Conebills (Conirostrum) are small, warbler-like thraupids with a sharply pointed bill. Cinereous Conebill is a relatively dull colored species. The upperparts are pale slate gray to brownish olive above, and with pale gray to warm buffy underparts; these differences in tone generally reflect geographic variation across the three recognized subspecies. All subspecies have a bold pale supercilium, and a whitish panel at the bases of the primaries that forms a conspicuous "speculum". The sexes are similar, but the three subspecies also vary subtly in size and proportions.
Within conebills, Cinereous Conebill is similar only to Tamarugo Conebill (C. tamarugense) of southern Peru and northern Chile. However, Tamarugo Conebill easily is distinguishable by its rufous supercilium, throat, and vent. Cinereous Conebill also could be mistaken for other tanager species such as Superciliaried Hemispingus (Thylopsis superciliaris), especially the populations of central Peru (subspecies insignis and leucogastra). The latter species is a small, warbler-like tanager roughly the same size as Cinereous Conebill; both species are gray above, dull white below, and with a white supercilium. The hemispingus, however, has a slightly thicker bill and lacks the pale speculum at the base of the primaries. See also female Rusty Flowerpiercer (Diglossa sittoides).
The following description is based on Fjeldså and Krabbe (1990) and Jaramillo (2011), and refers to nominate cinereum; see also Geographic Variation:
Adult: Sexes similar. Upperparts pale slate-gray, crown dusky. Narrow forehead line and conspicuous supercilium white or whitish. Remiges and wing coverts dusky (except for black primary coverts), most narrowly edged with pale gray. Greater secondary coverts tipped with white, forming a narrow wingbar. Bases of central primaries white, forming a small speculum, touching the white wingbar. Underparts pale gray, sometimes with a fain brownish or buffy wash; vent buff.
Juvenile: Pattern similar to that of adult, but the feathers of back and breast faintly dark tipped, and the supercilium and underparts are faintly tinged with yellowish.
Immature: Similar to adult, but with a buff wash on the wingbars and edges of the tertials.
Based on an assessment of museum specimen data (Chavez 2016), molt can occur throughout most months of the year in Peruvian populations of Cinereous Conebill in the region between 5º and 12º S; no molt data for subspecies fraseri (southern Colombia and Ecuador) are available. The vast majority of wing and tail molt was observed during July and August, for both subspecies. Other types of molt are more protracted, occurring during all months in which there are available specimen data. This sampling is preliminary, and more data are needed to understand the phenology of this species, including variation across latitudes and among subspecies. Data available to describe the breeding season of Cinereous Conebill are limited and primarily consist of gonadal measurements of museum specimens; these suggest that breeding commences between September and December, but it also may occur at other times of the year (i.e., May). The phenology of the observed molting pattern is consistent with the hypothesized breeding period as adult molting typically occurs pre- or post-reproduction. However, a more robust and systematic sampling effort for all three subspecies is necessary to reduce the uncertainty apparent in the current data.
Subspecies cinereum and littorale:
Irides: brown to dark brown.
Bill: gray to black; typically the maxilla is dark gray or black, the mandible gray.
Tarsi: dark gray or black; toes gray or black, toe pads ranging from light to dark gray
Data from specimens at the Museum of Southwestern Biology.
Bill: dark brown
Tarsi and toes: dark brown to black, with toe pads dark brown ror gray.
Data from specimens at the American Museum of Natural History.
Total length: 10.5-11.5 cm (fraseri largest) (Fjeldså and Krabbe 1990), 11-12 cm (cinereum, littorale; Schulenberg et al. 2010), 12.5 cm (Ridgely and Greenfield 2001b)
Linear measurements (data from Chavez 2016, based on specimens in the Museum of Southwestern Biology and the American Museum of Natural History):
Morphological measurements of museum specimens suggest that the three subspecies differ significantly in body size. No difference was found in culmen length. However, body mass, tarsus length, tail length, and wing length (chord) are largest in the highland subspecies cinereum and smallest in the coastal littorale. No body mass data are available for fraseri. Otherwise, fraseri is intermediate in size between cinereum and littorale, but shows no significant difference with cinereum for tarsus length. The larger individuals appear to occur in areas above 2500 m elevation where temperatures are substantially cooler than at lower elevations. These morphological characters do not vary within littorale across elevation.
Wing length (chord): In littorale, male wing chord length was found to average 3 mm longer than females, but no difference between sexes was observed in cinereum. Wing chord length is ca 5 mm longer in cinereum than in littorale [cinereum: mean 61.466 mm, SD 2.088, n = 10; littorale: mean 56.265 mm, SD 2.708, n = 54; fraseri: mean 58.713 mm, SD 2.529, n = 23].
Tail: Tail length is ca 7 mm longer in cinereum than in littorale [cinereum: mean 52.5, SD 1.779 mm, n = 10; littorale: mean 45.528 mm, SD 2.416, n = 53; fraseri: mean 50.282 mm, SD 2.9612, n = 23].
Bill length (culmen): Culmen length does not vary by subspecies [cinereum: mean 7.222 mm, SD 0.339, n = 10; littorale: mean 7.159 mm, SD 0.328, n = 54; fraseri: mean 7.025 mm, SD 0.494, n = 23].
Tarsus length: Tarsus length is significantly longer in cinereum and fraseri than in littorale [cinereum: mean 18.241 mm, SD 0.831, n = 10; littorale: mean 17.0 mm, SD 0.808, n = 54 ; fraseri: mean 18.264 mm, SD 0.667, n = 23].
Body mass varies by subspecies, with cinereum weighing over a gram more than littorale. Males and females in either subspecies do not differ [cinereum: mean 9.337 g, SD 0.264, n = 9 littorale: mean 8.125 g, SD 0.612, n = 42; t-test: t = 9.377, p=2.495e-10].