today. Also, many avian hybrid zones are known to be stable and longstanding despite the occurrence of extensive interbreeding. The American black duck and the local mallard are now very hard to distinguish by means of microsatellite comparisons, even if many specimens are sampled. Contrary to this study's claims, the question of whether the American haplotype is an original mallard lineage is far from resolved. Their statement, "Northern black ducks are now no more distinct from mallards than their southern conspecifics" only holds true in regard to the molecular markers tested. As birds indistinguishable according to the set of microsatellite markers still can look different, there are other genetic differences that were simply not tested in the study. In captivity studies, it has been discovered that the hybrids follow Haldane's Rule, with hybrid females often dying before they reach sexual maturity, thereby supporting the case for the American black duck being a distinct species. Nest predators and hazards The apex nest predators of the American black duck include American crows, gulls and raccoons, especially in tree nests. Hawks and owls are also major predators of adults. Bullfrogs and snapping turtles eat many ducklings. Ducklings often catch diseases caused by protozoan blood parasites transmitted by bites of insects such as blackflies. They are also vulnerable to lead shot poisoning, known as plumbism, due to their bottom-foraging food habits. Status and conservation Since 1988, the American black duck has been rated as least concern on the IUCN Red List of Endangered Species. This is because the range of this species is extremely large, which is not near the threshold of vulnerable species. In addition, the total population is large, and, although it is declining, it is not declining fast enough to make the species vulnerable. It has long been valued as a game bird, being extremely wary and fast flying. Habitat loss due to drainage, filling of wetlands due to urbanization, global warming and rising sea levels are major reasons for the declining population. Some conservationists consider hybridization and competition with the mallard as an additional source of concern should this decline continue. Hybridization itself is not a major problem; natural selection makes sure that the best-adapted individuals have the most offspring. However, the reduced viability of female hybrids causes some broods to fail in the long run due to the death of the offspring before reproducing themselves. While this is not