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The Yunnan nuthatch ( Sitta yunnanensis) is a bird species in the nuthatch family Sittidae. It was first described by William Robert Ogilvie-Grant in 1900 based on a male holotype. It is a small nuthatch, measuring in length on average and weighs between . The grey-blue upperparts contrast with the light, smooth, buffy lowerparts. It has a fine white eyebrow above a black eyestripe, which is distinct when the plumage is fresh, and exhibits a small degree of sexual dimorphism. A noisy bird, it produces simple, nasal sounds, sometimes in repetitive series.
Its ecology is not well-known, but it feeds on insects that it finds on pine branches. It can be found in the prefectures of Yunnan, Sichuan, and Guizhou in Southwest China, where it prefers sparsely forested pine forests and avoids dense spruce and fir forests. It is generally rare but can be locally common. It has a small range of . A 2009 study predicted that its population could decrease by 43.6–47.7% between 2040 and 2069.
The canadensis group, which is also sometimes called the subgenus Micrositta, contains six species – the Yunnan nuthatch, the red-breasted nuthatch ( S. canadensis), the Chinese nuthatch ( S. villosa), the Corsican nuthatch ( S. whiteheadi), Krüper's nuthatch ( S. krueperi) and the Algerian nuthatch ( S. ledanti). In 1953, the Dutch ornithologists Karel Voous and John G. van Marle thought that the Yunnan nuthatch formed a link between the canadensis and europaea groups, and simultaneously that it was very closely related to the canadensis group, of which it was a basal representative in terms of distribution and plumage. In 1957, Charles Vaurie postulated that the white-tailed nuthatch was the Yunnan nuthatch's closest relative. In 1998, Eric Pasquet studied the mitochondrial DNA about ten species of nuthatch, including the different species in the canadensis group, but the Yunnan nuthatch was not included in the study. In 2014, Éric Pasquet and colleagues published a Nuclear DNA and mitochondrial DNA-based phylogeny of 21 species of nuthatch and confirmed the 1998 study relationships within the canadensis group, adding the Yunnan nuthatch, which is found to be the most basal species of the group.
The species exhibits little sexual dimorphism, but the black of the female's eyestripe is on average less intense and its underparts are duller and grayer. The plumage is fresh from August, and gradually wears down until the following spring. In worn plumage, the whitish tips of the eyebrow feathers are worn away, and the eyebrow line becomes discontinuous or inconspicuous. The upperparts are duller, less blue. The tail and wing feathers also wear out, but the blue-gray tips of the rectrices persist at least until May. The underparts become duller and dirtier, turning grayish-white. The young is duller than the adult, with the eyebrow less marked and not extending to the forehead, sometimes almost absent and consisting of a lighter cap margin. The eye line is also less wide, the cheek is dirty gray and not white. The throat is whiter, but the rest of the underparts are duller, grayish cinnamon, though not as pale as in a worn plumaged adult. The upperparts are duller and grayer. The bill of young is shorter and has a pale base. Adults undergo a complete moulting from July to September, and perhaps sometimes a partial moult before the breeding season in January and February, which includes the breast.
The giant nuthatch can occur in the same type of habitat as the Yunnan nuthatch, but is much larger in size and lacks a white eyebrow. The chestnut-vented nuthatch is closer in size to the Yunnan nuthatch, but has reddish flanks and no white eyebrows.
The Yunnan nuthatch is a sedentism species. It inhabits pine forests with sparse undergrowth, and avoids dense spruce and fir forests. Occasionally, it occurs on small pine trees high, in open forests or among scattered groups of trees. In the summer, it lives at altitudes between above sea level (ASL), and travels down into valleys during winter to heights up to . However, it has been observed at between between November and January in Shuangbai County.
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