<p>Brood parasitism is a significant reproductive strategy among birds. Both the Scaly-sided Merganser (<i>Mergus squamatus</i>) and the Mandarin Duck (<i>Aix galericulata</i>) are tree-cavity-nesting waterbirds with overlapping ecological niches. Although field observations have previously suggested that Mandarin Ducks may parasitize the nests of Scaly-sided Mergansers, direct evidence of successful hatching of parasitic eggs has been lacking. During the 2024 breeding season in the Manjiang region of Changbai Mountain, Jilin Province, field monitoring documented a case where a Mandarin Duck laid an egg in an artificial nest box intended for Scaly-sided Mergansers. Using eggshell membranes collected after hatching, we performed species identification for all ducklings based on the mitochondrial 12S rRNA gene as a genetic marker. Combined with morphological observations, we confirmed that one duckling was a Mandarin Duck, while the rest were Scaly-sided Mergansers. Analyses of the mitochondrial D-loop region and eight microsatellite loci indicated that all Scaly-sided Merganser ducklings were full-sibling offspring from the same parental pair. This supports the genetic monogamy in this nest and the absence of intraspecific brood parasitism in this instance. Our study provides molecular-level evidence for a case of successful interspecific brood parasitism by a Mandarin Duck on a Scaly-sided Merganser, enriching our understanding of interspecific interactions among tree-cavity-nesting birds. The findings also offer insights for the conservation of endangered species like the Scaly-sided Merganser. Furthermore, we identified a sequence discrepancy in the Scaly-sided Merganser’s 12S rRNA compared to existing reference sequences, suggesting a need for re-sequencing its mitochondrial genome.</p>