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The blue nuthatch ( Sitta azurea) is a bird species in the nuthatch family Sittidae. It is a medium-sized nuthatch, measuring in length. The species, which shows slight sexual dimorphism, has dramatic plumage pattern unlike any other member of its genus. Its head is black or blackish-blue dark blue close to purple with azure feathers. The wings are edged with black. The throat and chest are white or a washed buff, contrasting with the upperparts and the belly of a very dark blue; the covert feathers are generally clear, blue-grey or purplish.
The blue nuthatch's ecology is poorly known, but it feeds on small invertebrates found on trees; reproduction takes place from April to June or July. It also forages in mixed-species flocks in larger groups. They can be found in the Malay Peninsula and on the islands of Sumatra and Java in Indonesia, where it inhabits subtropical or tropical moist lowland forest and subtropical or tropical moist montane forest above in altitude.
Three subspecies are accepted: S. a. expectata, S. a. nigriventer and S. a. azurea, which differ chiefly in the colour of their , chests and bellies. The species' closest relatives are the velvet-fronted nuthatch ( S. frontalis), the yellow-billed nuthatch ( S. solangiae) and the sulphur-billed nuthatch ( S. oenochlamys). The population of the species has not been determined but the species appears to be at low risk of extinction because of the extent of its distribution. It has been classified as of least concern by the International Union for Conservation of Nature.
The name "nuthatch", first recorded in 1350, is derived from "nut" and a word probably related to "hack", since these birds hack at nuts they have wedged into crevices.
The nuthatches constitute a genus, Sitta, of small passerine birds in the family Sittidae, typified by short, compressed wings and short, square tails with 12 feathers, a compact body, longish pointed , strong toes with long claws, and behaviourally, by their unique head-first manner of descending tree trunks. Most nuthatches have blue-grey upperparts and a black eyestripe. In 2006, the ornithologist Edward C. Dickinson proposed splitting Sitta in multiple genera on the basis of distinct morphological traits. He suggested as candidates the velvet-fronted nuthatch ( Sitta frontalis) and the blue nuthatch, the morphology of which he describes as "rather aberrant ... in spite of a character trait (white edges to wing feathers) shared with Sitta formosa", and that doing so might, in turn, require the beautiful nuthatch ( S. formosa) to be split off as well. He stated, however, that a molecular study would be warranted prior to any re-classification.
In 2014, Eric Pasquet and colleagues published a phylogeny based on examination of Nuclear DNA and mitochondrial DNA of 21 nuthatch species. The position of the blue nuthatch within the genus was not established with certainty, having a far lower correlation than many others in the model. Nevertheless, under the findings the species appears best represented by a clade comprising the velvet-fronted nuthatch and the sulphur-billed nuthatch S. oenochlamys (and presumably also the very similar yellow-billed nuthatch S. solangiae, which remains untested). These tropical Asian nuthatches are themselves a sister group clade to one comprising the subgenus Sitta ( Micrositta) (sometimes called the S. canadensis group), along with the brown-headed nuthatch ( S. pusilla) and the pygmy nuthatch ( S. pygmaea). These results were confirmed by a further study in 2020 by Martin Päckert and colleagues.
The three subspecies vary in the colour of their mantle, chest and belly. The upperparts are dark blue at the mantle or purplish in some subspecies. The rectrices are pale blue in the middle with a black border and contrast sharply with the dark areas of the coat. The throat and breast are white, or washed buff, especially in S. a. nigriventer. The belly and abdomen are blackish, contrasting with blue-grey or purplish . The bill is lavender, slightly tinged with green, and black at the tip; the legs are a pale blue-grey and the claws are slate-grey or black.
The species displays no significant sexual dimorphism, but the Japanese Ornithology Nagamichi Kuroda describes the female as having slightly duller upperparts. Juveniles resemble adults, but have a duller crown and ear coverts, as well as a brown cast that does not cover their entire body. The belly is a dull black and the undertail coverts are variably edged creamy white. The juvenile's bill is blackish, with a pink base. Adults experience a partial moult before the breeding season (February–March for S. a. expectata; March–April for S. a. azurea) involving the throat, chest and mantle; a complete moult takes place after the breeding season (March–April and August in Java in Malaysia).
Dark blue upper parts and azure coverts.
File:Sitta azurea nigriventer png.png| Sitta azurea nigriventer - Western Java
Purplish-blue upper parts, blue coverts and buffy belly.
File:Sitta azurea azurea png.png| Sitta azurea azurea - Central and eastern Java
Purple upper parts and violet coverts.
In Sumatra, the bird is found throughout the Barisan Mountains, and has been observed in the Gayo Highlands of Aceh province, the Batak Highlands of northern Sumatra, and at Mount Dempo in the south of the island. In a 1918 expedition by Robinson and Kloss, they commented: "from the commencement of heavy jungle on the valley slopes up to about 8,000 feet on Mount Kerinci this strikingly coloured little Nuthatch was very common, feeding on tall tree trunks in parties of six or seven."
The blue nuthatch is typically found on mountains, inhabiting subtropical or tropical moist lowland forest and subtropical or tropical moist montane forest. In Malaysia, it is found from to the highest point in the country at . In Sumatra, the species has been reported at an altitudinal range of between and , and on Java, between and . It has also been seen occasionally at lower altitudes on the plains of Java.
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