Can a sexually dimorphic learned birdsong be used for male-female recognition?

In many temperate zone songbirds, singing is solely a behavior of males. Tn these species, the singer's sex is encoded in the act of singing. However, in some species both sexes sing. For songs of these species to be used for sex recognition, the acoustic structure of songs of the two sexes needs to be dimorphic, and the dimorphism needs to be identifiable by the birds themselves. I examined the ability of acoustic sex recognition in a species in which the two sexes produce very similar songs, Northern cardinals (Cardinalis cardinalis). A field playback experiment was conducted to examine whether cardinals can distinguish the sex of the singer based on hearing a song. The results showed that cardinals responded differently to the songs of the two sexes, suggesting that songs of cardinals are used for acoustic sex recognition despite their remarkable similarity.