<p>Avian brood parasites have developed shorter incubation periods than their hosts, likely as an adaptation to their parasitic reproductive strategy. Increasing evidence suggests that the rapid development of parasitic eggs is a highly energy-intensive process. However, little attention has been directed towards examining the potential energy trade-offs and distribution among the different physiological mechanisms involved in embryonic development. The aim of this study was to compare the developmental differences in embryonic heart rate between parasitic and host eggs, focusing on the role of embryonic heart rate in the adaptation of parasitic egg development. Using artificial incubation control and an embryonic heart rate monitor, we measured the heart rates throughout the incubation period of the plaintive cuckoo (<i>Cacomantis merulinus</i>) and its four sympatric hosts. The results revealed that, although the embryonic heart rate of parasitic eggs did not appear earlier than that of host eggs, they nevertheless developed over a shorter incubation period. Although the embryonic heart rate increased over the course of incubation, the average daily embryonic heart rate in parasitic eggs was lower compared to that of the hosts throughout the incubation stage (except on day 4). Our findings suggest that the lower embryonic heart rate in parasitic eggs may be associated with an energy trade-off between different developmental processes in the rapidly developing embryo. This study highlights the important role of a low embryonic heart rate in facilitating the rapid development of parasitic eggs and proposes that this adaptation merits further verification in conjunction with the embryonic morphogenesis process.</p>