Abstract: Brood amalgamation is the phenomenon that the adults merge non-filial young into their family group and provide parental care to them. We examined the brood amalgamation in the Scaly-sided Merganser (Mergus squamatus) in Changbai Mountains, China. During the four breeding seasons in 2017-2020, there was （0.65±0.17） nests·km^-1 (n=4) and （0.39±0.05） broods·km^-1 (n=4) along Manjiang River in Changbai Mountains, with an averaged brood size of （8.5±2.8） (n=37). The occurrence of brood amalgamation was 26.5%±6.8%, with no interannual variation. The brood size did not significantly differ between broods with amalgamation occurrence (8.3±2.3) and other broods (8.5±3.0), but significantly increased after brood amalgamation (18.3±8.3). Brood amalgamation of Scaly-sided Merganser occurred in foraging sites or resting grounds, mostly (83.3%) before the ducklings were in 30 days of age, due to competition between adults or accident mixture of chicks. Brood amalgamation of Scaly-sided Merganser probably resulted from competition for chick-rearing territory by adults due to high nestling density, and lack of efficient care for chicks by single parent. We suggest using color marking and molecular techniques to further understand the mechanism of brood amalgamation in Scaly-sided Merganser and assess the effects of fitness losses and gains of the adults that donate and receive chicks.

Key words: Scaly-sided Merganser (Mergus squamatus), brood amalgamation, Manjiang River of Changbai Mountains, fitness.