Progressive postnatal hearing development limits early parent-offspring vocal communication in the zebra fnnch

Acoustic communication relies critically on the receiver’s ability to hear. In precocial birds, hearing can already be functional during embryonic stages in the egg, whereas in altricial species the limited available data indicate a gradual postnatal development with poor sensitivity in early hatchlings, implying even lower sensitivity in embryos. Recent research suggests that zebra fnnch embryos, despite being altricial, have functional hearing already in the egg to engage in parent-embryo acoustic communication and anthropogenic noise detection. However, their auditory sensitivity during early development remains unknown. Here, we measure auditory brainstem responses (ABR) over early postnatal development and show that zebra fnnch hatchlings have >54 dB lower threshold than adults to loud, broadband clicks. ABR responses emerge between 4-8 days after hatching. Auditory sensitivity develops progressively and reaches adult levels by day 20. Additionally, egg vibrations induced by sound remain far below detection thresholds of vibrotactile senses. Together these data contradict the notion of acoustic perception in embryos and early parent-to-embryo communication in zebra fnnches. The striking timing coincidence between maturation of the peripheral auditory system and the onset of song learning suggests that hearing functionality may gate the onset of vocal learning in zebra fnnches.