Abstract: To evaluate the offspring sex ratio ofMergus squamatusin Manjiang River, Changbai Mountain, and to understand their breeding strategy, population dynamics, and individual fitness, a total of 131 eggshell samples were collected from 13M. squamatusnests along the river from 2021 to 2024 using a non-invasive sampling method. Molecular biology techniques were employed for the sex determination of the offspring. Statistical analyses, including binomial distribution, Wilcoxon-Signed-Ranks test, and Kruskal-Wallis test, were conducted to investigate offspring sex ratio and the influence of clutch size on it. The results showed that the primary and secondary sex ratios for this species did not significantly differ from 0.5 （P>0.05）, whether at the population level or the nest level. This suggested that parental investment in male and female offspring ofM. squamatusmight be balanced along the Manjiang River in Changbai Mountain, which was consistent with Fisher’s theory predictions. Furthermore, the study revealed that clutch size did not exert a significant influence on the offspring sex ratio （P>0.05）, suggesting that parental modulation of offspring sex ratios through clutch size control might not be a prominent mechanism in this species. This study provided a database for future in-depth research in this area, aiming to support conservation efforts forM. squamatus.

Key words: Mergus squamatus, offspring sex ratio, clutch size, breeding strategy, conservation