Vocal signals mediate much of human and non-human communication. Key performance traits - such as repertoire size, speed and accuracy of delivery - affect communication efficacy in fitness-decisive contexts such as mate choice and resource competition 1 . Specialized fast vocal muscles 2,3 are central to accurate sound production 4 , but it is unknown whether vocal, like limb muscles 5,6 , need exercise to gain and maintain peak performance 7,8 . Here, we show that for song development in juvenile songbirds, the closest analogue to human speech acquisition 9 , regular vocal muscle exercise is crucial to achieve adult peak muscle performance. Furthermore, adult vocal muscle performance reduces within two days of abolishing exercise, leading to downregulation of critical proteins transforming fast to slower muscle fibre types. Daily vocal exercise is thus required to both gain and maintain peak vocal muscle performance, and if absent changes vocal output. We show that conspecifics can detect these acoustic changes and females prefer the song of exercised males. Song thus contains information on recent exercise status of the sender. Daily investment in vocal exercise to maintain peak performance is an unrecognized cost of singing and could explain why many birds sing daily even under adverse conditions 10 . Because neural regulation of syringeal and laryngeal muscle plasticity is equivalent, vocal output may reflect recent exercise status in all vocalizing vertebrates.