Simple SummaryThe Spectacled Fulvetta (Fulvetta ruficapilla sordidior) is an endemic bird species of the mountains in southwest China. The breeding biology of this species is largely unknown. In order to describe the breeding biology of this species, this study was conducted for two years on Humashan Mountain, a hill mainly covered with secondary forest located at the eastern marginal of Kunming, the capital city of Yunnan Province in Southwest China. A total of 16 nests were found, mainly located in dense shrubs. The breeding season is from March to June, with its peak period from April to May. The clutch size is 2–3, most of which are 3 eggs. The incubation period is 13–14 days, and the nesting period is 12–14 days. Both parents participated in the breeding process. We found a total of 16 nests, of which only 11 were active; 5 were successfully fledged with at least one (45.45%). This study is the first report on the breeding biology of the Spectacled Fulvetta, which has scientific value for the understanding of this species and other relative species in theFulvettagenus.AbstractThe Spectacled Fulvetta (Fulvetta ruficapilla sordidior) is an endemic bird species to the southwest mountains of China, distributing from 1250 to 2500 m in the widespread broadleaved evergreen forest and occasionally in secondary scrubs. The present study describes its breeding biology for the first time. Fieldwork was conducted in the springs of 2017 and 2018 on Humashan Mountain, a hill mainly covered by secondary forest located at the eastern marginal of Kunming, the capital of Yunnan Province in Southwest China. This bird was found to initiate egg-laying mainly in March, and most nestlings fledged in late April and May. There were 16 nests found in total, which were located mainly in the dense shrubs at a height of 0.99 ± 0.40 m (n= 15). Of the 11 active nests, clutch size averaged 2.73 ± 0.45 (n= 11). Focal observations were made on nests; the incubation lasted for 13.67 ± 0.47 days (n= 3) with a notably high nest attendance, i.e., eggs were incubated 84.23% of the observation time. Nestlings fledged at 13.00 ± 0.71 days (n= 4), and parents feeding frequency increased as the nestlings grew. Overall, the cumulative hatching and fledgling rates were 71.43% and 35.71%, resulting in a nesting success rate of 45.45%.