Females and young birds have brown plumage with black marks above, and a dull grey underside, and are distinguished from other species of sparrow by their large, distinct supercilium.

The male has a black or greyish-black crown and eyestripe, a grey nape and a small patch of white on the lower forehead.

The cheeks and underparts are pale grey, and the throat and chin are marked with a small black bib.

The juvenile resembles the adult female, but young males are more chestnut from an early age, with a trace of a black bib on the chin.

Calls are chirps, somewhat similar to those of other sparrows, the usual version made by males described as a "twangy" or "chew-weep", and that of females described as a "more sibilant" chisk.

[6] The Iago sparrow was first collected by Charles Darwin during the first stop of the second voyage of HMS Beagle at the island of Santiago (St.

[10] French ornithologist Émile Oustalet described a specimen from Branco as a separate species Passer brancoensis in 1883,[2][11] which was recognised as the subspecies Passer iagoensis brancoensis by W. R. P. Bourne, who claimed to observe differences between Iago sparrows from different islands.

[12] He later wrote that the variations he saw comprised two clinal trends, of increasing darkness towards the south, and of smaller size further from the continental coast.

It is common on most islands, excluding Fogo (from which it is absent) and Santa Luzia, Branco and Sal (on which it is scarce).

[3][15] The Iago sparrow is found commonly in a variety of habitats, including flat lava plains, coastal cliffs, gorges, and the edges of farmland, at altitudes of up to 1,200 metres (3,900 ft).

Though the size of its range means it may be at risk to unpredicted changes in its environment, it is assessed as Least Concern by the IUCN Red List.

[1] In May 2013 four vagrant Iago sparrows were seen at Hansweert, in the Netherlands, having flown onto a ship as it passed by the island of Raso.

On Cima, W. R. P. Bourne observed females remaining in flocks while males began to take up locations on rocky slopes from which they could sing.

When approached by a female, the displaying male will increase the intensity of his calls and hop around her while crouching with chestnut rump- and shoulder-feathers exposed.

The nest is an open structure made of grass, lined with feathers and hairs, packed densely for compactness.

Some ornithologists have reported this bird building domed nests in acacia trees, but these records may reflect confusion with the Spanish sparrow.