Scaly-breasted woodpeckers range in size between 22 and 26 centimetres (8.7 and 10.2 in). Their weight varies by subspecies; it is 63–75 grams for C. g. grammicus, 77–82 grams for C. g. verreauxii, and 75–87 grams for C. g. latifasciatus. They are a mixture of rufous, chestnut, and brown over most of their body. The head is short, and is described as "shaggy", with a pointed crest that has often has black streaks. The wings and upperparts of the bird are also barred in black, although the thickness of the bars depends on which race the individual is from. The primary feathers are blackish with a rufous edge, while the coverts are buff-colored and the underside of the wing is brown. The rump, vent, and undertail-coverts are unmarked, and colored buff, cinnamon, or yellow, while the tail is a plain chocolate color. The uppertail-coverts have some barring on them. The underside of the bird is paler than the upperparts, being colored tawny or cinnamon; the belly and breast have black bars and chevrons. The flanks of the species have scale-like markings colored brown or yellowish. The beak is mostly cream-yellow, while its base is bluish or greenish. The legs are grey, while the iris is red.
The species exhibits a slight sexual dimorphism: males have a red patch on their lower cheeks, while the females do not. Juveniles look similar to adults, but have darker and browner heads, and heavier barring on the back and the mantle. There is considerable variation among subspecies or races; C. g. grammicus tends to be heavily barred; C. g. verreauxii has lighter barring on its underside; C. g. subcervinus is cinnamon-colored on its rump and flanks; and C. g. latifasciatus is lighter than the other races, being cinnamon colored rather than chestnut on its head, and a pale buffish-brown on its underside. In general, birds from the northern Amazon basin are darker than those from the south. The scaly-breasted woodpecker closely resembles the waved woodpecker (Celeus undatus ), and has a similar call, but is differentiated by its darker appearance and unbarred tail. It also resembles the chestnut woodpecker (Celeus elegans ), but the latter is considerably larger.
Celeus grammicus is chiefly a forest species, being found in rainforest, including both terra firme forest and várzea forest in the Amazon, and subtropical forest, as well as in swamps, drier wooded savanna, and secondary forest. The species feeds on fruits, sap, and insects, including ants. It is seen foraging on tree trunks, branches, and vines, from the tops of the canopy to the middle regions. It often forages in small family groups, and frequently associates with mixed species flocks. The species does not drum very often. Its most common call is a two-noted nasal whistle, of which the first note rises and the second falls. This call is very similar to that of the waved woodpecker. The scaly-breasted woodpecker also produces a few other calls, including a series of rasping metallic "pring" notes.
The scaly-breasted woodpecker occurs across a large portion of South America. It is predominantly found in lowland regions, but is seen up to and above 1,000 metres (3,300 ft) above sea level, in southern Venezuela, Colombia, Ecuador, Peru, northern Bolivia, and the Amazonian region of Brazil. It has also been recorded in French Guiana. It is thought not to be a migrant species. Of the four races, C. g. grammicus is found in southeastern Colombia, northeastern Peru, southern Venezuela, and western Brazil; C. g. verreauxii is found in eastern Ecuador and northeastern Peru; C. g. subcervinus is found in the western part of the Brazilian Amazon basin, from the Rio Tapajos to the northern Mato Grosso; and C. g. latifasciatus is found in southeastern Peru, northern Bolivia, and in southwestern Brazil in the upper regions of the Rio Madeira basin. The species is thought to be relatively common in the Peruvian and Venezuelan parts of its range, and less common in Ecuador and Colombia. It is not well known, and little population data is available. Its habitat is thought to be in decline as a result of deforestation in the Amazon forest. The scaly-breasted woodpecker, and its sister species the waved woodpecker, replace each other across the Amazon basin. The scaly-breasted woodpecker is generally predominant south of the Amazon river and west of the Rio Negro.
