Studies of species recognition in male birds have often identified features that are distinctive, invariant markers of a species song but which can be altered without altering male response1-4. Such results raise the question of why natural selection has favoured the retention of these features in the song. We propose that such features are favoured by selection because females use them in species recognition even though males do not. There are excellent theoretical grounds for expecting that males and females of the same species would differ in their means of species recognition, as required by this hypothesis. We report here that female red-winged blackbirds (Agelaius phoeniceus) discriminate against abnormal songs previously shown to be acceptable to males of this species. Similar sexual differences in species recognition may be expected in animals other than birds, and in recognition systems based on cues other than vocalizations.
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