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Many hypotheses have been proposed to explain why in some species, males and females coordinate their behaviour to defend a territory. The sex of the individual initiating a joint display may shed light on the function of the display, but this variable has not been systematically explored. This study investigates whether male–female pairs in a socially monogamous cockatoo, the galah, respond differently to male-initiated and female-initiated joint threat signals, and whether one sex is more likely to initiate a threat response. Solo male defense calls, solo female calls, male-initiated paired calls, and female-initiated paired calls were presented to pairs of galahs at nest cavities during the pre-breeding season. Birds responded most strongly to female-initiated call bouts, regardless of the number of stimulus birds giving vocalizations. While paired birds coordinated their approach responses to the stimuli, males tended to initiate these responses. These results suggest that the sex of the initiating bird, rather than the number of calling birds, is most relevant to galahs when assessing threat near an active cavity. This study indicates that it is critical to consider the separate roles of the male and female in a joint display in order to fully understand that display's function. [ABSTRACT FROM AUTHOR]