Birds can behave aggressively towards conspecifics to defend a territory and to compete for access to food and mates. The effects of phenotypes, such as sex and age, in aggressive behaviour have not yet been fully understood. Moreover, differences in aggression levels can reflect alternative behavioural strategies (i.e. proactive–reactive axis). We aimed to understand the relation between aggressive behaviour and different phenotypes (i.e. age, sex, body size) and between behaviours present in the proactive–reactive axis (i.e. aggression, exploration and fear). We studied the behaviour of a highly territorial species that shows sexual dimorphism, the Sardinian warbler (Sylvia melanocephala), using four behavioural tests: mirror, exploration, predation and tonic immobility. The tests measured respectively aggression, exploration and fear (the last two). Our results showed that the studied behaviours were not related to each other, suggesting an absence of the proactive–reactive axis. Only body size was found to predict aggressive behaviour, which might signal fighting ability in this species. Males and females did not differ in their aggression levels, suggesting that both sexes might play a role in territory defence. These results may reflect the importance of territorial defence in both sexes for species evolution.

鸟类可以对种属采取激进的行动，以保卫领土并竞争获得食物和伴侣的机会。表型，如性别和年龄，对攻击行为的影响尚未完全了解。此外，攻击水平的差异可以反映其他的行为策略（即主动-主动轴）。我们旨在了解攻击性行为与不同表型（即年龄，性别，身体大小）之间以及主动-反应轴上存在的行为（即攻击，探索和恐惧）之间的关系。我们研究了显示性二态性的高度领土物种的行为，撒丁岛鸣鸟（Sylvia melanocephala），使用四种行为测试：镜子，探索，掠食和补品固定。这些测试分别测量了攻击，探索和恐惧（最后两个）。我们的结果表明，所研究的行为彼此不相关，表明不存在主动反应轴。发现只有身体大小可以预测攻击行为，这可能表明该物种的战斗力。男性和女性的攻击程度没有差异，这表明性别可能在领土防御中发挥作用。这些结果可能反映出领土防御对于物种进化的重要性。