Conspecific competition is thought to have a stronger selection effect on aggressive behaviour and its underlying physiology in birds than the heterospecific competition. We hypothesised that the hormone-behaviour interaction between testosterone and aggression could be influenced by severe heterospecific competition among secondary cavity nesters. Adult Eastern Bluebirds (n=34) were subjected to three challenge conditions that simulated territorial intrusions (STI) by heterospecific (House Sparrow) and conspecific (Bluebird) using live and stuffed decoys and their respective playback vocalisations. Late in the nesting cycle, Eastern Bluebirds responded to simulated territorial intrusions (STIs) by Conspecific and heterospecific. When compared to homospecific STI, the aggressive response elicited by a heterospecific STI was muted. Individuals with high aggression levels had slightly higher mean testosterone concentrations than those with low aggression levels. For all STI conditions, baseline T and aggression were positively correlated, but the relationship was strongest for the live heterospecific decoy. Furthermore, the live heterospecific decoy elicited a stronger aggressive response than the stuffed decoy, implying that other cues produced by the living bird, in addition to auditory and visual cues, influenced individual responses to a challenger. Because of the increased frequency of human-wildlife interactions, it is becoming increasingly important to understand how anthropogenic disturbance affects animal populations.
