Click-evoked ABRs appear and gradually mature after hatching in zebra finches.

(A) ABR recording setup. (B) Used sound stimuli. Red, phase inverted signal. (C) Representative, 95 dB SPL click-evoked ABR ante-(black) and post-mortem (grey). (D) Representative ABRs to 95 dB SPL clicks at tested DPH timepoints. The first negative peak (I) and the following trough are designated as wave I. Noise RMS is determined from the red area and signal RMS from the blue area. (E) Percentage of animals that exhibited a detectable (S/N > 2) click-evoked ABR. (F) Detection thresholds were measured with a set of clicks with decreasing 5 dB SPL steps. Arrow indicates the determined threshold for this 8 DPH example. (G) Average click thresholds at different DPH ages. (H) Click-evoked ABR wave I amplitude and (I) latency over SPL. Age groups are color-coded in D-I. Shaded areas in H-I = s.e.m.

Sound frequency sensitivity matures over postnatal development.

(A) Representative ABRs to 1 kHz and (B) 8 kHz tone burst stimuli over postnatal development. (C) Zebra finch ABR thresholds obtained with tone bursts (solid lines) and with tone pips (dashed line).) Shaded areas = s.e.m.

Intense sound stimulation induces zebra finch eggs to vibrate at very low velocities.

A Setup used to measure sound induced vibrations of eggs. A 94 dB, 0.25-to-10 kHz frequency sweep was played at the eggs to determine the vibration transfer function. B Egg vibration velocities at frequencies associated with song (left) and with heat whistles (right). Error bars = s.d. C Example vibration velocity transfer function of a zebra finch egg (mean ± S.D. (grey area) N=5) Green and red areas display the frequency ranges of song (Elie and Theunissen 2016) and heat whistles (Anttonen et al, 2025), respectively, that were used to calculate mean velocity in panel B.

Heat whistles are below hearing and vibration threshold levels in hatchlings and adults.

A In vivo heat whistles at 10 cm distance (Anttonen et al., 2025) (orange) are just on or below the adult hearing threshold based on behavioral audiograms (blue line; Yeh et al. 2023; green line, Okanoya and Dooling, 1987). Increasing the most sensitive adult audiogram by 54 dB as a conservative estimate of reduction based in clicks (Fig 1G), shows that both in vivo heat whistles and previously playback levels (Katsis et al., 2018) lay far below the estimated thresholds for a 2DPH old hatchling, even without an expected further increased high frequency threshold (horizontal arrow). Thresholds for embyos still in the egg are likely be shifted further up, and sound dampening by parents incubating the egg will reduce the heat whistle levels further down, making the gap even larger. B Egg vibration velocities caused by 67 dB song, 65 dB heat whistle playbacks (Katsis et al., 2018), and in vivo heat whistles (Anttonen et al., 2025) are far below with thresholds for human (Håkansson et al., 1985) and mouse bone conduction (BC) hearing (Chhan et al., 2017) and for human vibrotactile threshold at 0.2 kHz (Mountcastle et al., 1972).

ABR and behavior-based audiograms in adult zebra finches are consistent in sensitivity and shape.

The ABR based audiograms from four different laboratories including our pip-data are consistent in overall sensitivity and shape. The distinct low threshold at 6 kHz in the data by Amin et al. (2007) may reflect real biological variation between laboratory populations. The behavioral-based audiograms (Okanoya and Dooling, 1987; Yeh et al., 2023) are also consistent in shape and thresholds. The systematically lower threshold of the behavior-based audiograms varies from about 20 dB (downward arrows) in the 0.25 – 4 kHz range to 20-40 dB at 6 kHz.