Sound level detection from within avian eggs – are acoustic signals amplified or attenuated at different frequencies for developing embryos?

Over the past decade, novel research has rapidly advanced our understanding of how nest occupants communicate with avian embryos. Beyond the nest, studies investigating environmental acoustics, such as anthropogenic noise, have also identified impacts on avian embryos, with potential health and developmental consequences. However, how the egg itself influences sounds – whether it amplifies, attenuates or distorts them – remains largely unknown. We conducted a pilot experiment to explore this by measuring external acoustics inside two eggs of three bird species: Southern Cassowary (Casuarius casuarius), Emu (Dromaius novaehollandiae) and Chicken (Gallus gallus domesticus). A frequency sweep at a known amplitude was played outside the egg to a hydrophone or microphone inside the egg, with and without albumen. Data were analysed as the difference in sound pressure recorded internally and externally across frequencies through the creation of a random forest model. While external sound pressures were comparable inside the eggs, depending on the frequency, some minimal amplification did occur. Species also appeared to influence sound pressure differences within the egg, although the sample size was small (two per species). Our findings provide initial validation for previous research on external acoustics in avian embryos, highlighting the importance of investigations in this area to further elucidate the acoustic environment of the avian egg.