Hatching failure is a common problem for endangered birds, and kākāpō (Strigops habroptilus), are a particularly clear example. In these critically endangered flightless parrots around 61% of eggs fail to hatch, with 73% of these showing no sign of development. The low hatching success of kākāpō was previously attributed to fertilization failure, but undeveloped eggs can also result from embryo death during early stages of development. As the underlying causes of fertilization failure and embryo death differ, identifying the relative importance of these effects in driving hatching failure has implications for effective conservation management. The entire remaining population of 200 kākāpō is managed by the New Zealand Department of Conservation on several predator-free offshore islands. During the 2019 breeding season we preserved the yolks of undeveloped kākāpō eggs, then investigated their fertilization status using fluorescence microscopy. We found that the majority (72%, n=124) of undeveloped kākāpō eggs were fertilised, suggesting early embryo death is a much more important contributor to hatching failure than previously realized. We also found positive effects of artificial insemination on the numbers of sperm reaching the egg. We are continuing our investigation into kākāpō hatching failure during the 2022 breeding season, with the aim of building a more complete picture of how multiple mating, maternal traits, and other factors combine to explain fertilization failure and embryo death in kākāpō and other threatened bird species.