Communication depends on accurate reception of signals by receivers, and selection acts on signals to transmit efficiently through the environment. Although learnt signals, such as birdsong, vary in their transmission properties through different habitats, few studies have addressed the role of cultural selection in driving acoustic adaptation. Here, we present a test of the hypothesis that song-learning birds choose to copy songs that are less degraded by transmission through the environment, using swamp sparrows (Melospiza georgiana) as our study species. We found that all subjects discriminated between undegraded and naturally degraded song models, and learnt only from undegraded song models, demonstrating a role for cultural selection in acoustic adaptation of learnt signals.