and females. Our results show that females are the more aggressive sex, even though both sexes showed similar behaviours during conflicts. Female and male aggression was mostly unaffected by the sex of the intruder, although males sang more from a distance when encountering a male intruder. We conclude that females might have evolved to be more aggressive as they have to compete for nesting sites, and have invested more resources into reproduction. ABSTRACT: During the breeding season, aggression is expressed to gain access to resources such as territories and mates and protect offspring. Female aggressiveness has received much less attention than male aggressiveness, and few studies have examined female and male aggressiveness towards intruders of both sexes in the same species. We compared female and male aggressiveness towards same- and opposite-sex intruders during the egg-laying period in blue tits (Cyanistes caeruleus) using simulated territorial intrusions. For each sex, we examined the occurrence of different behavioural responses during agonistic encounters, and compared the intensity and individual consistency of intra- and inter-sexual aggression using same- and opposite-sex taxidermy mounts. Our results show that females are the more aggressive sex. Both sexes showed similar behaviours during simulated intrusions, although females were never observed singing and males never entered the nest box. In females, aggression was predominantly independent of the sex of the intruder, while males sang more from a distance during male–male encounters. The relative levels of aggression (pecking and perching on the mounts) during intra- and intersexual conflicts were consistent for females, but not for males. Females might be under stronger selection for aggressive phenotypes due to nest-hole competition and larger reproductive investments.