Sound is arguably the external cue most accessible to embryos of many species, and as such may
constitute an unrivalled source of early information. Recent evidence shows that prenatal sounds,
similarly to maternal effects, may shape developmental trajectories. Establishing whether parental
vocalisations are signals directed at embryos, or parental cues on which embryos eavesdrop, can
elucidate whether parents or embryos control developmental outcomes. Prenatal exposure to a
characteristic heat-related parental call was recently shown to alter zebra finch growth and fitness.
Here, we test the ecological context of this behaviour in the wild, and assess the information value
and specificity of this vocalisation for an embryonic audience. We show that wild zebra finches also
produce this characteristic call, only at high temperatures. In addition, in the lab, we demonstrate
experimentally that calling is specifically triggered by high air temperatures, can occur without an
embryonic audience, and importantly, is predicted by individuals’ body mass. Overall, our findings
reveal a specialised heat vocalisation that enables embryonic eavesdropping, by indicating high
ambient temperatures, and parents’ capacity to cope with such conditions. This challenges the
traditional view of embryos as passive agents of their development, and opens exciting research
avenues on avian adaptation to extreme heat.