Assessing animal welfare in invasive bird management: evidence for reduced stress hormones with lethal shooting

Refinement of management actions for invasive species requires identifying methods that minimize physiological strain on target animals. Glucocorticoid concentrations are commonly used to assess the activation of the hypothalamic-pituitary-adrenal (HPA) axis in response to acute stress. Using monk parakeets (Myiopsitta monachus) as an avian invader model, we aimed to assess the HPA response of two common control methods: shooting and live trapping followed by confinement until euthanasia. Plasma corticosterone (CORT), the primary avian glucocorticoid, was measured in 63 parakeets assigned to one of three groups: (i) shooting (N = 7), (ii) baseline (live-trapped birds in which blood samples were taken <3 minutes after capture, N = 6) and (iii) live trapping-confinement (live-trapped birds in which blood samples were collected 10-210 minutes after capture, N = 50). Differences among treatments were analysed using a generalized linear model with a gamma error distribution, and CORT dynamics over time were evaluated with non-linear logistic and quadratic models. CORT levels in shot birds were similar to baseline, indicating that their death occurred before a systemic HPA axis response was initiated. In contrast, live-trapped-confined birds showed markedly elevated CORT levels. The logistic model best described the stress response, showing CORT rising with time after capture until stabilizing while the stressor persisted. These results have important implications for management design. Since shooting has been proved effective in population reduction and we found it is associated with reduced CORT levels compared to trapping, we recommend that practitioners prioritize this method when controlling monk parakeets. By incorporating these objective metrics, managers can offer transparent, evidence-based justifications for the selection of control techniques, potentially reducing social conflict and fostering greater public acceptance of managing protocols for invasive species. Efforts should be made to inform the public about how different bird control techniques vary in terms of the physiological impact they may cause.