To avoid ending up with a less committed mate, some female birds listen for red flags in their suitor’s song. Male pied flycatchers change their song after mating, and females use these song features to tell if a male is already paired, researchers report July 1 in Ethology. The results are consistent with earlier research showing that most female pied flycatchers prefer to settle with an unmated male. To test whether females can hear the difference, the team played recorded songs from both single and mated males in large outdoor aviaries. But that preference disappeared when researchers doctored the bachelors’ songs to resemble that of a mated male.