Conspecific competition is assumed to exert stronger selection than heterospecific competition on the aggressive behavior and its underlying physiology in birds. We tested the hypothesis that hormone-behavior interaction between testosterone and aggression may also be shaped by severe heterospecific competition among secondary cavity nesters. Adult Eastern Bluebirds (n=34) were exposed to three challenge conditions that simulated territorial intrusions (STI) by heterospecific (House Sparrow)

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... conspecific using live and stuffed decoys with their respective playback vocalizations. Eastern Blue birds were responsive to simulated territorial intrusions (STIs) by Conspecific and heterospecific late in the nesting cycle. The aggressive response elicited from a heterospecific STI was muted compared to honspecific STI. Overall, highly aggressive individuals had slightly greater mean testosterone concentrations than individuals with low aggression scores. Baseline T and aggression were positively correlated for all STI conditions, but the relationship was strongest for the live heterospecific decoy. Furthermore, the live heterospecific decoy elicited a stronger aggressive response than the stuffed decoy suggesting that other cues produced by the living bird, in addition to the auditory and visual cues provided influenced individual responses to a challenger. We conclude: a. Conspecific competitors exert stronger selective pressure on the aggressive behavior than Heterospecific competitors. b. Stuffed decoys may not accurately reflect the relationship between hormones and behavior in an individual.