The six species of rock-sparrow were formerly lumped together in the genus Petronia, but current opinion recognizes that these are sufficiently distinct to be placed in three separate genera. The Pale Rock-sparrow (Carpospiza brachydactyla) differs from the others in its triangular-shaped wings, giving it an appearance in flight more like that of a lark (Alaudidae); it also builds an open cup-nest in a low bush. This and other characteristics, such as the fact that the post-juvenile moult is only partial, compared with the complete moult of other sparrows, have led H. Mendelssohn and others to suggest that it is not even a sparrow but, rather, a cardueline finch. Nevertheless, internal characteristics such as the cranium, and the structure of the horny palate and the digestive tract, confirm its relationships with the sparrows, although it is sufficiently distinct to be placed by itself in a separate, monotypic genus, Carpospiza. The story does not end there. The five remaining rock-sparrows fall into two distinct groups. One consists solely of the Common Rock-sparrow, which is confined to the Palearctic Region, is terrestrial in behaviour, and is associated with open, often barren, rocky ground. The second group contains the remaining four species, which occur widely over the Afrotropical, southern Palearctic and Oriental Regions and are more arboreal, living in open dry country with scattered trees, wooded savanna or open woodland; these are perhaps more appropriately termed "bush-sparrows" and are treated as belonging to a separate genus, Gymnoris, which is separated from Petronia on the basis of body structure and behaviour. Petronia has a short square tail with white terminal spots and longer wings, whereas the tail of Gymnoris is slender and slightly forked and lacks the white terminal spots; the bill of Petronia is heavy and conical, while that of Gymnoris is pointed and slender. Petronia, with its noticeably bouncy flight, is a highly social species, breeding mainly in loose colonies with up to 100 pairs, the nests at times separated by only a few metres. Its nest, usually placed in a crevice in a rock, a hole in an earth bank or a rodent burrow, is domed over like those of the Passer sparrows; it is largely terrestrial, and normally found in large flocks outside the breeding season. In contrast, Gymnoris species are much less social, at times in small flocks, but normally breeding in isolated pairs. They nest in tree cavities with usually only a pad of nest material, and are largely arboreal, using the pointed bill to obtain insects under tree bark, although they forage also on the ground.
