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The black-chested sparrow (Peucaea humeralis) is a small bird in the New World sparrow family Passerellidae. It is endemic to southwestern Mexico, where it is found in dry thorn forest and adjacent scrubby brush.

The species was first described by Jean Cabanis in 1851. There are no subspecies. The International Union for Conservation of Nature rates it as a species of least concern. Though it is estimated to number fewer than 50,000 mature individuals and its overall population trend is declining, it still inhabits a large range, its population is not fragmented, and the scrub habitat it prefers is not known to be decreasing.

Taxonomy
When German ornithologist Jean Cabanis first described the black-chested sparrow in 1851, he assigned it the scientific name Haemophila humeralis. He indicated that Haemophila was a revised spelling for the genus Aimophila, a name previously established by William John Swainson in 1839. Over the ensuing century, the species bounced through several genera, including Amphispiza and Embernagra, though most taxonomists kept it in the genus Aimophila. However, a genetic study published in 2009 revealed that the genus Aimophila was polyphyletic – that is, that some members assigned to the genus were not closely related to the others. The authors of that study resurrected the genus Peucaea (first created by John James Audubon in 1839) and moved eight species, including the black-chested sparrow, into the newly restored genus. The same study showed that the bridled sparrow is the black-chested sparrow's closest relative. There are no subspecies. The species is one of many belonging to the family Passerellidae, also known as the New World sparrows.

The genus name Peucaea is from the Ancient Greek word peukē, meaning "pine tree". The species ephithet humeralis is a Late Latin word meaning "of the shoulders" (from the Latin umerus). In the past, the black-chested sparrow was also known as the Ferrari-Perez sparrow.

Description
The black-chested sparrow is a small passerine, measuring 6 – in length, and weighing 24.8 –. At each life stage, the sexes look the same, though males average larger than females. The adult's head is blackish-brown, with a whitish and a white. The throat is also white, edged by a black and a black breast band. The underparts are primarily white, though buffier on the, and. The iris is brown.

Habitat and range
The black-chested sparrow is endemic to the interior and Pacific Slope of southwestern Mexico. Its range stretches from southern Jalisco and northeastern Colima southeast through Michoacan and Guerrero to the southwestern edges of Edomex, Morelos, and Puebla, and a small part of western Oaxaca. Though it is generally found at elevations ranging from 300 –, in some places it descends nearly to sea level.

Breeding
Both members of the pair build the nest. This is roughly 115 mm across, with an inner cup which measures some 60 mm across and 37 mm deep. Constructed of fine grasses on the outside and lined with more fine grass and animal hair, it is generally located within 12 in of the ground in thick weeds. Occasionally it is placed in a small tree. The female lays three unmarked eggs, which are a pale bluish-white. These measure between 20 – long and 16 – wide. The black-chested sparrow is known to be a host for the bronzed cowbird, a brood parasite.

Vocalizations
Black-chested sparrows sing in duet, with both members of the pair vocalizing. The song, which is rapid with no pauses, has been transcribed as "che-ti-ti che-ti-ti chi-chiti chititi, etc." The alarm call is a short, metallic "pit".

Conservation and status
The International Union for Conservation of Nature (IUCN) rates the black-chested sparrow as a species of least concern. Though its numbers are relatively small – estimated at fewer than 50,000 mature individuals – and thought to be decreasing, the species is still found across a large range in southwestern Mexico and its declines are thought to be small. Its populations are not fragmented, and the shrub habitats that it prefers are not known to be declining. It is found in a number of protected areas, including El Veladero National Park and El Tepozteco National Park.