Defending as a unit: sex- and context-specific territorial defence in a duetting bird

Behaviours such as territorial defence represent functionally integrated traits that underlie multiple behavioural variables such as physical and acoustic responses. Characterizing the multivariate structure of such traits is fundamental to understand their evolution. In bird species that form stable pair bonds and are territorial year-round, both sexes are expected to defend their territory; however, the role that each sex plays in defending their shared territory remains largely unknown. Evidence for the sex-roles during territorial defence is mixed and sex- and context-specific characterizations of territorial defence embracing the multivariate nature of the trait are currently lacking. Here we investigated sex- and context-specific variation in a hypothesised latent variable called "territorial defence" and tested whether duets were part of territorial defence in a wild population of rufous hornero (Furnarius rufus). To do so, we combined a simulated territorial intrusion approach during nest building and provisioning contexts with a structural equation modelling approach. Our results showed that, in males and females, the six measured behavioural variables were linked by a single latent trait, territorial defence, in both contexts. Flights over the decoy and duet songs were equally good proxies of territorial defence. Although males were defending more the territory than females, pair members showed a positive correlation in their behaviour. The structural equation modelling framework enabled us to capture a complex correlation pattern among behavioural variables, expanding upon a classic body of research on territorial defence. Thus, the combination of classical behavioural approaches with sophisticated statistical analyses brings new exciting possibilities to the field of behavioural ecology. Significance statementTerritorial defence is a key behaviour in territorial species as it plays a major role in an individuals reproductive success and survival. Additionally, territorial defence has been proposed as one possible evolutionary driver of duetting behaviour, one of the most fascinating vocal behaviours in birds. As behaviours are evolutionary characters, they must be studied in a multivariate framework. In this study we focused on characterizing territorial defence during a simulated territorial intrusion in an integrative manner using a classical territorial intrusion framework. We did so in male and female rufous horneros (Aves: Furnaridae) across two breeding contexts, while simultaneously testing theoretical predictions about the role of duetting behaviour as key part of territorial defence. Overall, our study provides for the first time a sex- and context-comparison of the multivariate, latent variable "territorial defence" in duetting birds, while highlighting the potential of combining field behavioural approaches with structural equation modelling.