Mallards belong to a cluster of closely related ducks that diverged recently and never evolved strong reproductive barriers. Members of this group can pair and produce fertile young, so where ranges overlap, gene flow follows.
The consequences are clearest on islands. In New Zealand, mallards introduced for hunting from the late nineteenth century now vastly outnumber the endemic grey duck, and genetic studies find hybrid ancestry through most of the surviving population. In Hawaii the endemic koloa maoli faces the same problem, with hybrid swarms on several islands and the least-introgressed birds concentrated on Kauai. Similar mixing is documented between mallards and American black ducks in eastern North America.
What remains contested is how much of this is genuinely a threat and how much reflects long-standing natural exchange. Black duck and mallard genomes are so similar that separating recent human-driven hybridisation from ancient shared variation is difficult, and researchers disagree on where the boundary lies.