When subspecies matter: resident Superb Fairy-wrens (Malurus cyaneus) distinguish the sex and subspecies of intruding birds

The widely accepted functions of complex bird song - to defend a territory or attract a mate, or both - have generally been tested in northern hemisphere species in which males produce the song and females choose the singer. In our study species, the Superb Fairy-wren (Malurus cyaneus), both males and females sing a solo song throughout the year. We compare the chatter song in males and females of two genetically distinct subspecies, and test if resident birds respond to the sex and subspecies of the intruder song. Compared with island birds (M. c. ashbyi), mainland Superb Fairy-wrens (M. c. leggei) produced songs with lower frequency and fewer elements. Compared with females, males produced longer songs with more elements. Resident birds showed acoustical discrimination for the sex and subspecies of the intruder bird. The response of resident pairs was positively correlated, but each sex showed a solo response. Resident males were the first to respond to male intruders, and resident females were the first to respond to female intruders. Fairy-wrens had the strongest response towards (1) intruders of the same subspecies and (2) male intruders. The finding of signal divergence and acoustical discrimination in males and females makes this a model system to test the mechanism of reproductive isolation when both sexes sing.