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Simple Summary: Trophic niches, as one of the important dimensions of niche theory, show the nutritional requirements of species; they can reflect their position in the ecosystem trophic level, and functional status, as well as the nutritional relationships between species. Similarities in body structure and ecological needs may lead to interspecific nutritional competition, and species may undergo niche differentiation through different foraging strategies. The body tissues of consumers contain stable isotope signatures that reflect their dietary information, and stable isotope analysis has been widely used in animal diet research. In this study, stable isotope techniques were used to assess the dietary information and nutritional niches of Sitta yunnanensis and Sitta nagaensis coexisting in Yunnan Province. S. yunnanensis and S. nagaensis mainly preyed on six orders, including Orthoptera, with overlapping food resources but different dietary ratios. Their two niches were differentiated, with S. yunnanensis having wider trophic niches. This study provides new insights into the food resources and trophic niches of S. yunnanensis and S. nagaensis, and the mechanisms behind their coexistence were consistent with the niche hypothesis. Sympatric closely related species may experience interspecific trophic competition due to ecological similarity; they may isolate in terms of diet or habitat use as a strategy to avoid competition. The body tissues of consumers contain stable isotope signatures information that can be applied to infer their dietary information. In this study, δ13C and δ15N stable isotopes were analyzed to determine the dietary information and trophic niches of sympatric coexisting Sitta yunnanensis and Sitta nagaensis. The results showed that the food sources of S. yunnanensis and S. nagaensis were from six orders, including Orthoptera, and the cumulative contribution rate was 99.97%, with the two species eating similar diets but at different rates. The larger δ13C of S. yunnanensis indicates that it had a wider range of habitats for feeding, while the difference in δ15N values was not significant (p > 0.05), indicating that both species feed on similar nutrient levels. As determined by Bayesian ellipses, the isotopic niches of S. yunnanensis and S. nagaensis were differentiated; the isotopic niche width of S. yunnanensis is 2.69‰2, which was larger than that of S. nagaensis (0.73‰2), indicates that differentiation between the two species in diet or habitat use reduced competition. Trophic niche differentiation and differences in foraging proportions may be the principal resource allocation mechanisms behind S. yunnanensis and S. nagaensis coexistence. [ABSTRACT FROM AUTHOR]