<p>Human-facilitated introduction of invasive species is increasing rates of anthropogenic hybridization, further threatening endemic species. In fact, the endangered Hawaiian Duck (<i>Anas wyvilliana</i>; ‘Koloa Maoli’), which is the remaining endemic duck species on the main Hawaiian Islands, is now threatened by genetic extinction through ongoing hybridization with feral Mallards (<i>Anas platyrhynchos</i>). In addition to hybridization, the Koloa adult populations are known to be heavily male-biased which can further reduce population growth rates in threatened species. However, it is unknown if the sex-bias occurs at hatch, or is from a female bias in post-hatch mortality that could be managed. Towards this, we tested for bias in offspring sex ratios among Koloa, feral Mallards, and their hybrids and investigate their respective mitochondrial DNA (mtDNA) diversity. Despite the reported male-bias among adult Koloa populations, we did not recover a significant deviation in clutch sex ratios from 50:50. Although we also recovered no sex-ratio bias among feral Mallard × Koloa hybrid populations, feral Mallard eggs did show a significant male bias. Next, we recovered previously reported mtDNA haplotypes across the Islands and discovered areas outside of Kaua ‘i were dominated by Game-Farm Mallard haplotypes found around the world, which confirms these to be the source of current feral and hybrid populations. Moreover, we did not find significant change over time in the ratio of Game-Farm Mallards verses Koloa found at each location. Finally, we found that both species do perform inter and intra-specific brood parasitism, which could provide for further Mallard introgression. We conclude the male-biased sex ratio among adult Koloa are therefore driven by increased post-hatch female mortality, and that re-establishing Koloa outside of Kaua ‘i likely hinges on the ability to manage these feral Mallard and hybrid populations.</p>