Untangling behaviours: independent expressions of female–female aggression and snake-like hissing in the blue tit (Cyanistes caeruleus)

Abstract

Abstract Aggression plays a crucial role in deterring predators and securing resources to promote fitness. Nevertheless, studies focussing on female aggression remain scarce. In songbirds, aggression is prevalent during the breeding season, when same-sex individuals compete for limited resources. Additionally, females of some bird species exhibit snake-like hissing behaviour during incubation presumably to lower predation rates and improve fitness. Such behaviours may co-vary, forming a behavioural syndrome that could constrain trait expression. Here, we investigated a resident population of blue tits (Cyanistes caeruleus), to examine the repeatability and covariation of female–female aggression and hissing behaviour, aiming to determine if these constitute a behavioural syndrome. We quantified female–female aggression during simulated territorial intrusions and measured number of hissing calls in response to a simulated predator intrusion into the nest box. We found that both female–female aggression and hissing behaviour were repeatable traits, and that older females approached the intruder less. However, we found no evidence of covariation between female–female aggression and hissing behaviour. Thus, our findings suggest that female–female aggression and hissing behaviour, although both displayed in a nest defence context, are evolutionarily independent traits in the blue tit.