Simulated territorial intrusions (STIs) represent a commonly used experimental manipulation to test behavioural and hormonal responses of birds towards conspecific intruders. They are typically either conducted with live birds in a cage or with stuffed decoys. To our knowledge, nobody has tested whether these two different kinds of stimuli elicit the same kind of behavioural and hormonal response. We compared the reactions of European robins to STIs with stuffed and live decoys to see whether these stimuli are perceived in similar ways. We conducted STIs by placing a stuffed or a live decoy in a territory, played-back robin song and recorded the behaviour for at least 10 min. Then, the focal bird was caught, and a blood sample was taken to measure hormone concentrations. Males challenged with a stuffed decoy responded with more threats and movements around the decoy than males that were exposed to a live decoy. Furthermore, males challenged with a stuffed decoy had significantly higher corticosterone levels than males challenged with a live decoy. Androgen levels did not differ between treatments. The differential behavioural and corticosterone response of robins to stuffed and live
decoys suggests that robins may perceive stuffed decoys as more threatening than live decoys. Future investigations using STI experiments should be aware of the potential impact different kinds of decoys may have on the behavioural and hormonal response of birds during STIs.