Robust behavioral effects of song playback in the absence of testosterone or corticosterone release

► We asked if song playback affects plasma testosterone (T) or corticosterone (CORT). ► Playback elicited strong aggressive responses, but did not elevate plasma T or CORT. ► Lack of T or CORT elevation found in 2 populations varying in breeding season length ► Two song classes (high- and low-amplitude) did not differentially affect hormones.