Deciding to fight, regulating investment: hierarchical territorial defence in the Lanner falcon

Territorial defence is a key component of avian reproductive strategies, yet its regulation across breeding stages and competitive contexts remains poorly quantified in many raptors. In long‑lived species, territorial behaviour should balance immediate risk management against long‑term reproductive costs. Using a two‑step hurdle modelling approach that separates the decision to initiate defence from the level of investment once defence occurs, we investigated how breeding stage and competitor density shape territorial behaviour in the Lanner falcon (Falco biarmicus), a fragmented Mediterranean raptor facing multiple sympatric competitors. We addressed statistical challenges including complete separation in binary response models and overdispersion in count data through penalized likelihood methods and simulation‑based diagnostics. Across 155 behavioural observation sessions from 39 breeding attempts in eastern Sicily, we recorded agonistic responses to conspecific and heterospecific intruders during courtship, incubation and chick rearing. The probability of initiating defence was strongly shaped by competitor identity and density: Ravens (Corvus corax) consistently triggered aggressive responses, indicating a low tolerance threshold, whereas responses to Peregrine falcons (F. peregrinus) varied with breeding stage. Complete separation in standard logistic models required Firth’s penalized likelihood, which confirmed the strong positive effect of Raven density and revealed a significant decrease in attack probability during chick rearing. In contrast, defence intensity was largely independent of competitor density and was primarily regulated by reproductive timing, peaking during courtship and declining through incubation and chick rearing. Substantial overdispersion in the Poisson model for defence duration cautions interpretation. Territorial defence showed parental symmetry, with males and females contributing equally, and defensive investment was not associated with fledging success or secondary sex ratio. Overall, findings indicate a hierarchical, state‑dependent structure: external threats determine whether defence is initiated, while internal reproductive state constrains investment, suggesting territorial defence functions as flexible risk management rather than a direct driver of reproductive output.